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Community and Q&A

Upsize my generator or add propane backup?

Nathaniel Hieter | Posted in Mechanicals on

Rebuilding my home after a loss.
Pole-mounted solar array survived (~14,000kwh/year)
Energy efficient construction/design.
Manual J puts heating load at just under 24k.
Climate zone 5 (but right near the edge of 6).
Sadly, radiant heat is a design requirement.
(We had it in our old house and the rest of the family is reluctant to give it up.)
No access to natural gas.
My hvac supplier is a very talented mechanical engineer with years of experiencing installing solar/gshp/ashp. (and a family friend)

Right now we are leaning towards a water-to-water gshp (with MultiAqua heads for cooling). I looked long and hard at Chilltrix, but I’m not convinced (yet) that it is mature/durable enough. Regardless, I am heating with electricity no matter which way I go.

I live out in the country and it is not uncommon for us to lose power in the winter months. Very rarely is it more than ~4hrs, but…… the freak snow storms do occur. Our previous home was heated with a mix of oil-based radiant (ouch) and a well-placed wood stove. Power outages just meant I had to be diligent about keeping the stove running. No big deal. However, due to the nature of the loss, any wood/coal/pellet stove in the new house is out of the question.

We had a 7.5kw generator (propane) previously that was relatively inexpensive and easy for me to install. It really worked great. My original plan was to simply replace the one I had, but with moving to electricity as our heat source the old generator is insufficient.

Unfortunately, the cost of generators does not seem to scale exactly linearly. Or at the very least, it seems that there is a discontinuity when one jumps from something that is “portable” to something that is marketed as “whole house”.

I’ll almost certainly just upsize the generator, but the price difference did make me flirt with the idea of an inexpensive heat source in parallel with the gshp such as a tankless water heater. I would never drive the buffer tank with such a unit full time, but one or two days a year…….. Anyway, I’m sure there are several reasons why this is a BAD IDEA so I imagine this thread won’t be very long.

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    Before you go too far down the path of installing a tankless water heater to supply space heat, you might want to read this article: Using a Tankless Water Heater for Space Heat.

    When you have a power outage, the best backup heat source is a wood stove. If you can't use a wood stove (and I understand why you can't), the second best heat source is a propane space heater with through-the-wall venting. (Choose an Empire model that doesn't require electricity.) If your house is fairly airtight and has a good thermal envelope, a single space heater heater almost anywhere on your first floor will keep your house from freezing.

  2. User avatar Expert Member
    Dana Dorsett | | #2

    If you build the house with low enough loads, the size of the heat pump will be low enough to use with a portable generator.

    If you have 7500 watts of generator and reserve say 2000 watts for critical circuits such as refrigerators, lights, TV and internet (so's you can binge watch Korean TV dramas on NetFlix until the power comes back on :-) ) you still have more than 5000 watts that could be applied for space heating. What's the peak draw on the GSHP system?

    And, 5000 watts (x 3.412 BTU/hr per watt) is 17,000 BTU/hr, which is more than the design heat load of many Net Zero Energy type houses, and about half the heat load of MY not-so-super-insulated 1920s antique. It's more than enough to keep a few rooms warm while camping out in front of the TV.

    A propane tankless is a ridiculously expensive thing for a back-up, and the system needs to be designed, not hacked to really work. Neither an electric tankless nor a propane tankless is necessarily designed to handle the higher flows of heating systems. If going that route, look at electric boiler.

    The stated 24,000 BTU/hr design load would be fully covered by 7000 watts of electric boiler, if you wanted to integrate it into the system and run all zones full tilt. But that leaves you no margin for the other power uses. But on a zoned system you can probably stay comfortable in several rooms with only 3-5000 watts of electric boiler. A 4.5- 5kw electric boiler can be had for under a grand, (eg: Electro Industries EMB-S-5 ) with valves pumps and dumb controls it might be DIYable for under $1500, but there's still a bit of design work to do. Do you really need to have the backup fully integrated into the system?

    For about $50 you can buy a pretty good 1500 watt finned radiator style electric space heater. For $150 you can have three of them and still be at only 4500 watts of peak draw, and move them to the rooms you need to keep warm, as long as you have an available socket powered by the generator. Just don't put more than one on a 15-20 amp circuit breaker. That's my go-to backup solution for when heating systems fail, and that would be reasonable solution for even a multi-day power outage a house with a 24,000 BTU/hr design heat load and a 7500 watt generator.

    In a PassiveHouse the best backup would be to put on a sweater, &/or invite a bunch of friends over for a party! :-)

  3. Nathaniel Hieter | | #3

    Space heater!

    (slaps forehead)

    Good grief, my head was up some place dark and smelly.

    Thanks guys!

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