Geothermal HVAC

[saijoe]

I am wondering about the cost effectiveness of Geothermal HVAC.  It seems like a pretty mustachian idea, but I don't believe I've seen it as a blog or forum subject.

How much does it cost to install?

Are there areas of the country where it's more effective?

[CowboyAndIndian]

A ground loop or a well based geothermal  is effective all parts of the US since it is dependent on the ground temperature and the  ground temperature at about 5 feet below ground is constant year round at about 55 degrees F.

When I priced it about 5 years ago, it was about 2-3 times the price of a regular system, so I did not go ahead with it. I believe that the prices will become competitive when it is about 1.5 times the price of a regular system.

IMHO, it is better than a  regular system since

• It should last longer since all components are indoors
• It is safer, no burning of any fuels to create the heat
• It is cheaper, especially if coupled with solar panels

[ShoulderThingThatGoesUp]

I know several people who have it and love it, but it's certainly not cheap.

[Vibrissae]

I had a geothermal system installed...ugh, I think it must have been about 8 years ago or so. Not cheap! It cost around $40,000, but I got special loans because it was a green energy thing: one small loan that was entirely covered by an energy rebate; one large loan that was interest free for the first year (paid off by the end of that year); and another loan with 0% interest that I'm still paying off (the principal is around $6000 right now; I'm paying $83.33/month).

I've been super happy with it, though. I have a fairly small house that I don't heat or cool excessively (64-68 in winter, mid to upper 70s in summer), and the system definitely does the trick. I never have to worry about oil or gas prices, just the electricity that the system uses to run itself and to supplement the geothermal in extreme temperatures. (I'm in New Jersey, so not too much in the way of extremes here.) At an extremely hazy estimate--I don't have the actual numbers here with me--my geothermal heat+electric is somewhere around half of what my oil heat+electric was. I can probably bring that down further now that I'm living alone.

[rulesofacquisition]

I'm in the Mid Atlantic US and have been selling these for about 10 years. Here, they are either open loop (use well water) or vertical closed loop (circulating an antifreeze solution around pipes in 200 ft deep holes). Our water quality is poor so generally closed loop - costs about double a conventional system. It was a better deal with the tax credit but you may have help availablle thru your utility company or on a state level.

That said, I'm upgrading a 1950's farmhouse, and did a hybrid system for the first floor (heat pump w/gas backup) and a ductless for the second floor. If your house needs other help, like insulation or sealing air leakage, you get more bang for your buck that way, and not going with the most expensive HVAC system out there.

[BudgetSlasher]

We looked into it soon after we bought this house. After state and federal incentive it came out to be somewhere around 10-11 years of fuel costs at that time, from the start one of the installers we were working with made it clear that we were on the cusp of whether it made sense.

That was 10-11 year of fuel cost just for the purchase price, the break even point moved further into the future when factoring in interest and potential lost investment returns.

We've lived in the house a while and made some upgrades that have cut fuel use ~15-20%. I've got another few to get done over the next few years as time and money allow. Maybe once it is all done we will re-examen the possibility. If our improvements can drive our heating load low enough, we could use our existing well as a standing column heat exchange and not have to drill a new well or deepen the existing one (it would probably reduce cost by 40-50% to use the existing well).

Another consideration is that we were looking to upgrade a functional system, so the cost was all allocated to the geothermal. If our current system were to deteriorate to the point of needing to be replaced, well then the math changes again; a third of the cost of geothermal would already be spent.

If I were not somewhere the design value was below zero, I would look at high efficiency mini-splits as an alternative to geothermal. Their COP is rising and their cost is less than geothermal.

[Fishindude]

We have a well based, open loop system.   It's probably cut our heating and AC bills in half.
Heat is a little weird because the air coming out of the registers is not hot, like when you are using a gas fired furnace.  It's got electric coils in the plenum to provide additional heat when the water based heat isn't cutting it.   We run a wood stove too as supplemental heat, so we can get near it if we want to get real warm.

[CowboyAndIndian]

I'm in the Mid Atlantic US and have been selling these for about 10 years. Here, they are either open loop (use well water) or vertical closed loop (circulating an antifreeze solution around pipes in 200 ft deep holes). Our water quality is poor so generally closed loop - costs about double a conventional system. It was a better deal with the tax credit but you may have help availablle thru your utility company or on a state level.

