Back Up Heat to Heat Pumps

Mainer

I live in Maine. My wife and I purchased a 2500 square foot home heated by a circa 1989 oil boiler with hydronic baseboards in all but a 300 square foot segment above the garage and sealed off from the rest of the home by an insulated door. The home is on a slab with no accessible attic, so storage space is at a premium. The home is very well insulated. In fact, it is so well insulated that the builder used precious space for a heat exchanger. The boiler and oil tank are in a utility room that is attached to the unheated garage. The domestic water enters into the home through the utility room, so that needs to be heated above freezing.

We have installed heat ductless heat pumps to heat the home, including an interior unit in the previously unheated above garage segment.

My issue is how to provide a secondary heat source to augment the heat pumps for the infrequent really cold days when the heat pumps cannot obtain enough heat from the outdoors. The existing oil tank and boiler (as well as the domestic stone lined hot water tank which gets heat from the boiler) are nearing or at their life expectancy and will need to be replaced. Natural gas is not available. Do I replace the existing oil boiler with 1) a propane boiler, 2) another oil boiler, or 3) an electric boiler? Do you have any other suggestions? How should I heat domestic water? A hot water heat pump? Please recall that the utility room needs to be kept above freezing. The existing oil boiler accomplishes this just by ambient heat. The oil tank consumes space which could be better used, but a propane tank outside of the home presents its own disadvantages since the home is in a community with restrictions.

EBEBRATT-Ed

Unless you like adding electric baseboard I think you have answered most of your own questions. What kind of heat pumps do you have ? I am assuming they are air-air split systems? Do they have auxiallary electric heat??

If the boiler is to be shut down most of the time you probably do not want to use the boiler as a dhw source.

If you go with a propane boiler you could use a standalone propane water heater.( Your water heater is electric now I assume)

or a heat pump water heater which will cool the space it is installed in.

If the oil tank is sound you can stay with oil.

The fact that the heat pumps won't heat the house at low outdoor temperatures means the boiler has to be sized at the full heat loss.. can't be used as a booster.

Seriously, most everyone here will disagree but electric heat may be the best option. You need to analyze the heat loss, degree days and analyze how much the electric heat will be used

The cost of boilers, hot water tanks, oil tanks and service may be more that it will cost to run the electric heat depending on how much you run it.

need more info

Mainer

The heat pumps are air mini-splits with no internal resistance electric. The secondary heating source is an oil fired boiler with baseboards. Domestic water is from a 40 gallon stone-lined tank heated indirectly from the oil boiler. Thanks for your thoughts. I am reluctant to duplicate the existing baseboards with electric baseboards.

njtommy

I would probably stay with an oil boiler for back up.

Propane is nice especially if you go high efficiency, but given your only running it for the really cold days Your efficiency would most likely only be around 87% any way. Which puts you into a high efficiency oil boiler like a 3 pass Basi boiler.

Solid_Fuel_Man

If you want the oil tank out you have the option of propane, buried oil tank, or buried propane tank. I'm not a huge fan of burying anything... i would avoid electric baseboard. If you wanted Electric backup and already have hydronic, an electric boiler would be my only consideration. Where in Maine are you located? I'm in Aroostook County. Our Electric rates are $0.15 per kWh, making Electric resistance heat verrrrry expensive.

Taylor

Aaron_in_Maine

I would do oil. Propane pricing is based on the amount of gallons you burn. The less you use the more expensive it becomes. It sounds like you won't burn very much.