Author Topic: Need help designing a radiant heat system  (Read 1420 times)

intellectsucks

  • Bristles
  • ***
  • Posts: 252
Need help designing a radiant heat system
« on: September 14, 2020, 12:50:55 PM »
Cold weather is coming quick to my area, and I have the time and money to install a radiant heat system in my house.

This MMM article (https://www.mrmoneymustache.com/2014/02/16/the-radiant-heat-experiment/) makes me confident that I can tackle the install, however it was pretty light on the processes that went into the DESIGN of such a system.

How do I calculate the size of such a system and it's components?

Thanks in advance for your advice and help.

intellectsucks

  • Bristles
  • ***
  • Posts: 252
Re: Need help designing a radiant heat system
« Reply #1 on: September 16, 2020, 02:57:14 PM »
Did some research and found what seems to be some useful info:

https://www.usboiler.net/heat-loss-calculator.html
Used this heat loss calculator to get an idea of what type of heat loss I'm using.  Here are my inputs from my measurements and their chart:
Length 77 (area1) 22 (area2)
Width 79 (area 1) 20 (area2)
Height 7 (area 1) 11 (area2)
Windows and Doors sq ft 78 (area1) 50 (area2)
Windows and Doors HM 41
Net walls HM 6
Ceiling HM 4
Floor HM 2 (I plan on insulating the floor very well when doing the install)
Infiltration HM 1.61

This gave me an estimate of around 114k BTU heat loss

Based on that, does this water heater seem like overkill (my house does not have gas, so I need an electric one)? https://www.homedepot.com/p/Rheem-Performance-36-kw-Self-Modulating-7-03-GPM-Tankless-Electric-Water-Heater-RETEX-36/300800822?source=shoppingads&locale=en-US&mtc=Shopping-VF-F_D26P-G-D26P-26_10_WATER_HEATERS-RHEEM-NA-Feed-LIA-NA-NA-New_Engen&cm_mmc=Shopping-VF-F_D26P-G-D26P-26_10_WATER_HEATERS-RHEEM-NA-Feed-LIA-NA-NA-New_Engen-71700000070895281-58700006241204832-92700056340155208&gclid=CjwKCAjw74b7BRA_EiwAF8yHFBo6OlziMol9sO8WCnWkF5qAebBwoN8Z-Bvs-R5VRD7NWfTbaDbacBoCNAAQAvD_BwE&gclsrc=aw.ds

How do I figure out what size water pump I will need?

If I overestimated my needs in either area (or if/when I improve my heat loss efficiency) will it cause problems for the components?

Thanks again for the thoughts and attention.

robartsd

  • Magnum Stache
  • ******
  • Posts: 3084
  • Location: Sacramento, CA
Re: Need help designing a radiant heat system
« Reply #2 on: September 16, 2020, 03:29:31 PM »
I'm not sure how to calculate your circulating water pump requirements, but it will depend on how you layout your system (size and lengths of tubing, number and type of joints) as this will determine the resistance to the flow of water. You need to figure out your target flow rate, calculate the resistance of your layout at that flow rate, then select a pump that will overcome that resistance.

I doubt that efficiency gains will be much of a problem for your system components. It should take enough time to circulate all the water in the system that I wouldn't be concerned about short cycling the equipment. The minimum on cycle length shouldn't really change with efficiency gains.

I'm interested in the type of system featured on builditsolar.com. Not sure if anyone there posted information on how they calculated the pump size.

Dusty Dog Ranch

  • Stubble
  • **
  • Posts: 230
Re: Need help designing a radiant heat system
« Reply #3 on: September 17, 2020, 07:12:54 PM »
We did most of the radiant system at our former house (back in 2003), and our best resource (and where we bought materials) was the Radian Company. They are actually supportive of DIY folks. https://www.radiantcompany.com/

uniwelder

  • 5 O'Clock Shadow
  • *
  • Posts: 53
  • Age: 40
  • Location: Virginia
Re: Need help designing a radiant heat system
« Reply #4 on: October 02, 2020, 09:13:40 AM »
Sorry, but this project sounds like a terrible idea.  You're going to be spending a good chunk of time and money installing a complex electric resistance heating system with the only benefit of warm floors.  It worked well for MMM because he's hooked to natural gas.

