Heating System For a Small Off-Grid Cabin
Hello, first post here.
I'm in the design phase for a small (500sf) off-grid cabin in far northern NH. Looking to design for future retirement where operational cost then is prioritized over installation cost now.
It will be well insulated, sitting on an ICF crawl space (5'-ish headroom) which will house mechanical equipment.
Power is provided by PV solar with a DC backup generator (Kohler g4213). At least a 20KWh battery bank.
We will be visiting the cabin occasionally during the winter and do not want to winterize when away. Figuring on keeping the heat at about 50F when un-occupied.
Planning on radiant heat (Warmboard or similar) using an AWHP (eyeballing a Spacepak CC32-18) as primary heat. Thinking of using a small tankless propane boiler as backup. There will likely also be a small woodstove for use when we are there. DHW provided by indirect tank.
At -20F outdoor, 50F indoor, heat loss ls about 10,000BTU. I obtained the hourly historical temperature, solar and AWHP data and dumped it into a spreadsheet. If my math is correct, there are about 200 hours a year that the battery would be pushed below 20% (generator would start), and about 130 hours where the AWHP just couldn't supply the needed BTUs. At some point of falling temperature & battery depletion, it would be significantly cheaper to switch to propane heat to preserve the battery rather than starting the generator. 24 hours of propane would be about $8, while generating electricity to feed the AWHP would be closer to $20. If we are there on super cold days, we can run the woodstove to keep up.
Looking at the AWHP data, in cold weather the heating capacity (and COP) drops off as water outlet temperature rises. The lowest temperature given in the spec is 95F, but I see in the manual the outlet temp could be set all the way down to 54F. Does it make sense to run loop temperatures lower than 95F if the thermostat is set for 50F, thereby squeezing more efficiency out of the AWHP? I have not been able to obtain any additional data from Spacepak regarding capacity / COP at lower than 95F outlet temperature.
Please tell me if this approach is crazy / thoughts on a better idea.
Thank you.
Comments
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have you considered ground water to tank water heat pump at all? Check out Water Furnace for the geothermal heat pump section. They may have something small enough for that space. The other parts for the buffer tank, and DHW tank and radiant floor/wall sections can be put together for a more efficient use of the refrigerant portion of the system. Not that you will need it but, Water Furnace also has air handlers for air conditioning for those three days in the summer that might be uncomfortable for us old folks.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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A hp will be a pricey system for such a small space, if that matters? That 10K heat load is on the coldest days, about 200 hours you figure? Your heat load may be more in the 5- 8K range most of the winter.
Be nice if you could find one of the small Honda or ECR co gen units. May as well get some heat if you run a generator..
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
"If my math is correct, there are about 200 hours a year that the battery would be pushed below 20% (generator would start), and about 130 hours where the AWHP just couldn't supply the needed BTUs. At some point of falling temperature & battery depletion, it would be significantly cheaper to switch to propane heat to preserve the battery rather than starting the generator. 24 hours of propane would be about $8, while generating electricity to feed the AWHP would be closer to $20."
So in 24 hours of propane operation you save $12, or 50 cents an hour. And you expect 200 hours a year. So you save $100 a year by having the propane backup. Unless it's a simple ventless heater or something the payback on that is essentially never.
I'm not a fan of fuel backups to heat pumps, they just don't tend to pencil out.
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This sounds super-risky. You need to keep the heat on to keep the pipes from freezing, but you're relying entirely on energy produced or stored on site, because you're off-grid. It's really an open-ended question of how much you have to store to get through the worst possible scenario.
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Ed, I considered geothermal, but was put off by the ground loop install cost. I own an excavator, but don't really have the time to install on top of building the cabin. I know a half-dozen folks that have geo and they all hate it. I suspect it was poor design / install, but still. This will be a DIY, so a monobloc AWHP seems easy. I had a hard enough time getting an excavator and well driller out to that site.
