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Community and Q&A

Boiler for Off-Grid Modular Home

rtrask | Posted in Mechanicals on

It’s getting down to decision time, and the more I know the more confused I get.   We took advantage of the hot real estate sold our house in town and are building on 40 acres in the Colorado mountains at 8000 feet. It is a cold valley in the winter the low heating design temp is -10 climate zone 7b.

We purchased a modular home and paid for up graded insulation R21 on the walls R50  in the attic. Over a full basement with Nudura  ICF form concrete walls.   Nudura says R23.5 insulation for the basement walls, R10 in the basement floor.  I had hot water heat with base board radiators, and was determined to install hydronic radiant floor heat in the new house.   We have installed the pex in the basement, and we are ready for the pour of the basement slab.

I have FLU 419 backhoe / front end loader, so I am able to do a lot of the excavation.  I had thought about installing a geo-thermal heat pump, but when I priced getting grid power it made more sense to go 100% solar.  So I decided I should go with a condensing boiler with propane.  We have really hard calcium water so I intend to install a closed loop system and use a indirect fired water with a non salt based whole house water softener to condition the water for the heater  / all domestic water.  I have had good luck with a Triangle tube Solo 110 at my old house.  All set,  …. or so I thought.

The specs from the modular home manufacture says.
Heating  structure is  30930 BTU
Load equipment 35022 BTU
Heating Area 4509 sq/ft

I did a Cool Calc report because they rated the basement wall at only R14, and no insulation in the basement floor, but mostly because the numbers seemed so low.   My Cool Calc came out 29370 BTU.  This is a lot smaller than I expected.

I am considering putting a biomass heater in the garage to heat that as well.  If get a pellet boiler, I could likely heat both the garage and the house.  I would have to pipe the hot water into the house but I am confident that could be worked out.

If I stick with propane what size?  Modulating boilers will not have the same problems with short cycling but more than double is not a good choice.

Any input on choice of boiler would be greatly appreciated.

Sorry for all the details, but it’s the way my mind works.

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Replies

  1. Expert Member
    Zephyr7 | | #1

    If you have really hard water, you're probably going to want that salt style softener. Most of the non-conventional softners have very little ability to actually soften water, so they are likely to be ineffective with very hard water. Note that salt-type water softeners don't put salt in the water, they just replace the calclium iones with sodium ions.

    For a boiler system, I would recommend not using your well water, even with a softener. Either order water to fill it (i.e. get a delivery from a supplier), or use RO water with the correct mix of corrosion inhibitors. Do NOT use RO water directly as that will cause problems. Keep a small supply of makeup water on hand just in case, which can be easily stored in one of those 5 gallon jugs for an office water cooler. Well water in general is not usually very good for a closed system heat plant.

    If you use a remote boiler in a detached building, be careful what type of insulated pipe you run underground. So-called "wrap pipe", that has two PEX lines and bubble-type insulation wrapped around them, isn't very good long term. You're much better off with one of the insulated PEX products that uses a solid foam material such as InsulPEX. These hold up much, much better over time.

    Keep in mind that any wood-fed boiler is going to require a good amount of work on your part compared to a propane or oil fired system. Pellets are better in this regard, but you'll still be putting in effort to keep the hopper filled. Insurance companies typically want to see some kind of conventional heating system too, so you'll need an oil or propane system as your primary, then use the wood fired boiler as a supplement. This keeps the insurance people happy, and also means if you want to take a break from moving wood around (or you're out of town), the oil/propane system can keep your home from freezing.

    There is a lot of good info on hydronic systems at hearth.com, you might want to try asking there too.

    Bill

    1. rtrask | | #5

      Hi Bill,
      Thanks for your thoughtful reply. You have reopened the whole house water softener debate. I was told, and it is backed up by research that salt based water softeners are bad for septic systems. As I did more research I found that there are a lot of conflicting opinions and research conclusions. The problem seems to not be from the treated water, but from the back flow. The other issue is because of the sodium added to the drinking and cooking water. While my blood pressure is pretty low, we intend to live here till the end, and heart health was a part of why I decided to go with a non conventional water softener. I have not bought one yet, so I will do some more research. If I do use a salt based water softener I will likely get a RO filter for drinking and cooking.