Thanks rulesofacquisition. That is very interesting to know.

• Would the price of an open loop system be the same as a closed loop system?
• For the conventional system, I am assuming you are considering the cost of A/C as well as heat since the geothermal will replace both. Is that right?

That said, I'm upgrading a 1950's farmhouse, and did a hybrid system for the first floor (heat pump w/gas backup) and a ductless for the second floor. If your house needs other help, like insulation or sealing air leakage, you get more bang for your buck that way, and not going with the most expensive HVAC system out there.

+1.

[rulesofacquisition]

I'm in the Mid Atlantic US and have been selling these for about 10 years. Here, they are either open loop (use well water) or vertical closed loop (circulating an antifreeze solution around pipes in 200 ft deep holes). Our water quality is poor so generally closed loop - costs about double a conventional system. It was a better deal with the tax credit but you may have help availablle thru your utility company or on a state level.

Thanks rulesofacquisition. That is very interesting to know.

• Would the price of an open loop system be the same as a closed loop system?
• For the conventional system, I am assuming you are considering the cost of A/C as well as heat since the geothermal will replace both. Is that right?

That said, I'm upgrading a 1950's farmhouse, and did a hybrid system for the first floor (heat pump w/gas backup) and a ductless for the second floor. If your house needs other help, like insulation or sealing air leakage, you get more bang for your buck that way, and not going with the most expensive HVAC system out there.

+1.

Open loop vs. closed loop well costs depend on how many loops for closed loop you are using and whether or not there is an existing well or wells that can be used as a supply or dump well for the open loop. This is probably pretty variable by region - we can get plenty of water at less than 60 feet and have no rocks. The closed loop is based on one 200 foot deep loop per ton of AC, or fewer, deeper wells if space is an issue, for the soil in our specific area. Open loop tends to be less overall, but is less reliable, and not recommended if water has low pH, sediment or iron, etc.

[KarefulKactus15]

I dont know your location, but air source Heat pump are getting pretty efficient.   

Where I live,  the GEO thermal thing isnt financially appealing to me when compared to traditional heatpumps over an operating life.


Perhaps in northern areas where the air source heat pumps cant operate in extreme cold it becomes a better alternative?

[CowboyAndIndian]

I dont know your location, but air source Heat pump are getting pretty efficient.   

Where I live,  the GEO thermal thing isnt financially appealing to me when compared to traditional heatpumps over an operating life.


Perhaps in northern areas where the air source heat pumps cant operate in extreme cold it becomes a better alternative?

Yup, in the North East, the air source heat pumps give up the ghost in Jan/Feb and your heating is basically electric and expensive as hell.

The geothermal heat pump is a far better solution IMHO.

[BudgetSlasher]

I dont know your location, but air source Heat pump are getting pretty efficient.   

Where I live,  the GEO thermal thing isnt financially appealing to me when compared to traditional heatpumps over an operating life.


Perhaps in northern areas where the air source heat pumps cant operate in extreme cold it becomes a better alternative?

Yup, in the North East, the air source heat pumps give up the ghost in Jan/Feb and your heating is basically electric and expensive as hell.

The geothermal heat pump is a far better solution IMHO.

I know a few people who have air source heat pumps and utilize them for heat during the shoulder seasons, but not a single one of them rely on resistance heating for the dead of winter, they are all backed up by oil/propane/wood stove.

[CowboyAndIndian]

I know a few people who have air source heat pumps and utilize them for heat during the shoulder seasons, but not a single one of them rely on resistance heating for the dead of winter, they are all backed up by oil/propane/wood stove.

I could see using a wood stove, but backing up with Oil/Propane would mean having two systems to maintain and does not make sense.

[eddiebolt]

What prices are you all looking at with these jawns? I saw a billboard on I-44 in the middle-of-nowhere Missouri that said "$7000 for Geo-thermal" which is what a complete install of a new conventional Heat and A/C unit would cost, but not finding that in Springfield. We're roughing it with the space heaters tonight to avoid off-hourly rates for repair costs, but it's got me thinking about replacement.

Anyone done any research about the amount of area needed to do a horizontal loop system or digging and installing the pipe loops themselves? If I don't have a backhoe? If I rent a backhoe do I save any money?