Bob, To clarify, the 10K is when the thermostat is turned down to 50F. Turned up to 70 it's more like 15,000 BTU.
DCContrarian, You make a good point about the payback. I forgot to mention, I'd be using the system for DHW as well when we are there, so would need the propane boiler on cold days when the HP couldn't keep up. something with a low minimum fire, like a Lochnivar WHB055N or similar. I'm thinking that would give us capacity on cloudy days when the family is there and multiple showers are happening. I considered a small direct vent wall heater for backup, but I need a way to make DHW anyway.
As for the risk, I'll have remote telemetry. If the system faults out I can hop in the truck and be there in three hours.
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One 4x8 solar thermal panel with a 50 gallon solar tank might cover the DHW needs. A resistance element for no- sun days.
If you could get the tank above the collector it could be a no pump thermo-siphon system.
Here is an interesting ground mount ST in Wisconsin. The array shades the AC.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
For off grid, you have plan a bit differently. All those 24/7 loads add up very quick when you have a weeklong storm. A modcon plus pump you are probably looking at around 150W draw so 25kWh a week. I have a cottage in northern Ontario with a 10kW array and there are snowy weeks when the array output never goes above 100W.
Generators have a nasty habit of not starting when you are away, so not something you can rely on to keep your place from freezing.
You are on the right track in making the structure efficient first. With good design and a bit of south facing windows you can keep the place above freezing down to pretty cold temperatures.
For the rest of the time, you need something that doesn't require power to run to heat. That is typically a through the wall vented propane heater.
You can also reduce the amount of heat you need if you move all your plumbing into a central core area of the house and only heat that.
As for space heat the rest of the time, the tried and tested is a high efficiency wood stove.
If you want to run space heat off your PV, you can use a wall mount minisplit connected to a load shed output on your PV setup. This way if there is enough sun, the minisplit would heat the place but once the batteries run low, it gets disconnected and propane heat takes over.
If you really must have hydronic floor heat especially with a crawlspace, I would do a direct vented water heater in thermosyphon to oversized pipes in the floor. No power needed for the operation.
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I would make it as easy as possible to winterize, so that's always available as a Plan B. Ideally that would mean you could just turn off the well, open all the taps and put RV antifreeze in all the drains when you go, and just turn the well back on when you return. In the basement crawl space create an insulated area below the frost line and put the well pump there.
If you have hydronics, put glycol in them.
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Have you considered a Solar H20 slab battery system? Ultimate simplicity, and enough heat stored to weather a long storm or power outage. These are simple systems without the need for limited life span "appliances". Electricity use would be minimal, so batteries could likely run the needed pumps for several days. Siggy has some useful info on the subject.
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This is one of those that feels like it should work, but doesn't. You have to do the math (building energy use, energy lost from buffer thorugh insulation and overall amount of energy stored in the buffer) to see where exactly you'll end up. I can guarantee you the buffer will be much bigger than you think and the overall losses will be higher than not having the buffer at all.
A buffer also only works if you have excess energy to store in it. This is very rarely the case in the wintertimer.
Seasonal storage is also one of those ideas that feel like it should work but end up completely impractical.
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That is a bit more engineered Bob Ramlow sand bed storage concept.
Bob has been promoting and installing sand bed radiant, throughout Wisconsin, charged all summer with solar thermal for maybe 30 years now.
His systems are not top insulated, so they radiate through the winter at a bit of an uncontrolled rate
Spending many years at the MREA event and meeting owners of these sand bed homes they love them, and learn to live with the idiosyncrasies.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
The article says the sand is two feet thick. It also says sand is 100 lbs per cubic foot and has a specific heat of 0.2 BTU/lb/F. They recommend heating it to 110F. (Left unsaid is how you keep from roasting in the summer with an underfloor temperature of 110; no insulation is shown. Also left unsaid is how exactly you get heat into and out of the sand).