Also, for calculating your heat load, what kind of equipment and size does your house currently use? 

Update---- from looking at your last posting about getting your oil furnace operational, it seems like you might have functional oil again?  You want to replace it with radiant floor heating?  What's the btu burner capacity of the furnace and how often does it kick on in the cold of winter?  I would trust sizing for a new system based on what you currently use (adjusting based on whether its short cycling or running appropriately).  During the coldest time, if it only runs 10 minutes, then shuts off for 10, its probably oversized 2x and your new system should be sized that much smaller.
« Last Edit: October 02, 2020, 09:23:02 AM by uniwelder »

intellectsucks

  • Bristles
  • ***
  • Posts: 252
Re: Need help designing a radiant heat system
« Reply #5 on: October 05, 2020, 02:55:18 PM »
Sorry, but this project sounds like a terrible idea.  You're going to be spending a good chunk of time and money installing a complex electric resistance heating system with the only benefit of warm floors.  It worked well for MMM because he's hooked to natural gas.

Also, for calculating your heat load, what kind of equipment and size does your house currently use? 

Update---- from looking at your last posting about getting your oil furnace operational, it seems like you might have functional oil again?  You want to replace it with radiant floor heating?  What's the btu burner capacity of the furnace and how often does it kick on in the cold of winter?  I would trust sizing for a new system based on what you currently use (adjusting based on whether its short cycling or running appropriately).  During the coldest time, if it only runs 10 minutes, then shuts off for 10, its probably oversized 2x and your new system should be sized that much smaller.

Thanks for the input. 

A couple reasons why I'm doing this: Our floors are so cold for most of the winter that it's difficult to walk on them without shoes or slippers.  There is very little insulation down there now and most of it isn't really functional anymore (old fiberglass batts that are falling, mostly shredded and in some cases have water damage).  I'm going to be down there installing new insulation anyway, so I'm figuring on adding in a relatively small amount of extra work to yield an exponentially huge increase in comfort.

Our living room also has high, sloped ceilings with a landing that leads to the second floor.  That landing is generally very warm in winter but the actual living area is cold.  Radiant floor heating will heat the area much more efficiently from a comfort standpoint than our current one.

After talking to the Radiant Company (thanks for the reference Dusty Dog Ranch!!) they seemed to indicate that using a propane fuel tankless water heater would be a better option than electric due to cost and capacity, though I'm still digging into that claim.  I'm not planning on REPLACING my existing oil heater, but rather SUPPLEMENTING it.  Perhaps I'm inferring something you didn't imply, but I get the impression that you seem to agree that using electric as the main heating source isn't very efficient from a cost standpoint.  I'm having trouble getting cost comparisons between LP gas and electric/BTU.  Can the difference be so huge?  Adding a propane tank that is sufficient to power run the system for a while, running lines to the heater and venting it are all complexities that I wasn't planning on dealing with.


uniwelder

  • 5 O'Clock Shadow
  • *
  • Posts: 53
  • Age: 40
  • Location: Virginia
Re: Need help designing a radiant heat system
« Reply #6 on: October 05, 2020, 04:27:24 PM »
Cost of LP vs electric is unknown to me.  I had the impression LP was almost as expensive as electric for heat, but I'm not sure.  The other issue is the electrical draw that water heater demands at full blast---- 150 amps at 240 volts---- probably demanding a service upgrade to your home.  Most houses generally have 200 amp incoming service for everything under the roof.  Perhaps you don't actually need that hefty of a water heater, not sure.

If you only plan on using this as a supplement and still rely on your oil furnace, perhaps you can use a smaller water heater and not worry too much about your electric bill.  I assume your current system is forced air.

Papa bear

  • Handlebar Stache
  • *****
  • Posts: 1530
  • Location: Ohio
Re: Need help designing a radiant heat system
« Reply #7 on: October 05, 2020, 06:24:51 PM »
Oh man.  Do you have any experience in hvac at all? I remodel houses.  My brother has a few places with boilers.  And Iím left scratching my head on the design of these things. 

I think you need to get to a supply house for radiant heat and find someone you can pay to consult you for what you need.  Thereís zones, flow rates, heat loss, different manifolds, mixing valves, recirculating pumps, you need to keep them at the correct temp or you boiler can condensate, youíll have primary and secondary loops. And that was all from calling one of the suppliers.  I said, well eff this to my brother, hook back up those rads, to hell with in floor heating on these places. 

