Improving hot water heat pump tank efficiency with free dryer heat
Hi. I have been thinking about how to improve my GE electric hot water heat pump efficiency, particularly in the winter months. It is located in a very well insulated utility room off the garage (larger than recommended minimum volumes). The by-product of using the heat pump is cool, dry air, which in turn creates a cold room calling for accessory heat to warm the room back up to temp for the heat pump to then work efficiently again. I find the room temp is fluctuating up and down in conjunction with the use of heat pump this not making it as efficient as it could be. So my question is: could I seasonally vent my dryer into this room (assuming proper filtering of the incoming air and insuring on a daily basis that the hot water tank filter remains clean)? My dryer sits against an adjoining wall and currently has its ductwork traveling through this room already. I could install a 4″ Y and in the winter months divert the dryer exhaust into that room. Free heat. Thoughts?? Thank you.
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Replies
Matthew,
Adding moisture to the interior of your house can cause problems, including condensation on your windows. The extent of these problems varies, depending on how often you operate the dryer and the size of your house. Beware.
The room in question is in the garage, completely separate from the house. With our little amount of use of the dryer and the large amount of cool/dry air the hot water tank produces, I think it would be offset. We could coordinate our hot water demand/usage with dryer operation.
What's your climate, and what is the wall construction of the garage? Insulated ?
Being separate from the house is good for not putting the house at risk from moisture, but you could still have condensation on some part of the wall of the garage. If you do play with this, I recommend a humidity/temperature meter in the garage. Take a look at it before and after drying a load of wash.
If you get one of the new heat-pump dryers, you'll get more heat and less water vapor out.
Very cold climate. Northern Maine. Room is 2x6 insulated with Roxul and 2" polyiso. Will do with the humidity monitoring. Just bought a new dryer-traditional type! Thanks.
Try it and let us know, easy to compute the BTUs from each appliance with run times on the dryer and water usage on the heat pump. Put in a $10 temp and humidity gauge and see what happens. Experimenting is great why not do it. I just watched a 3 year old pour water into different containers, empty refill repour for hours, maybe she will be a chemist someday!
Polyiso on the outside of the 2x6 roxul wall, or vice versa? With the polyiso on the inside, you have nothing to worry about unless you get up to very high humidity. With polyiso on the outside, you might have some condensation on the inside of the polyiso when it gets really cold outside, because the R-value of the polyiso drops at low temperature, as per Martin's Sept. 4th blog, for example. The allowable humidity inside would be a function of the interior and exterior temperatures...kind of complicated, but I guess you could make a chart to consult and tape it up on the wall next to the gauge. Let us know if you want help constructing such a chart.
Charlie-
Polyiso inside of Roxul. Yes a chart would be handy.
Great to hear that the polyiso is on the interior. That means you really don't need a chart. The walls will be fine. Condensation would form first on the windows and doors. So just keep an eye on them. The other concern might be the attic or roof. A well vented attic with insulation on the attic floor, well sealed from the main space, it will be fine. Otherwise I can comment if you describe the construction.
Clogging the coils on the HPWH with lint is another potential issue with this approach. It's hard to make dryer exhaust truly lint-free, and when the coils on the water heater are wet with condensation it becomes pretty sticky for any free-floating cotton & polyester fibers that pass that way.