The OP said they wanted a cabin of 500 square feet. So that would 1000 cubic feet of sand, or 100,000 lbs. Taking it from 110F to 70F is 40F, or 8 BTU/lb, which would release 800,000 BTU.
A gallon of heating oil nets about 100,000 BTU, so that's equivalent to eight gallons of heating oil. I paid $4.11 at my last fillup, so that's $32.88 per year in heat provided by the sand.
I'll leave it as an exercise to calculate the payback period.
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I was merely pointing out that sandbed systems are out there. Even stumbled across the earlier rock bed forced air systems in Missouri from time to time.It is not something I would consider or recommend.
The numbers look better when 50% or more of those systems were incentivized. Wisconsin had 3 pools of money available for ST for a number of years. State, federal and the Milwaukee Shine program. Hydronics often got lumped into the solar rebate $$. Although I don’t think that was the intent.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
Can't find a reference but I have seen experiments (I think a Scandinavian country) where they have big, well insulated, containers of sand they heat to 100's in the summer using PV and then pull the heat out in the winter.
It begs the question of how well insulated does it need to be and how to get the heat back out. Guess if you make the store really big (neighbourhood system) this could work.
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I guess if you make it big enough it would be an earth-source heat pump.
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Found the link
The store energy at up to 600c using a closed loop fluid and heat exchangers. If water is the fluid this is comparable to temperature and pressure used in modern turbine steam plant.
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Viessmann Ice Storage system, with some PV fence
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
OK, 600C is about 1100F, so instead of getting $32 worth of storage you get 25 times as much, or $800. For that, you need to figure out a way of putting heat into the bank at 1100F, and getting usable heat out. You also need to insulate the bank with something that can stand 1100F.
The challenges of storing heat in the summer and retrieving it in the winter are just too daunting. Particularly at residential scale. Now, I'll give two examples of heat storage schemes that seem to be practical. First, many people who heat with wood seem to have systems that allow them to load the stove once per day, store the heat in a large water tank, and meter it out over the course of the day. Second, on a commercial basis there are solar installations that store excess production during the day in molten salt banks, which they retrieve at night.
Daily heat storage as a big advantage in heat storage in that you use it every day rather than once a year. The molten salt systems have the advantage that they rely on phase change to store the heat, rather than just the heat capacity of the material. And doing it on an industrial scale is more practical than on a residential scale.
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Since we're veering onto the subject of thermal mass storage, it is possible to design the structure itself with thermal mass walls and south-facing glazing for solar gain. The "Earth ship" concept developed by Michael Reynolds in the Taos area uses discarded tires filled with rammed earth and arranged in a U-shape for walls, the opening facing south with a wall of sloped glazing for solar gain. The earth-filled tires buffer the daytime heat in summer, and the night cold in winter.
It's better adapted for drier climates like the desert Southwest, but in theory the exterior could be waterproofed with stucco, etc, for wetter Northeast climates.
When I lived in Colorado and was younger (25 years ago) I had plans to buy some land and build one to live in, but then other things happened and took me in a different direction.
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At this time last year I was in Taos and visited the Earthships. I had two thoughts on the tour: First, these things are beautiful, they really did some clever things with the architecture. Second, the science -- or "science" -- is load of hooey. I probably ask more probing questions than the average tour visitor, but I got them to concede that they aren't as self-sufficient as they try to claim.
I also visited Taos Pueblo. Adobe houses are often held up as an example of successful application of the principles (or "principles") of "thermal mass." It was a pretty raw day, not super cold but mid-30's, sunless and a stiff breeze. Just about every building in the place was downright miserable to be in. The ones without fires were bone-chilling, even the ones with fires were cold and smoky. The only building in the place that was bearable was the church, which had a propane heater.
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Yet another option.
The very first ground up home I built in 1984 had a slab garage and mudroom on the main floor. Above that I poured a 1-1/2" thick thin slab over PB tubing, on a framed floor. Covered it with glue down wood parket tiles. 10 years in that home with 2 dogs and the floor held up fine.