If you figure this out, please post more.  I really struggled to find proper information online.


Sent from my iPhone using Tapatalk

Well Respected Man

  • Stubble
  • **
  • Posts: 202
  • Location: About Town
Re: Need help designing a radiant heat system
« Reply #8 on: October 05, 2020, 08:50:24 PM »
You can also attach a tankless system (with or without a tank) to your oil burner. Our radiant heat included such a system, and provided domestic hot water as well. Are you using the oil burner for domestic hot water and/or radiators? If so, it should be possible to add the tankless system. If not, you would need to replace the furnace with a boiler, which would then run the radiant heat, domestic hot water, and optionally, radiators.

robartsd

  • Magnum Stache
  • ******
  • Posts: 3084
  • Location: Sacramento, CA
Re: Need help designing a radiant heat system
« Reply #9 on: October 06, 2020, 09:55:56 AM »
1 kWh is about 3400 BTU. 1 gallon propane is about 91000 BTU. Resistive electrical heating at $0.20/kWh costs about the same as efficient propane heating at about $5.00/gal.

big_owl

  • Pencil Stache
  • ****
  • Posts: 895
Re: Need help designing a radiant heat system
« Reply #10 on: October 08, 2020, 03:44:54 PM »
Using electric resistance heat to heat water then using that water to heat your home seems a whole lot more complicated and expensive than just using electric resistance heat to heat the air in your home.  It doesn't make sense.

Get a natural gas line and the math changes, but barring that just buy a very nice heat pump system. 

robartsd

  • Magnum Stache
  • ******
  • Posts: 3084
  • Location: Sacramento, CA
Re: Need help designing a radiant heat system
« Reply #11 on: October 09, 2020, 09:08:23 AM »
Get a natural gas line and the math changes, but barring that just buy a very nice heat pump system.
Or look into solar thermal.

intellectsucks

  • Bristles
  • ***
  • Posts: 252
Re: Need help designing a radiant heat system
« Reply #12 on: October 09, 2020, 12:57:40 PM »
Regarding the benefits of radiant floor heating: overall it is generally a much more efficient heating system than forced air.  Since heat rises, forced air systems tend to heat the room from the top down.  In my situation, that means an absolutely MASSIVE space needs to be heated before it reaches the lower parts of the living area.  This means that I have to keep my thermostat around 75 or 76 on the first floor to get the living room to be around 70, and even then the floors are uncomfortably cold.  Radiant floor heat heats the space from the bottom up, meaning that the majority of the heat is lower down where people are, not up near the ceiling and I can run the heat at a lower temperature with much more comfort.

Regarding efficiency of the system, electric water heaters tend to be much more efficient than gas ones (Natural gas or propane), topping out at around 98% efficiency, whereas gas systems tend to be in the 75-85% efficiency range.  After digging into the pricing, propane runs in the $3.50-$4/gallon range in my area, but those prices can swing wildly.  Electricity is around .25/kwh and I plan on converting to solar power within the next few years anyway, so I'll probably end up going with the electric one.

The Radiant Company was extremely helpful and basically calculated all of the components that would be needed for my system based on a sketch of the area being heated and some additional info.  Their numbers mostly line up with what I was coming up with from my own research so I'm pretty sure I'm on the right track in regards to system design.  Barring any major complications, I'll probably be starting the install in the next two weeks and will update more then.  Thanks for the advice and input everyone!

uniwelder

  • 5 O'Clock Shadow
  • *
  • Posts: 53
  • Age: 40
  • Location: Virginia
Re: Need help designing a radiant heat system
« Reply #13 on: October 10, 2020, 04:51:35 AM »
Since you seem pretty decided about this, your panel capacity and incoming service will handle the load?

big_owl

  • Pencil Stache
  • ****
  • Posts: 895
Re: Need help designing a radiant heat system
« Reply #14 on: October 10, 2020, 05:20:09 AM »
Since you're using the high efficiency figure of electric resistance heat as a selling point it leads me to believe either you maybe need to do some more research or are being sold a bill of goods.. Of course resistance heat has high efficiency....but you're paying for the inefficiency of the electric company's 3 on 1 GTG and the natural gas it uses for fuel.   I have resistance radiant heat in several of my bathrooms. Our master bath required 220V @ 10A just for the bathroom. And our electric prices are half yours per kwh.  As mentioned above, what did the radiant company determine for your service size?

