Adapting MMM's radiant heat design to a single bedroom

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Hello!

MMM retrofitted a house with a radiant heat floor system.  Here's the diagram.
https://www.mrmoneymustache.com/wp-content/uploads/2014/01/radiant_system_1600px.jpg

I'd like to adapt this to a single bedroom of my house.   This one particular room is a converted garage.  The previous owner actually laid down all of the piping (PEX!) for a radiant floor system, but then had to rip out the heating bits to pass inspection when he sold the house to me.  I'd like to start fresh using MMM's approach as a template.

MMM installs a pretty fierce $2000 water heater in his diagram, because he's heating his whole house, but I was wondering if I could get away with using one of these $175 point-of-use tankless electric heaters instead as a dedicated heat source for just this room.  https://www.amazon.com/Rheem-RTEX-04-Heating-Residential-Tankless/dp/B01N7U5S0G?SubscriptionId=AKIAILSHYYTFIVPWUY6Q&tag=duckduckgo-ffab-20&linkCode=xm2&camp=2025&creative=165953&creativeASIN=B01N7U5S0G

The max flow rate on that Rheem unit is 0.5 GPM, which sounds lowish, but... is that a big deal?  I measured how much water fits into the under-floor system in my house.  Less than 5 gallons of water!  So, that means from a cold start, with 10 minutes of work the heater could have heated all of the water to the set-point at least once.   Sounds pretty good?

Is my proposed adaption cray-cray? Or zany and effective at shockingly low cost?

[J Boogie]

The glaring question here is why did the heating bits (valves? boiler?) have to be removed to pass inspection. You gotta find out.

Sometimes red tape is a nuisance and sometimes it's for good reason. Building code is almost always for good reason.

If you've already got the PEX ran through the floor, then the most laborious part is already done (whether its done properly is a good question though - transfer plates, insulation, etc).

Definitely have a plumber/heating professional come out and look at it, probably have them bid on completing it.

I think the stand alone dedicated water heater you've selected is the right idea.

I don't think MMM was wise to tie his PEX loops into his potable water piping. He discards them as being uptight or whatever but man sometimes you gotta listen to professionals. To be honest I wouldn't use his approach as a template. He's a pro when it comes to personal finance, but he's spread too thin across other disciplines to be regarded as an expert in those fields.

“A little learning is a dangerous thing; drink deep, or taste not the Pierian spring: there shallow draughts intoxicate the brain, and drinking largely sobers us again.”

A decent house is too valuable to use as a sandbox to learn plumbing (unless you're doing low risk, by-the-book stuff).

[trammatic]

I'd say go for it.  As a converted garage, I imagine it's on a slab right?  I'd fill the pipes with water and let them sit for a couple of days to check for leaks.  Then, add in your water heater and a pump and see what happens.  To help prolong the life of the heater and pump, I'd look into what fluid is used for closed-loop systems--distilled water? Any sort of additive to prevent growths?

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The glaring question here is why did the heating bits (valves? boiler?) have to be removed to pass inspection. You gotta find out.

Sometimes red tape is a nuisance and sometimes it's for good reason. Building code is almost always for good reason.

...

I can't be sure since it's so long after the sale and the previous owner's been blowing me off, but I expect it's because they had their system looped into the house's main water heater (as MMM does) but they didn't use all the right components to protect drinking water.

[a_scanner_brightly]

I'd say go for it.  As a converted garage, I imagine it's on a slab right?  I'd fill the pipes with water and let them sit for a couple of days to check for leaks.  Then, add in your water heater and a pump and see what happens.  To help prolong the life of the heater and pump, I'd look into what fluid is used for closed-loop systems--distilled water? Any sort of additive to prevent growths?

They were actually still full of water when we bought the place.  Although I suppose it's possible when we drained them and "only got 5 gallons", it's because that's what hadn't leaked out yet. :P

Good call on additives.  Do you know off the top of your head how closed loop systems compensate for loss due to evaporation?   Do I need to periodically "inject" some water in every few years?

[Cadman]

It’s time to grab a good book on radiant heating and dig in. How many loops are we talking? Is there a manifold? Is this just one large loop?

You’re going to want to perform a leak test using air pressure. You can get the Schrader valve fitting, Pex rings and crimper at the local box store. Seal one end off, pressurize the other and let sit. You can even get a dedicated gauge to attach for this purpose. The same technique is used to verify plumbing, but I don’t recall max PSI off the top of my head for a closed system.

I would NOT use any on-demand heat source. They’re designed for intermittent use and would not only wear out prematurely, but would likely use a lot more energy than a well-insulated tank-type heater. If a boiler is planned, a small water heater makes an excellent storage tank, or can be brought into service in time of emergency, though probably not critical for a garage slab.

You’re also going to want to use an appropriate antifreeze even if you don’t expect the slab to ever drop below freezing. Radiant antifreeze has additives to help lube your pump and cut down on contaminant buildup. When done correctly, a good closed system should almost never need a top-off, assuming all air is purged.

In selecting a circulation pump, it will be helpful for you to know loop length and diameter (assuming this is just one large loop). There’s some math involved as you need to ensure good heat exchange, run the numbers to correctly size the pump, and attempting to avoid system turbulence.

I'm hoping they used barrier-Pex and not regular, cheaper, potable grade. One is designed for radiant heat and the other for cheapskates.

[a_scanner_brightly]

Thanks for the useful feedback.  Question on this part:

I would NOT use any on-demand heat source. They’re designed for intermittent use and would not only wear out prematurely, but would likely use a lot more energy than a well-insulated tank-type heater. If a boiler is planned, a small water heater makes an excellent storage tank, or can be brought into service in time of emergency, though probably not critical for a garage slab.

I don't really understand why keeping water heated in an insulated tank is a win.  The purpose of the heater is to keep water in the entire system warm, including incrementally warming recirculated water.  I would think eliminating the tank entirely is the way to get the most efficiency.  e.g. the water out in the system is "the tank".

Also a point-of-use tankless water heater is only about $175.  Even if I have to replace it every 3 years isn't that still cheaper than the alternatives?

[Cadman]

The major problem is short-cycling. On demand units are optimized to bring ground temp water up to 120F+. For radiant heating, you have a much smaller temp delta you're trying to maintain. At that point it becomes a control-loop problem (cut-in, overshoot, oscillating). These also typically have higher head pressure requirements so you might need a higher pressure ($$) pump. I'm not saying it can't be done, there are some units rated to do this and not void their warranty, but you might spend more than a conventional tank heater ($300-400) and not be satisfied with the final result.

Ultimately you need to do the calcs to see what makes sense and only then will you know what to buy.

[J Boogie]

If it's a dedicated water heater, then it can be shut off and turned on again in the winter when it will be needed.

Otherwise water heaters run year round so we can take warm showers and wash dishes with hot water. Run your water heater too low and you risk legionnaire's disease. But that's not a concern if you never come into contact with the water.