      For whatever boiler I buy I intend to run a distilled water glycol mix through it and get a mini water feeder. Perhaps it is overkill but in the end better safe than sorry. I can't remember the exact numbers but when I had it tested it was 3 0r 4 times what they said was acceptable. when I took the sample the well had not been pumped for many years, so it is likely not as bad as was tested.

      I will keep in mind what you said about the extra work associated with a pellet stove, but I think it depends on the model you get and the hopper size.

      Thanks for the tip on hearth.com.

  2. GBA Editor
    Martin Holladay | | #2

    R. Trask,
    Forget the boiler. You want a conventional wood stove or propane-fired space heaters with through-the-wall venting.

    More information here: "How to Design an Off-Grid House."

    1. rtrask | | #6

      Thanks for the tip, I will read the article. I have 30K watt of LiFePO4 batteries and I can add storage if I need to. I have a 15KWatt diesel generator for emergencies and times when I need 3 phase power. Solar panels are pretty cheap and Colorado has a lot of sunny days, so I think I do not need to be too stingy on my electrical usage.

      1. Expert Member
        DCContrarian | | #12

        With 4500 sf floor area and a 30K BTU heating load you're at 6.7 Btu/SF. That's a floor temperature of about 80F which is barely going to be noticeable. Radiant floor heat in that situation is a lot of money without a lot of impact.

        If you're not being stingy on electrical usage, just put in resistive heating. Baseboards are cheap, in-floor is a lot cheaper than hydronic.

  3. Charlie Sullivan | | #3

    Colorado gets better winter sun than a lot of places do, so it's probably possible to provide some of you heat through some combination of heat pumps (air-source or ground source) or solar collectors heating water, with the propane only being a backup.

    1. rtrask | | #7

      I like the way you think, as I tend to believe in multiple redundant systems. I will have to think and read more about more of a passive system. I will still need to supply electricity to the pumps.
      Heat pumps return a lot of bang for the bucks, but I think they would be too much of an electrical draw to rely on in an off grid situation..

      1. Charlie Sullivan | | #11

        Modern ECM pumps use maybe 1/4 the electric energy of traditional ones. You can also configure the plumbing for minimal resistance. And even a traditional system has pumping energy that's a tiny fraction of heating energy.

  4. Expert Member
    Akos | | #4

    Having powered my modcon with ECM circulators through a longish power outage off batteries, I can say that it is a no go for off grid. These are 24/7 loads that add up to a lot of energy which means inevitabely it will need to run off the generator. If the genny fails to start, which will inevitabley happen when you are away in the middle of winter, you now have no heat.

    If you must have hydronic, I would look at something like a combi rated non powered direct vented water heater in the basement with thermosyphon gravity fed rads upstairs.

    For extra solar capacity, you can look at a very large resistance tank as a diversion load. This gives you the cheapest energy storage and free hot water. If you still have excess heat, you can always pipe excess heat to the house through a small pump and heat exchanger.

    1. rtrask | | #8

      Akos, how big of a solar panel array were you using, how big was your battery storage, and how many zones were you running your pumps for? The thing about radient heat is that the water generally does not need to be that hot when circulating through the pex. I think pumping water to disperse the heat should be more efficient.

      I appreciate your post, it has given me a lot to think about.

      1. Expert Member
        Akos | | #10

        The battery was just a bit over 5kWh. I got about a day out of it including some lighting loads and this wasn't even in the middle of winter so the heat was not at full tilt. No solar.

        The setup is two zones of rads and four zones of floor heat. The whole thing is pumped by two ECM pumps. Not a lot of pumps but add on the load from the combi and it adds up.

        How hot the water is doesn't effect your pumping loads, it is all about GPM and loop length. You can always reduce the losses by going with larger pipes but it will still add up. 24/7 loads are a killer in off grid.