Well Respected Man

  • Stubble
  • **
  • Posts: 202
  • Location: About Town
Re: Need help designing a radiant heat system
« Reply #15 on: October 11, 2020, 06:24:32 AM »
Yes, see the BTU calculations above. Even though the electric heater itself is 100% efficient almost by definition, the production of that electricity costs energy, and that process is far from 100% efficient, adding to the cost of the output. Add in the line losses, and the overall efficiency drops even further. Even with the most efficient fossil fuel generation, you are looking at 40% efficiency by the time the electricity is available to you. With a typical mix, it could be 30-35%. Those costs are reflected in the price of electricity.

I will second the statement about radiant being warmer for than forced hot air, however. I switched from a cranky old oil-fired radiant system (with forced hot water baseboard in some places) to forced hot air connected to electric heat pumps. I used to set the radiant part to 64, and that was ok in the places where the radiant worked. Now I set the heat to about 70, and it's ok. The first winter was a massive shock at my electric bills. I installed a wood stove (which provides a lot of nice radiant warmth) before the next winter, and switched away from the pricey wind power option. My winter electric bills would still shock and sicken every Mustachian, but they are 1/3 of that first year.

With the combination of a lower thermostat setting plus water heated by electricity, your bills will be somewhere between the extreme lowest and highest, but I would guess that it will be higher than your current system.


robartsd

  • Magnum Stache
  • ******
  • Posts: 3084
  • Location: Sacramento, CA
Re: Need help designing a radiant heat system
« Reply #16 on: October 12, 2020, 10:01:52 AM »
Energy Star tankless gas water heaters must be at least 90% efficient. Condensing tank gas water heaters can also be about 88-90% efficient. At energy prices you stated ($4/gal propane, $0.25/kWh electric)  $1 would provide about 20,475 BTU's of usable heat from propane or 13,600 usable heat from electricity. I imagine that the electric heater would have lower upfront costs.

intellectsucks

  • Bristles
  • ***
  • Posts: 252
Re: Need help designing a radiant heat system
« Reply #17 on: October 14, 2020, 01:43:01 PM »
Thanks for the input guys.  So if I'm understanding the concerns between electric and propane regarding efficiency it can summed up like this: both are burning fossil fuel, but the propane water heater is applying the majority of it directly to heating the water; the electric company is using the fossil fuel to generate electricity and with a lot of energy being lost during the process of generating and getting it to my house.  If this is an accurate summary, then it's not much of a factor for me for a few reasons: 1. the electric grid has been slowly moving towards using more and more renewable energy sources and will continue to do so. 2. Propane costs are volatile and can sometimes have big short term cost increases.  3. I plan on converting the majority of my electricity to solar in the next few years, so the electricity cost may be higher and more fossil fuel dependent now but will drop to near zero in the near future.

Regarding the cost difference for running the various systems: my current heating costs are somewhere between $1500 and $2000 per year using oil heat.  Switching to the radiant flooring as primary heating should give me a cost of around $1000-1200 for electric and $800-1000 for propane, based on the numbers that robartsd estimated.  A difference of $200-$300/year isn't big enough to sway me to the propane system, especially since the propane heater will cost $600-$700 more upfront.  I think even these estimates are high though, because in order to stay comfortable in the area we spend the most time, the thermostat needs to be set to 75-77.  I expect to be able to run the thermostat much lower using the radiant heat and be just as, if not more comfortable, so that should yield some significant cost savings as well.

I'm pretty dead set on the radiant floors, and the discussion that's taking place here has only made it seem like a better idea in my mind.  If I've miscalculated anything that you can see, feel free to correct me and it may sway my decision but otherwise I plan on ordering the PEX this week and installing in the next week or two.  I could probably still be swayed about the LP tankless vs electric tankless but unless I've made a major miscalculation, electric seems to be the best way to go in my scenario.

Thanks again for all the input and advice!!

