# Too many Heat Pump Water Heaters in a basement?

I recently met with the owner of a local rural inn to discuss performing a comprehensive energy assessment for him. He has been looking at replacing his existing propane boiler and three 80 gal. electric water heaters and has received a proposal for that from a contractor. That contractor is proposing to install a Weil-McLain GV-90 plus THREE- 80 gal. Heat Pump Water Heaters (HPWHs). The basement (which is only under the original building) is roughly 30' x 30' by 8' high, with a bare 1800s stone foundation.

I am concerned that the HPWHs will cool down the basement and default to electric resistance heat when the demand (occupancy) is high. I understand the basics of the enthalpy of the air, but is there a way to calculate how much the temperature of the space will decrease as the HPWHs are doing their thing?

Lower air temperatures and electric resistance heating will notably decrease any efficiency of this renewable technology. Is there a chart or formula that can take into account the size and recovery rate of the water heater, the volume, average temperature and RH of the ambient air to determine the effects of running a given HPWH in a given space?

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### Replies to This Discussion

Hi Brad, as you know, a heat pump's capacity and COP depend largely on ambient temperature. But in situations like this where heat pump operation will have an out-sized impact on ambient, you essentially create a feedback loop.

Unfortunately, there's no way to model this, at least that I can imagine. Even if we had in-depth sensible and latent performance data per temperature bin for the HPWH (which we don't) and an accurate hourly thermodynamic model of the basement (including heat transfer from rooms above), the biggest variable in determining the supplemental fraction cannot be known: DHW consumption and especially draw patterns.

I would look at this problem differently. You're essentially talking about installing 3 small air conditioners. Assuming basement is mostly below grade, the extent that the basement remains above earth temperature depends on heat produced by the boiler -- either through jacket loss, heat transfer from heated rooms above, and/or direct supply, which I think would be necessary to prevent condensation and mold.

The main point is that in all cases, you're essentially using propane + compressor electricity to heat the water.

For most of the year, the difference between intentionally heating the basement to maintain, say, 65F, and allowing it to cool to, say, 55F, is the amount of heat lost to the earth. Without lifting a pencil, it's obvious to me that the additional boiler heat required to maintain a higher ambient (due to higher delta-T across foundation walls and slab) would almost certainly be more than offset by increased water heater performance.

A lot of words to get us to a much simpler question: whether the economics of a heat pump water heater makes sense when the majority of the heat is produced by burning propane most of the year. It's an interesting question and one that I think can be readily calculated, although not without additional data, in particular, cost of propane, marginal electric rate and HPWH performance data.

With basement thermostatically maintained at a given set-point, we still have no way to predict the supplemental fraction since that depends on occupant behavior. But the results of the primary analysis may be such that it becomes obvious that a propane water heater would cost less to operate than a HPWH, even if supplemental fraction is low.

If you post the proposed HPWH model number and relevant energy costs, I'm game for doing a thumbnail analysis here in the forum, as this is a question I have pondered. When this has come up in my work, it was in locations where there's a significant cooling load, in which case there's no way to do the analysis. I've seen some studies on this. The problem is the results can't be extrapolated to a general case or modeling technique. But in a heating only climate, I think we can get to a useful answer through the analytical method.

BTW, with 3 water heaters, I'm thinking this might be a great application for the Sanden CO2 HPWH, which has an outdoor evaporator.

The HPWH model proposed is an 80 gal State, Model HPX80. Estimate propane at \$2.30/gal. and electric at \$0.15/KwH.

The basement is NOT intentionally heated. The existing SlantFin Cat.I boiler will be replaced with a Weil-McLain GV-90, which is Direct Vent. The energy code calls for insulating all of the heating piping in the basement (none in presently insulated). The upgrade would provide little heat to the basement.

My experience tells me that this is a recipe for disaster and, as you noted, will not be very efficient. I proposed replacing the electric water heaters (insulated with fiberglass, so another contributor to ambient heat) with a propane tankless heater and may be one HPWH. Occupancy in the inn will vary from day to day and season to season, with days and weeks with low occupancy followed by a week or weekend of high occupancy.

The HVAC company providing the proposal claims that there will be no problems and the HPWHs are the way to go. They have not yet agreed to have me perform an energy assessment, in spite of several emails back and forth.

It's irrelevant that the basement isn't heated at present. My point is that if you install 3 HPWH's down there, it makes no sense to NOT intentionally heat it. The heat pumps must get their heat from somewhere. Without a heat source other than indirect transfer from above, the room will get progressively colder over the winter, risking mold, condensation, and killing the performance of the water heaters. So the only way to make this work would be to pre-heat the air with the boiler. You don't really have a choice.

If the contractor doesn't buy that, ask him where the heat for the water comes from. A heat pump doesn't create heat (aside from some residual mechanical waste heat). It simply takes heat from the air and transfers it to the water.