[trammatic]

I have a horizontal closed-loop system in PA, and strongly recommend.  They have a few features that are often overlooked:

• Geothermal heat pumps can be very DIY friendly.  If you have enough yard, horizontal closed loops just require an excavator to dig trenches.  (See MMM's last post in renting one.)  Then it's simply a matter of laying pipes down and filling in the trenches.  On the inside, you'll need a recirculating pump and a Geo heat pump.  If you're replacing a system, you can buy one with the same tonnage rating yourself because the refrigerant is all self-contained (like a window unit) and so no HVAC tech is required.  Also a number of stores will sell them direct to consumer.  Geo heat pumps have two ports for the antifreeze solution to come in and out, and ducting connections. I just looked online, and a 3 ton system for about $3,000 shipped.  The excavator rental and all the other parts can be found for probably around $1000.  So for $4000, you'll get a much more efficient system.
• Geo pumps can last much longer than air-source pumps for a couple of reasons.  1) There is no "outside" unit subject to the weather.  All components are out of the sun and elements. 2) the refrigerant loop is much shorter since it's self-contained in the unit. It's also installed to factory specs and not installed in place.  I've seen window A/C units from the 70's still running.
• Replacement costs go way down because of the other two.  When my unit gives up the ghost after a 15 or 20 year lifespan, I'll buy another unit and spend about 3 or 4 hours installing myself.  No problem.
• Geo pumps can also have a water heater option that use the heat pump to help with residential hot water usage.  Keeping the A/C at 77 in the summer is more than plenty to fill all of our hot water needs.  We also use it in the winter where it has to pull more heat from the ground, but is much more efficient than a traditional water heater.

All that being said, as another poster said above, insulate and fill gaps first!  Get the big wins out of the way first.

[CowboyAndIndian]

Open loop vs. closed loop well costs depend on how many loops for closed loop you are using and whether or not there is an existing well or wells that can be used as a supply or dump well for the open loop. This is probably pretty variable by region - we can get plenty of water at less than 60 feet and have no rocks. The closed loop is based on one 200 foot deep loop per ton of AC, or fewer, deeper wells if space is an issue, for the soil in our specific area. Open loop tends to be less overall, but is less reliable, and not recommended if water has low pH, sediment or iron, etc.

RulesOfAcquisition, could you give us a sample pricing? A comparison to a standard gas heating+ac in your area (so labor costs are comparable) would give us a good comparison.

[CowboyAndIndian]

I have a horizontal closed-loop system in PA, and strongly recommend.  They have a few features that are often overlooked:

• Geothermal heat pumps can be very DIY friendly.  If you have enough yard, horizontal closed loops just require an excavator to dig trenches.  (See MMM's last post in renting one.)  Then it's simply a matter of laying pipes down and filling in the trenches.  On the inside, you'll need a recirculating pump and a Geo heat pump.  If you're replacing a system, you can buy one with the same tonnage rating yourself because the refrigerant is all self-contained (like a window unit) and so no HVAC tech is required.  Also a number of stores will sell them direct to consumer.  Geo heat pumps have two ports for the antifreeze solution to come in and out, and ducting connections. I just looked online, and a 3 ton system for about $3,000 shipped.  The excavator rental and all the other parts can be found for probably around $1000.  So for $4000, you'll get a much more efficient system.
• Geo pumps can last much longer than air-source pumps for a couple of reasons.  1) There is no "outside" unit subject to the weather.  All components are out of the sun and elements. 2) the refrigerant loop is much shorter since it's self-contained in the unit. It's also installed to factory specs and not installed in place.  I've seen window A/C units from the 70's still running.
• Replacement costs go way down because of the other two.  When my unit gives up the ghost after a 15 or 20 year lifespan, I'll buy another unit and spend about 3 or 4 hours installing myself.  No problem.
• Geo pumps can also have a water heater option that use the heat pump to help with residential hot water usage.  Keeping the A/C at 77 in the summer is more than plenty to fill all of our hot water needs.  We also use it in the winter where it has to pull more heat from the ground, but is much more efficient than a traditional water heater.

All that being said, as another poster said above, insulate and fill gaps first!  Get the big wins out of the way first.

Good info trammatic. I did not even think of the DIY aspect of this. Also the hot water is free when you use your air conditioner.

Also, I believe that there is special equipment which can put a pipe below ground without digging a trench. Not sure if it can do so 5 or 6 feet below. I have seen Verizon using it when they installed fiber optic in my neighborhood. No damage to our front lawn.