J Boogie

  • Handlebar Stache
  • *****
  • Posts: 1360
Re: Need help designing a radiant heat system
« Reply #18 on: October 14, 2020, 02:58:56 PM »
DON'T ORDER PEX YET!! Why do you want to use electricity to heat water to heat your floor? Get rid of the middle person. Heat your floor. Just lay some mats in the places your feet spend the most time.

You might have a massive headache on your hands trying to DIY a hydronic radiant system. That's for pros or underemployed geniuses, seriously. And I'm not sure there's much evidence to suggest that electricity heating water that heats a floor is significantly more efficient than electricity heating a floor directly. Lay the mats, enjoy the warmth.

Here are some experts discussing a similar idea.

https://www.greenbuildingadvisor.com/question/electric-hydronic-radiant-heat

uniwelder

  • 5 O'Clock Shadow
  • *
  • Posts: 53
  • Age: 40
  • Location: Virginia
Re: Need help designing a radiant heat system
« Reply #19 on: October 14, 2020, 03:47:37 PM »
Intellectsucks-----   I keep repeating myself (third time) and others have mentioned this as well, but do you have panel capacity and sufficient incoming power for the electric water heater you plan to install?  The one you originally talked about requires 160 amps at 240 volts--- thats pretty huge and for most houses would require an electric service upgrade.  You have yet to address this question in any of your responses so far and its rather important.

big_owl

  • Pencil Stache
  • ****
  • Posts: 895
Re: Need help designing a radiant heat system
« Reply #20 on: October 14, 2020, 04:16:28 PM »
Intellectsucks-----   I keep repeating myself (third time) and others have mentioned this as well, but do you have panel capacity and sufficient incoming power for the electric water heater you plan to install?  The one you originally talked about requires 160 amps at 240 volts--- thats pretty huge and for most houses would require an electric service upgrade.  You have yet to address this question in any of your responses so far and its rather important.

Yeah it says 4x40A double pole breakers... Yikes!   Could end up spending thousands just to upgrade the incoming service.  If the house is currently heated with oil it would lead me to believe it might be an older house with a smaller service anyway.

intellectsucks

  • Bristles
  • ***
  • Posts: 252
Re: Need help designing a radiant heat system
« Reply #21 on: October 15, 2020, 12:52:03 PM »
Hey guys, particularly Uniwelder, thanks for being persistent regarding the service box issue, I hadn't given it the careful scrutiny that I should have earlier.  The amp rating on the home depot model would likely eliminate it as an option but the Radiant Company suggested a model that is rated for 60 amps.  I have an upgraded service box in my home and an electrician friend is coming over soon to check to make sure that my service can handle it.  If it can't then that would probably be the deciding factor swaying me towards the LP heater.

J Boogie, I'm not really understanding your suggestion.  My plan is to use the radiant system to contribute a pretty significant amount to my home heating; are you suggesting putting electric heating mats under the existing flooring or just carrying around the equivalent of heating blankets and using them as heated area rugs?  Neither of those would work well in my situation.  Maybe I'm misunderstanding what you're suggesting?

Thanks again for the input guys!!  It has all been golden!!

nedwin

  • Stubble
  • **
  • Posts: 101
Re: Need help designing a radiant heat system
« Reply #22 on: October 15, 2020, 01:11:00 PM »
Intellectsucks-----   I keep repeating myself (third time) and others have mentioned this as well, but do you have panel capacity and sufficient incoming power for the electric water heater you plan to install?  The one you originally talked about requires 160 amps at 240 volts--- thats pretty huge and for most houses would require an electric service upgrade.  You have yet to address this question in any of your responses so far and its rather important.

Yeah it says 4x40A double pole breakers... Yikes!   Could end up spending thousands just to upgrade the incoming service.  If the house is currently heated with oil it would lead me to believe it might be an older house with a smaller service anyway.


This cannot be emphasized enough.  I found an online calculator for determining the electrical service needs and added this water heater to my home's demands just to see the results.  Just adding this heater to my house required the electrical service go from 125 amps to 250 amps.  This is a huge jump!

You say you're pretty dead set on this, but you might want to consider some less expensive alternatives first.   I was able to obtain a home energy audit through my utility provider in the past (at reduced cost), and it was very helpful for prioritizing energy improvements for my home.  The full cost was about $350, I think it would be helpful even at that price.  Do you have this available?