During the 1990s we did a lot of gyp pours in condo projects around the ski town of Utah. The gyp provided fire and soundproofing. It was a no-brainer to add radiant at that point. It was a selling and rental feature.
So a "low mass" thin pour is a nice in-between from a dry wood system or a 4" slab.
We would be sure the builder accommodated the weight by beefing up the floor joist to eliminate bounce, on thin pours.
A local redi-mix company can supply a "pea gravel" mix, a 3/8" aggregate. That can be poured down to 1-1/2".
But it would not be a great finish floor. The thin pours tend to develop "road map" type thin cracks. With pea gravel they triple the amount of fiber in the mix to prevent crack from shifting.
Double plate the bottom of the walls with a 2x6 under a 2x4 and you get a carpet nailing strip.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
@DCContrarian said:
"At this time last year I was in Taos and visited the Earthships. I had two thoughts on the tour: First, these things are beautiful, they really did some clever things with the architecture. Second, the science -- or "science" -- is load of hooey. I probably ask more probing questions than the average tour visitor, but I got them to concede that they aren't as self-sufficient as they try to claim."
I agree, there was a large degree of oversell of the benefits. For me the attraction was the low cost (but a lot of work to fill the tire walls with earth) passive solar design, and I liked that you could turn the interior front of the house into a year-round growing area for plants and vegetables.
You still need heat, water, electric, but you can use less of all of them with thoughtful design.
I suppose another option for the OP would to go the "passive house" route and build a super-insulated box, essentially, as his cabin. The passive house standard is gaining traction here in the US, but it is more costly to build. But for a 500 sq ft cabin, the additional cost would not be that much, and then you can keep the interior above freezing with the equivalent of a hair dryer.
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Hi, I like heading towards Passivhaus design without necessarily going that far. Here's an article on "the pretty good house". Doing this makes heating/cooling a far easier proposition.
Yours, Larry
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The "Pretty Good House" movement was a reaction to the Passive House movement, to try and get most of the benefit at a fraction of the cost and complexity. I highly recommend this book to anyone contemplating building a house:
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This was my thought as well. The places that I've had to winterize were all a pain, and every time I've thought about what would go into making a place easy to winterize.
- Make all pipes in habitable space easy to drain. Have everything arranged so that you close the water main and open a few taps and bam, empty pipes.
- I've wondered if one could create an insulated utility space coupled to soil, so that slow/annoying to drain tanks would be effectively below the frost line.
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It used to be common to build "well houses" to contain above-ground shallow well pumps. Open to the soil on the bottom, insulated on the tops and side. A rule of thumb I've heard is an inch of foam is worth a foot of soil, so whatever your burial depth is, use that to figure out how much insulation you need.
Nowadays most people just use submersible pumps instead.
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Another thing you can do is put a thermostatic switch like this one:
in the enclosure with your pump or tank. Often just a 100-watt incandescent bulb is enough to keep it from freezing.
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Thanks for all the input everyone!
Right now, the plan is to over insulate. It's only 500sf and very simple plumbing wise, a kitchen and bath. The plumbing will be arraigned to gravity drain with just a valve or two. The well already has a drainback device at the pitless (well tank is currently in a conex box and has gone through winter ) I can dump the pressure tank and the lateral from the well by opening one valve. Hydronic loop(s) will be glycol.
The plan (for the first winter at least) is to winterize before we leave, as if we were shutting the heat off. That way it won't be an emergency if the heat / batteries fail.
I'll have at least 30KWH of battery storage / 12KW array with a DC output generator. The nice thing with that generator is it can be set to run at it's most efficient load and bring the bank up to full charge. 30KW should be more than a week of running the boiler.
I'm making provisions for a powerless standing pilot millivolt direct vent heater, should the whole power system prove to be unreliable.
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