When you got geothermal, did you price out the cost of a regular system?

[rulesofacquisition]

Open loop vs. closed loop well costs depend on how many loops for closed loop you are using and whether or not there is an existing well or wells that can be used as a supply or dump well for the open loop. This is probably pretty variable by region - we can get plenty of water at less than 60 feet and have no rocks. The closed loop is based on one 200 foot deep loop per ton of AC, or fewer, deeper wells if space is an issue, for the soil in our specific area. Open loop tends to be less overall, but is less reliable, and not recommended if water has low pH, sediment or iron, etc.

RulesOfAcquisition, could you give us a sample pricing? A comparison to a standard gas heating+ac in your area (so labor costs are comparable) would give us a good comparison.

For a 3 ton closed loop geothermal (2 stage Climatemaster) you're at about $18,000, and for a 3 ton 14 SEER heat pump with electric backup (American Standard) you're at about $8,500. This includes duct, wells for the geothermal, and all materials and labor except the high voltage electrical.

I noticed horizontal loop mentioned, I have no experience with that or pond loop, but it would be more DIY. Also, DIY assumes you will need to learn to design the duct, not rocket science, but bad duct can cause equipment failures (example: restriction causes evap coil to freeze up and liquid refrigerant gets back to compressor, compressor fails).

[Midwest]

We had a goodman 14 seer 3 ton heat pump, furnace and electric backup installed for $5200 last spring.  Geothermal was in the $18k range.  If the difference had been closer, I would have considered geothermal.

[BudgetSlasher]

I have a horizontal closed-loop system in PA, and strongly recommend.  They have a few features that are often overlooked:


If you're replacing a system, you can buy one with the same tonnage rating yourself because the refrigerant is all self-contained (like a window unit) and so no HVAC tech is required. 

Two points:

1) I've heard from multiple people who have replaced heating systems that the equipment they replaced was oversized. Either someone added an unnecessary fudge factor, used a rule of thumb, didn't know how to properly calculate the need, or upgrades have been made to the house in terms of insulation. I know when we explored the option our system was 10,000 BTU oversized and we were told that we could probably install a smaller unit (and field) if we upgraded the attic and basement insulation and focused in on some air sealing which would reduce the field size and the cost of the install.

2) While it may be different in different states, in this state, and it appears under federal law, you can release pre-charged refrigerant out of the compressor into the lines of a split system. It is a more complicated process than many would want to DIY, to do it properly you would probably want a vacuum pump and nitrogen gas and of course there is the issue of warranty.

[BudgetSlasher]

I know a few people who have air source heat pumps and utilize them for heat during the shoulder seasons, but not a single one of them rely on resistance heating for the dead of winter, they are all backed up by oil/propane/wood stove.

I could see using a wood stove, but backing up with Oil/Propane would mean having two systems to maintain and does not make sense.

It makes sense here for a couple reasons,

1) Design values are negative, my location design value is -4, and we see -15 to -25 once or twice a year. The design values are beyond the limits of many heat pumps and even Mitsubishi's hyper heat and Fujitsu's RLS3H series don't appear to operate and the observed lows. Shoulder season is more late-September to mid-December and late-April on. With no heat pumps on the coldest days one would rely no supplement with the stove (see #2), depending on your schedule and the house that can be inconvenient at the least. Even if the heat pumps did operate at our lows their BTU output would be severely compromised and I would need even more units.

2) Some people around here have trouble with homeowners insurance around here when there "only" source of of heat is a wood stove.

3) Most of the installs I know of are retrofits, meaning that the existing heating system was already in place. And it was cost prohibitive or impossible to install ASHP sufficient to take the full heating load year round. 

4) Wood stoves require maintenance as well, such as chimney sweeping and modern stoves will require the catalytic part be replaces from time-to-time (around here that would be quite frequent if wood was your only source of heat). If I were to heat exclusively with wood, I would need 7 cords of wood. The opinion around here is that the chimney should be swept at least once in the off-season and once in the season, that would amount to more than my once a year boiler service.

5) Some of the people I know with heat pumps are not capable not people who should be lugging wood or bags of pellets into the house, bending, and stooping most every day for several months to supplement heat pumps.

[CowboyAndIndian]

For a 3 ton closed loop geothermal (2 stage Climatemaster) you're at about $18,000.

That is awesome, I think the price was about double that when I requested a quote.