I would re-insulate the basement/crawl space, work to air-seal the home as much as possible (spray foam and caulking where appropriate), and consider additional attic insulation depending on its current condition before adding more BTU.  There's a small part of my dining room that is cantilevered over the foundation and it was very poorly sealed and insulated.  I could easily feel the temp difference on both sides of the foundation.  I removed the old insulation and had spray foam added, the difference was night/day.

Have you tried a ceiling fan or other fan to better circulate warm air in your living room?  Finally, socks and slippers are a hell of a lot cheaper than what you are proposing.

Edit - Just saw your most recent post re electrical service, sounds like you're on it.
« Last Edit: October 15, 2020, 01:13:56 PM by nedwin »

J Boogie

  • Handlebar Stache
  • *****
  • Posts: 1360
Re: Need help designing a radiant heat system
« Reply #23 on: October 15, 2020, 01:23:11 PM »

J Boogie, I'm not really understanding your suggestion.  My plan is to use the radiant system to contribute a pretty significant amount to my home heating; are you suggesting putting electric heating mats under the existing flooring or just carrying around the equivalent of heating blankets and using them as heated area rugs?  Neither of those would work well in my situation.  Maybe I'm misunderstanding what you're suggesting?


Sorry, I breezed through your posts and misconstrued your plan to remove insulation (presumably from below) as a plan to tear out your flooring. I was thinking of something like Ditra Heat being run if you were to replace the flooring and install new tile.

big_owl

  • Pencil Stache
  • ****
  • Posts: 895
Re: Need help designing a radiant heat system
« Reply #24 on: October 15, 2020, 04:47:17 PM »

J Boogie, I'm not really understanding your suggestion.  My plan is to use the radiant system to contribute a pretty significant amount to my home heating; are you suggesting putting electric heating mats under the existing flooring or just carrying around the equivalent of heating blankets and using them as heated area rugs?  Neither of those would work well in my situation.  Maybe I'm misunderstanding what you're suggesting?


Sorry, I breezed through your posts and misconstrued your plan to remove insulation (presumably from below) as a plan to tear out your flooring. I was thinking of something like Ditra Heat being run if you were to replace the flooring and install new tile.

We have ditra heat in our bathrooms. Works great there but would be hard to apply to the whole house for sure.   

robartsd

  • Magnum Stache
  • ******
  • Posts: 3084
  • Location: Sacramento, CA
Re: Need help designing a radiant heat system
« Reply #25 on: October 16, 2020, 12:15:11 PM »
DON'T ORDER PEX YET!! Why do you want to use electricity to heat water to heat your floor? Get rid of the middle person. Heat your floor. Just lay some mats in the places your feet spend the most time.

You might have a massive headache on your hands trying to DIY a hydronic radiant system. That's for pros or underemployed geniuses, seriously. And I'm not sure there's much evidence to suggest that electricity heating water that heats a floor is significantly more efficient than electricity heating a floor directly. Lay the mats, enjoy the warmth.

Here are some experts discussing a similar idea.

https://www.greenbuildingadvisor.com/question/electric-hydronic-radiant-heat
I can't see any reason to prefer a tankless electric water heater feeding a hydronic system over using electric heat mats directly in the floor; other than to keep the option to change how to heat the water in the future. If trying to maximize the DIY aspect, I'd lean towards using builditsolar.com as a guide to designing and building a solar thermal hydronic system; for the OP's intention to use electric power and add grid tied solar electric, electric heating mats seem like a better option than hydronic.

intellectsucks

  • Bristles
  • ***
  • Posts: 252
Re: Need help designing a radiant heat system
« Reply #26 on: October 16, 2020, 12:58:28 PM »
DON'T ORDER PEX YET!! Why do you want to use electricity to heat water to heat your floor? Get rid of the middle person. Heat your floor. Just lay some mats in the places your feet spend the most time.

You might have a massive headache on your hands trying to DIY a hydronic radiant system. That's for pros or underemployed geniuses, seriously. And I'm not sure there's much evidence to suggest that electricity heating water that heats a floor is significantly more efficient than electricity heating a floor directly. Lay the mats, enjoy the warmth.

