HPWH for space heating.

[Ehouse]

I have a large(15x15) room that I want to heat and supply hot water separately from the rest of the house. I know some DWH's can be used for space heating. What do you think of using a HPWH for this. It would supply a fan coil to the room heating duct ( only about a 5' run). I'm guessing it would deplete the available heat in the basement too quickly. What say you?

 

[woodgeek]

The economics on HPWH are poorer on a cost/BTU during the heating season, and great outside the heating season. About half the BTUs come from the space heat, and half from electric kWh. So in the winter, a BTU of HW costs about the average of your space heating BTU cost and electric resistance heat BTUs. The rest of the year, they cost the average of elec BTUs and 0, IOW, half that of a resistance elec unit.

In your example, IF your winter BTUs are free (wood), available in excess and can get to the unit, then you are heating the space for half the price of electric (+ wear and tear on the unit). IF you want it to be really independent of the existing space heating system, e.g. heat the room while the rest of the house is going cold, then forget about it.

If this is a frequently used room, I wouldn't do it, since you would 'wear out' the unit. If it is an infrequently used room, then the heating cost is unimportant.

 

[Ehouse]

The economics on HPWH are poorer on a cost/BTU during the heating season, and great outside the heating season. About half the BTUs come from the space heat, and half from electric kWh. So in the winter, a BTU of HW costs about the average of your space heating BTU cost and electric resistance heat BTUs. The rest of the year, they cost the average of elec BTUs and 0, IOW, half that of a resistance elec unit.

In your example, IF your winter BTUs are free (wood), available in excess and can get to the unit, then you are heating the space for half the price of electric (+ wear and tear on the unit). IF you want it to be really independent of the existing space heating system, e.g. heat the room while the rest of the house is going cold, then forget about it.

If this is a frequently used room, I wouldn't do it, since you would 'wear out' the unit. If it is an infrequently used room, then the heating cost is unimportant.

Thanks geek. Wear and tear on the unit I hadn't considered.

 

[woodgeek]

In a warmer climate like CA or FL I would say no problem.

Let's do some math. Say we have 50 gallons = 400 lbs water at 120°F, and you installed low-temp radiation that worked to 100°F (like radiant, or oversized hydronic radiators, or a fan+coil), that is only 8,000 usable BTUs stored in the tank, and maybe an hour or two to recover?

If it will be seldom used, you could go for an electric heater and electric DHW. If frequently used, it might be nice to look at minisplits, but I would think in NY you would need some electric baseboard backup.

 

[Ehouse]

I Heat with about 80% wood and 20% Propane DV parlor stove, but both are small and in the half of the house we are using now. I have a 40Kbtu fan coil in the old furnace plenum charged by an LP boiler. I don't use it for heat at the moment but the fan is on a vari-speed control and I use it to move heat around through the existing ductwork. I have an extra 24Kbtu fan coil and was thinking of ways to use it for an independent system to open up the other part of the house, hence the HPWH idea. Now I'm thinking it's better to run it as a separate zone off the boiler and place it either in the duct to the room, or in the plenum for a two stage set up. Upgrading the woodstove and DV stove for more btu's is in the plans and would work as well.