If the room is not heated, at some point, it will get cold enough that the surrounding earth begins to contribute. Clearly you can't let it get that cold. So that means that for most of the year, virtually all of the source heat would be provided by the boiler anyway, so you might as well keep the room warm enough to make the water heater sing. As I said, the only difference between that, and allowing the ambient to drop, is it imposes a higher delta-T across the foundation. And that difference should be more than offset by higher water heater performance, since the compressor acts as a heat multiplier.

With the information you provided, I should be able to work up a thumbnail analysis to see if this even makes sense. I'll try to get it done this weekend. In the meantime, take a look at the Sanden. BTW, I still owe you a reply in Allison's block on the sealed crawlspace project :-)

Thanks for the reply, David. The bottom line is that you agree that installing 3 HPWHs in that basement would be foolish.

I looked at the Sanden. It is taking its heat from outside, and must be protected from freezing. It is not distributed to Vermont (none of the 3 major HVAC/Plumbing Supply wholesalers where I have accounts). Otherwise it is interesting, but appears to be more appropriate for a warmer zone.

The bottom line is that you agree that installing 3 HPWHs in that basement would be foolish.

Yes, now that I've taken time to make sure I understand the physics, I'm in full agreement.

Consider that with an air-air heat pump or A/C, the heat rejected by the condenser is roughly the same as the heat absorbed by the evaporator. A HPWH works exactly the same way, except condenser heat is transferred to water instead of air. The evaporator picks up that heat from the ambient air.

I'm not convinced it ever makes sense to put a HPWH in conditioned space in a cold climate, since you end up essentially paying twice to heat the water. An unheated basement might work economically IF there's some way to make up the losses in time with solar gains through glazing and/or sun-exposed walls. But if the basement is fully below grade, its only heat source -- direct or indirect -- is the building's heating system. And that clearly kills the economic rationale for a HPWH with an indoor evaporator. It's like bailing out a boat by tossing the water back into the boat!

BTW, don't even think about venting a HPWH evaporator outdoors in your climate since that necessarily induces an equal amount of infiltration. You'd be shooting yourself in the foot.

Regarding Sanden... US distribution is fairly new so I imagine they make it easy to set up an account, or would sell direct. The plumber would have to be on board, though.

The current model is good to -15F, and the forthcoming Gen3 is good to -20F with 10% higher COP.  Only the water lines require freeze protection (burial). Check out the cold climate study posted on Sanden's pros page (based on an earlier model).

Sanden HPWH's are pricey. You'd need to do a life-cycle cost comparison to see if it makes sense, based on est. annual DWH consumption.  At \$2.30/gal, a 90% propane heater would have an operating cost of ~\$2.80 per 100,000 BTU's delivered to the water. At \$0.15/kWh, the Sanden costs less than half that to operate. The big question is whether the inn could get by with two units.

BTW, since the Sanden HPWH has higher output capacity, there's no electric element. But it would be fairly straightforward to use the GV-90 for supplemental capacity should it become necessary.

1st,  any time you use a HPWH in a cold climate, you are using heat generated by your heating appliance. To avoid that you would need to insulate between the conditioned area ( basement ceiling) and the HPWH's, otherwise, as David pointed out, you are using energy that is keeping the structure warm.

2nd,  The long recovery time in a cold basement would most likely not align with high demand periods associated with lodging establishments. On-demand is a good solution, however, you need to understand the high demand capabilities and the low demand capabilities of the on-demand to make sure all needs are covered. The low flow/low demand capabilities of a larger single on-demand machine may indicate you need 2 or three smaller on-demands rather than one large machine.

Ed wrpte: any time you use a HPWH in a cold climate, you are using heat generated by your heating appliance. To avoid that you would need to insulate between the conditioned area ( basement ceiling) and the HPWH's,

Isolating a HPWH from conditioned space won't work unless there's another heat source.

Theoretically, let's say you perfectly isolate the HPWH from a heat source. The HPWH will eventually cool the air to the point that mold will grow. Here's the math:

30x30x8 = 7200 ft3 x 0.075 lbs/ft3 = 540 lbs of air

540 lbs x 0.24 btu/lb (specific heat) = 130 BTU per degree

Let's say the HPWH's capacity is 10,000 BTU/hr. Since it must extract 10,000 BTU/hr from the air, each heater would drop the temperature 1 degree per 77 hours of operation, in addition to foundation losses. Let's say the heaters average 5 hrs/day each. Three heaters would drop the air about 6F/month.

But even with insulation on the floor, there's still heat transfer to the basement (depends on delta-T and R-value). So to the extent the HPWH can operate without overcooling the space, that heat will come from the house. For most of the year, that heat must ultimately be replaced by the boiler.

If the basement gets cool enough to grow mold, then heat transfer must be increased by removing some insulation or by direct supply. Either way, the boiler ends up providing the HPWH's heat, which makes no sense economically.

As I said, the only way I can see installing a package HPWH in a basement would be if there's significant make-up heat from the sun via glazing or sun-exposed above-grade basement walls. Or perhaps if building above has significant passive solar gains.

Interesting I have not heard of these types of water heaters out east here, nor have seen them mentioned in any articles on the internet.  Thanks for posting this.

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