Replies

1. Expert Member
   BILL WICHERS | Dec 11, 2019 05:39pm | #1

   Are you really asking how many DEGREES the TEMPERATURE in your basement is likely to drop with your heat pump water heater running? The way you ask about BTUs is unclear. My GUESS Is the temperature of your basement probably won’t drop much, but exactly how much depends on a lot of things (are the walls insulated, how much airflow is there through the finished basement, etc).

   You probably won’t notice a big chill effect if that’s what you’re concern is.

   Bill

2. Expert Member
   Dana Dorsett | Dec 11, 2019 05:58pm | #2

   Heat pump water heater typically deliver heat to the water at about 15,000 BTU/hr. About 2/3 of that heat is coming from the room air, so it's sort of like a 3/4 ton air conditioner, but only while it's actively RUNNING. Unlike a room air conditioner it isn't going to run hours-long duty cycles to heat up the tank. The actual duty cycle depends on how much water is being used.

   At typical hot water use levels for a family of 4, the average temperature in a 900 square foot basement would drop by something between 1F - 2F, not more. If an average drop of ~1.5F cooler makes the difference as to what would be deemed "...too cold...", you're already too close to the edge- the basement needs to be either actively heated or better insulated, independently of the HPWH.

   Insulating and air sealing the foundation walls, foundation sillss & band joists to at least the IRC 2018 code minimum would be a good idea whether there's an HPWH in the basement or not. How best to do that with a finished basement depends on the details of how the exterior walls are currently finished, the local climate zone, foundation type, etc.

   [edited to add]

   Correcting myself on one aspect, the ~15,000 BTU/hr is usually only in "hybrid" mode, when there is a resistance heating element running (which doesn't lower the room temp). In heat-pump-only mode the rate is much lower, but the net effect on average room temp is as-described.

   eg:

   The Rheem HPWH heat pump compressors are only delivering 4200 BTU/hr, ~2/3 of which is coming from the room air, so it's only pulling heat from the room air at about 2500-3000 BTU/hr. That is about half the rate of a smallest window air conditioners.

   https://s3.amazonaws.com/WebPartners/ProductDocuments/c2ba7801-ae70-4c6c-be6b-f5c2a3647577.pdf

   See the chart on page 2. With the 5000 watt resistance element it adds another 5000W x 3.412 BTU/watt-hr = 17,060 BTU/hr, raising the total to (17,060 + 4200 =) 21,260 BTU/hr in full hybrid mode for fast recovery.

   With the 2250 watt heating element the resistance heater adds 2250 x 3.412= 8530 BTU/hr to the 4200 BTU/hr for a total of 12,730 BTU/hr.

   But if set up for heat pump only it will indeed knock a degree or so off the average basement temp, while delivering maximum water heating efficiency & maximum basement dehumidification.

3. 
   user-7591582 | Dec 14, 2019 01:08am | #3

   Thanks! Had a Rheem dealer come by today. Options are to just have it vent to the finished part of the basement or vent to exterior or the home. It would be pulling air from the finished basement via louvered door. It says the Rheem HPWH compressor is around 4000 BTU. Does that mean that 4000 BTU is the amount of cold air exhausting into the basement? So only would drop the temp a few degrees? Would you all vent it to finished part of basement or vent it to outside?

4. 
   grmp945 | Dec 14, 2019 09:44am | #4

   I'm not an expert, but it is possible that a good portion of the 4,000 BTU will be used for "latent heat" removal - i.e., for converting water vapor in the air (humidity) to liquid water. If your basement suffers from high humidity, you might notice a distinct de-humidification effect rather than much in the way of a reduced temperature effect.

   A good article related to this issue is here:

   https://www.energyvanguard.com/blog/60024/Latent-Heat-is-the-Wicked-Witch-of-the-South

   However, I also think that the dehumidification effect will vary depending on the model of heat pump water heater. The lower the fan speed of your model, the more latent heat removal. I have a GE Geospring and have been disappointed by the fact that the fan remains on after the compressor has stopped; this results in evaporating the liquid water on the coils back into the small room and drastically increasing the relative humidity (this is exacerbated by the fact that the room temperature has decreased due to the cooling effect).

5. 
   Jon_R | Dec 14, 2019 10:38am | #5

   In heat pump mode, it is going to run for hours after a single shower, taking about 3000 Btu/hr from the room. How much this cools some space is highly dependent on how well insulated the space is, thermal mass and how much airflow you have to upstairs. If you had to pull all of the heat through a 900 sq ft R2 ceiling, then about a 6F drop in basement temp (more with stratification).

6. 
   user-7591582 | Dec 14, 2019 06:11pm | #6

   Thank you all for the posts! Bottom line, would you vent to exterior (and deal with depressurization of the house), or just vent to the finished basement and occasionally have to turn up the heat every now and then when we go down there to watch a movie or have out of town guests staying with us?

1. 
   chrislee991 | Dec 14, 2019 09:00pm | #7

   I would not vent to the outside as you will be exhausting heat from your home, aside from other issues. We have a passive house with a Midea hybrid heater. It draws through a grill from the living room and has a ducted exhaust through two vents in the bathroom and an adjoining bedroom. Our ERV is supplying air to the bedroom and exhausting from the bathroom which provides mixing. The temperature effects are barely noticeable or measurable, if at all, even after sustained hot water use with the ERV off. We shared your concerns while building but followed advice provided and are quite happy with the outcome.

2. 
   Jon_R | Dec 14, 2019 10:13pm | #8

   I'd try it (vented to the inside) and if the temp drop bothers you, add a thermostatically controlled damper to sometimes increase heat delivery to the space.