[Syonyk]

Yup, in the North East, the air source heat pumps give up the ghost in Jan/Feb and your heating is basically electric and expensive as hell.

The geothermal heat pump is a far better solution IMHO.

It definitely depends on where you are, but the newer air source units continue working down to 0F or so - you may get a bit of backup heat, and they run 8-10 hours a day, but we've got a 2000 sq ft newer manufactured home with a 3.5 ton unit, and our heating costs on pure electricity were around $100/mo extra for a brutally cold Dec and Jan (quite below freezing, down below 0F a few times, I think the low over the winter was -11F or so).  It's not as extreme as some places, but it's colder than usual for the area.

What I've learned about heat pumps in the last year is that the stock thermostats are so insanely stupid they're criminal.  They get weird and panic about a few degrees difference and turn the backup coils on when they have no reason to do so - the thermostat that came with my unit, when it was 70F outside, would freak out and turn backup coils on if I wanted to heat the house to 90F or so for zero reason whatsoever (new construction, I was trying to bake out some of the volatile organics before we moved in - I cooked it, vented it, repeated that for... oh, a week or so).

I believe this is the reason for the "conventional wisdom" of not letting a heat pump system cool the house down at night - they'll be stupid in the morning.

I spent the $200 on a Nest, and it runs my unit a lot more reasonably.  It'll start heating at 4AM to get the house up to temperature by 8AM, and while it may use the backup coils a tiny bit (mostly for defrost, though occasionally if it's really cold it will run them to get up to temp), it does most of it on the compressor.  Based on the costs to heat this winter, I'm reasonably certain that the Nest paid it's own cost in the past few months.

I know a few people who have air source heat pumps and utilize them for heat during the shoulder seasons, but not a single one of them rely on resistance heating for the dead of winter, they are all backed up by oil/propane/wood stove.

Where is this?  We, admittedly, have quite cheap power out here.  I'm getting some quotes for propane backup, though, because I'm curious.  I'd like to be able to heat without as much generator if I needed to in the dead of winter.

[KarefulKactus15]

Just a thought about thermostats being stupid, you are very correct!      To remedy this I recently bought a newer honeywell Wi Fi thermostat.  (100$ , nothing fancy) 


Dicking around in the options I was able to find a Smart Heatpump mode.   It became MUCH smarter, it will do anything in its power to avoid using the electric backup heat unless I MANUALLY turn the temp up.       The programmed heating and cooling routines are much smoother now with no electric heat trying to rapidly raise the temp. 

Used to It would fire the electric heat to have the house warm by 5am when I wake up, in smart mode it starts the heat pump at like 2-4am (depending on recent weather) and avoids strips. 

[CowboyAndIndian]

Yup, in the North East, the air source heat pumps give up the ghost in Jan/Feb and your heating is basically electric and expensive as hell.

The geothermal heat pump is a far better solution IMHO.

It definitely depends on where you are, but the newer air source units continue working down to 0F or so - you may get a bit of backup heat, and they run 8-10 hours a day, but we've got a 2000 sq ft newer manufactured home with a 3.5 ton unit, and our heating costs on pure electricity were around $100/mo extra for a brutally cold Dec and Jan (quite below freezing, down below 0F a few times, I think the low over the winter was -11F or so).  It's not as extreme as some places, but it's colder than usual for the area.

Good to hear that air source heat pumps have improved so much.

But I still believe that a geothermal heat pump will be better. It is easier to pump heat from 55F than from 0F.

Syonyk, I remember your man cave/hut with Solar panels. Did you also do solar for your house to run the heat pump?

[Syonyk]

Dicking around in the options I was able to find a Smart Heatpump mode.   It became MUCH smarter, it will do anything in its power to avoid using the electric backup heat unless I MANUALLY turn the temp up.       The programmed heating and cooling routines are much smoother now with no electric heat trying to rapidly raise the temp.

Yeah, I run my Nest in "Max Savings" mode regarding the coils.  It uses them when it has to, but it doesn't do the "Woah, 7AM, better turn on the coils so I can have things warm by 8!" lunacy that spins your power meter up.

Used to It would fire the electric heat to have the house warm by 5am when I wake up, in smart mode it starts the heat pump at like 2-4am (depending on recent weather) and avoids strips.