Here are some experts discussing a similar idea.

https://www.greenbuildingadvisor.com/question/electric-hydronic-radiant-heat
I can't see any reason to prefer a tankless electric water heater feeding a hydronic system over using electric heat mats directly in the floor; other than to keep the option to change how to heat the water in the future. If trying to maximize the DIY aspect, I'd lean towards using builditsolar.com as a guide to designing and building a solar thermal hydronic system; for the OP's intention to use electric power and add grid tied solar electric, electric heating mats seem like a better option than hydronic.

My understanding of the electric heating mats is that they need to be installed OVER the subfloor, and in this case would require me to remove and replace the existing flooring over approx 1500 sq ft, a project whose scale and cost is NOT feasible right now.  Perhaps I'm misunderstanding and heating mats can be installed under the subfloor the same as hydronic systems?

jeninco

  • Handlebar Stache
  • *****
  • Posts: 1774
  • Location: .... duh?
Re: Need help designing a radiant heat system
« Reply #27 on: October 18, 2020, 12:29:04 PM »
We're thinking about pulling the carpets from a high-ceilinged room and I'd like to add radiant heat mats (probably under slate tiles). Is there anything special I should start thinking about while in the initial planning for this? MrINCO would really, really like warm floors in that room...

robartsd

  • Magnum Stache
  • ******
  • Posts: 3084
  • Location: Sacramento, CA
Re: Need help designing a radiant heat system
« Reply #28 on: October 19, 2020, 04:19:27 PM »
My understanding of the electric heating mats is that they need to be installed OVER the subfloor, and in this case would require me to remove and replace the existing flooring over approx 1500 sq ft, a project whose scale and cost is NOT feasible right now.  Perhaps I'm misunderstanding and heating mats can be installed under the subfloor the same as hydronic systems?
This isn't the ideal placement for either system as there is more material to block the heat from getting to the room. The electric mats are easy to make thin enough to easily incorporate above the subfloor, so I doubt anyone makes electric systems designed to go between the joists. Hydronic systems can be incorporated above the subfloor using specially routed boards, but this also raises the finished floor significantly, so putting it below is often easier. What is your plan for insulating below the hydronic system?

intellectsucks

  • Bristles
  • ***
  • Posts: 252
Re: Need help designing a radiant heat system
« Reply #29 on: October 20, 2020, 10:25:17 AM »
My understanding of the electric heating mats is that they need to be installed OVER the subfloor, and in this case would require me to remove and replace the existing flooring over approx 1500 sq ft, a project whose scale and cost is NOT feasible right now.  Perhaps I'm misunderstanding and heating mats can be installed under the subfloor the same as hydronic systems?
This isn't the ideal placement for either system as there is more material to block the heat from getting to the room. The electric mats are easy to make thin enough to easily incorporate above the subfloor, so I doubt anyone makes electric systems designed to go between the joists. Hydronic systems can be incorporated above the subfloor using specially routed boards, but this also raises the finished floor significantly, so putting it below is often easier. What is your plan for insulating below the hydronic system?

In either case, I'd need to pull up the existing flooring which is NOT happening, hence why I'm going with the hydronic system.  Obviously the less material between the heating source and the room the better, but everything I've read indicates that hydronic systems work very well even when installed under the subfloor.  Everything I've read about electric heating mats indicates that they need to go OVER the subfloor, so they're not an option at this point.

I have a bunch of foam board insulation from a previous project that I'm going to be using.  The radiant company told me that I need to keep it approx 1" below the PEX lines and make sure that it has aluminum on the side facing the PEX.  I've found that foam board doesn't cut super precisely, so I plan to cut it a little shorter than my joists, and then use spray foam to seal it in place.  Since I already have a ton of foam board (10 or 12 sheets), I'm reluctant to purchase fiberglass batts, but I suppose I could still be swayed if it would make a significant improvement to the performance of the system.  And yes, I understand that I'm going to need to purchase some additional insulation as well.

Dusty Dog Ranch

  • Stubble
  • **
  • Posts: 230
Re: Need help designing a radiant heat system
« Reply #30 on: October 20, 2020, 10:39:18 PM »
We did the under-subfloor installation and used the aluminum fins that help the heat spread from the pex, then used fiberglass roll (or batts, can't remember) to fill the rest of the space between the joists. Tried rigid board for the insulation but it was impossible to cut it to fit right.