  5. Paul Wiedefeld | | #9

    Propane boiler wise - all modulating options at maximum fire will exceed your load, but with turndown, you’ll be okay.

    That said: your heat load is minuscule on a BTU/sqft basis, so why mess around with a boiler at all? The floor will not be warm. Like low 70s on the coldest hour of the coldest day.

  6. Expert Member
    DCContrarian | | #13

    My overall impression is you're making this way too complicated. What you want is solar, and a backup electricity plan for when the sun isn't shining -- which is probably a fossil fuel powered generator. Then you want a heat source that runs off of your solar electric. When there's no sun heat can either be the same heat source running off of your backup electric, or a backup that runs off of the same fossil fuel as the backup electric.

    1. GBA Editor
      Martin Holladay | | #14

      "Then you want a heat source that runs off of your solar electric." Really? And what is the basis for your recommendation of the use of electric heat for an off-grid house? Have you lived in such a house for decades, as I have?

      During cloudy weather, you will waste a lot of fuel if you have to run a gasoline-powered generator to operate an electric appliance that produces space heat. It makes much more sense to burn the fossil fuel directly to produce space heat -- and you need an appliance that can operate without electricity, which is why I recommend the used of a propane-fired space heater with through-the-wall venting.

      1. Expert Member
        DCContrarian | | #17

        OK, I was thinking out loud. I'm reading what the OP is proposing and it has solar PV, and batteries, and a diesel generator, and a propane boiler, and wood heat -- and I'm thinking, that's too many pieces. I'm trying to get more utility out of the solar PV, but you're right, electricity is the toughest piece of the puzzle and you don't want to use too much of it. Basically minimize your electricity use, then size the PV array to mostly meet you needs, size the batteries to give some backup capacity and use a generator for the rest. It might be worth having resistive heat for when solar production exceeds demand, you can absorb some of it for heating the house and domestic hot water.

        I would use one fuel for the generator, heating and hot water.

    2. Expert Member
      Zephyr7 | | #15

      A typical engine-generator set (a basic generator running of propane or gasoline in this case) is around 16% efficient. That means 84% of the input BTUs from the fuel goes out from the radiator and exhaust in the form of waste heat that did NOT heat your home. While it is possible to make a small-scale combined cycle plant and recover a lot of that waste heat, there are no commercial systems available at this small scale (as far as I know), and it's suprisingly expensive to do a DIY job of it.

      I'm going to back up Martin's statement with some numbers. You want to maximize your fuel supply's usefullness. If you were to go with propane, the generator gets you electricity, but wastes around 84% of the energy content of the propane. A typical 90% efficient furnace only wastes 10%, and you can probably run the electric part of that furnace off of your solar system -- at least for a while. If you use Martin's recommended type of heater that doesn't even need electricity, you're even better off here, since you might not even need the generator to run now at all.

      Every time you convert energy from one form to another, you lose some of it. This is one of the pesky laws of physics that makes everything hard. If you use electric heat, you convert from chemical energy to thermal energy, then thermal to electrical, then electrical back to thermal. Three conversions. If you burn the fuel directly, you only have ONE conversion -- chemical to thermal -- which means less losses.

      You could use an electric resistance water heater as a dump load for excess solar output on sunny days, since water heaters of this type are relatively inexpensive, and they can act as a sort of thermal battery. For longer term heat, you really want to avoid the extra energy conversions and heat by burning your fuel directly and not running it through your electrical system.

      Bill

  7. plumb_bob | | #16

    For off grid living, a wood burning stove is essential in my mind, even if it is not your primary heat source. When all else fails (electricity, fossil fuels) you can still have heat and a place to cook.
    Labour- yes getting the wood is hard work, but you can pay some local fella with a truck and a saw to supply you with wood, the cost is really not very much when compared to other fuel sources. Or maybe you are dumb like me and enjoy that type of work.
    Carbon- if you are using naturally killed trees (standing dead) as fire wood this works out as carbon neutral as the tree will fall and rot if not burned.
    I have seen home-built water jackets for hydronic heating using a wood stove.

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