Sounds right.  Ours does much the same - the compressor will start at 4 to have the house up to temp by 7 or 8, whenever I have it set.  You can tell how often I fiddle with it.  I had to turn off the "smart learning" thing because my wife would turn the temperature up a bit in the afternoon and it would start doing that every day, so it's just on a static schedule now, paired with the "Auto Away" mode (if nobody walks past the thermostat in a while, it decides nobody is home so lets temperatures wander more).

As far as I can tell, I'm only paying about $100/mo in heating costs through the dead of winter, maybe $150.  I don't obsess over the power bill, since we've got a full electric house and a baby in cloth diapers (though she's potty training, so less diaper laundry!).  I did mean to add a valve to the dryer to vent in the house this winter and never got around to it.  Next year...

Good to hear that air source heat pumps have improved so much.

I was impressed this winter.  :)  It did a far better job than I expected in terms of heating on the compressor - the Nest will indicate when the backup coils are on, and it would turn them on after a few hours of compressor use (I believe this may be the compressor starting a defrost cycle), and on rare occasions if it was really cold out (the Nest, being generally local temperature aware, tries to learn the system and can figure out that if it's below X degrees outside, the compressor isn't worth running).  I couldn't tell you what it's decided about our system, but I was fearing $400 electric bills, and I'm only barely over $200.

But I still believe that a geothermal heat pump will be better. It is easier to pump heat from 55F than from 0F.

Certainly, but that doesn't always mean it's worth doing.  If you have a mild-ish climate, can dig your own coil trenches, and do things that way, it may be worth it but if you need a vertical well punched down a couple hundred feet, it not pay back in a reasonable amount of time.  A $10k well saving me $50/mo for a few months a year, for instance, isn't likely to be worth it (and given the hillside of basalt I live on, dropping my own subsurface coils in would probably require blasting).

If I were paying $500+ in electric for winter heating, yeah, I'd be looking into some other options, and I've got someone wandering by next week to give me a quote for a propane furnace to replace the coils, but other than giving me more grid independence in heating (I can run the blower on a generator, a generator that will run the compressor is a lot more expensive, and forget about running the coils), I'm not sure the payback will be there, so I may not do it.

Syonyk, I remember your man cave/hut with Solar panels. Did you also do solar for your house to run the heat pump?

Excuse me, 100% dedicated office space. :p  It's a work space, not a man cave.  I don't even have one retro metal sign on the wall!  Just license plates.  Which don't count.

I haven't done solar for the house yet.  I plan to, but the style of system I want is a bit more expensive than most systems (string inverters with battery backup capability) - I don't like microinverters that won't run without a ~infinite grid attached.

Since most banks wouldn't talk to me about the mortgage (eew, manufactured... um, you know, that's not really something we do...), and those that would felt that taking an eternal period of 3 months off between jobs meant I was no better than an unemployed bum, we spent most of the planned "improvements" cash on the initial purchase instead.

And my gut feeling without doing the math is that the cost of a geothermal well, invested in solar panels instead, will come out ahead in the long run.  I've got net metering available for now, so I can push in the summer and pull in the winter, and if (when?) my state gets rid of that, I'll probably do some seasonal thermal storage in my crawlspace to help with the heating - storing many thousands of gallons of well insulated hot water, then running that through a radiator in my heating stack, is well within my capabilities. :)

[BudgetSlasher]

I know a few people who have air source heat pumps and utilize them for heat during the shoulder seasons, but not a single one of them rely on resistance heating for the dead of winter, they are all backed up by oil/propane/wood stove.

Where is this?  We, admittedly, have quite cheap power out here.  I'm getting some quotes for propane backup, though, because I'm curious.  I'd like to be able to heat without as much generator if I needed to in the dead of winter.

Maine, my electric runs about $0.1451/kwh (but that will vary depending on who your T&D utility is and how large a usage your fixed cost are spread across). You can look here http://www.maine.gov/mpuc/electricity/delivery_rates.shtml, but the rate range from a low of $0.104 to a huge $0.799 on some of the remote islands; the State averages $0.148/kwh.

In comparison fuel oil runs around $2.28/gal and propane $2.58/gal (at least according to the governor's energy office).

Its a large state with a fair variance in climate zones (USDA 5b to 3b) and home construction, so a guy who is looking to back up a low of -10 to -15 in a well insulated house is going to have a different plan than the fellow trying to back up for a -30 to -35 low in an 1800s farmhouse.