HPHWH in a home with no basement
We’re building an all-electric home in CZ6. The home doesn’t have a basement. We’d really like to use a heat pump hot water heater, but we’re concerned about the energy penalty from operating one in a conditioned space. We’ll be heating the home with a wood stove and heat pump and I have no idea how to figure out if the energy savings from a HPHWH outweigh the energy penalty of cooling down a conditioned space during our cold winters. We’re also concerned about the noise.
Our hot water heater will be installed in a ~100 sq ft mechanical room. This mechanical room has a long narrow pantry on the opposite side of the wall where the hot water heater would be going. One thing I’ve considered to minimize the energy penalty is venting a HPHWH into the pantry and adding some insulation to the pantry walls to try to contain the colder air inside that room. Has anyone tried something like this?
Our other concern is noise. If we were to install a HPHWH above grade, would it make sense to try to soundproof our mechanical room a bit? We live out in the woods and I’d like to try to keep our mechanicals as quiet as possible.
To summarize, here’s my questions:
1. Are the energy savings from a HPHWH greater than the additional space heating energy required to make up for the cooling from their exhaust?
2. If so, are there any potential issues with venting the exhaust to another room?
3. Would an above-grade HPHWH be noisy when outside our mechanical room?
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Replies
HPWH should always be operated in conditioned space.
there are options with remote evaporators that supposedly work well in cold climates, pricey but perhaps an option. SANCO2 and Arctic Heat Pump come to mind, but I'm sure there are others.
If air conditioning isn't needed, you might want to consider Harvest Thermal (https://www.harvest-thermal.com/) which provides both space heating and hot water from the same SanC02 compressor.
I'm not sure if they are installing outside of California yet though.
You don't need the extra "H." They are called heat pump water heaters. They heat cold water, so if anything they should be called heat pump cold water heaters.
1. No, there is a "vampire load" in that in cold weather the water heater uses heat produced by another device, but they are very efficient, and in warm weather you get "free" cooling, so although the total system efficiency is lower than it would be without a water heater, it's still the most efficient way to get hot water using electricity.
2. On two projects I have backed the refrigerator up to a mechanical closet with a heat pump water heater. On other projects I have added wall grilles to provide airflow. Both seem to work reasonably well, though the mechanical space can get quite cold. I don't see why you couldn't duct the air rather than letting it circulate naturally.
3. Sound levels seem to vary. They are supposed to be about as loud as a typical refrigerator, which makes sense as they are mechanically similar. But there are reports of much louder units. Insulating and otherwise providing sound damping is a good idea, and may or may not be enough.
Let's talk about efficiency and then some other practical issues.
The heat pump should always be installed in conditioned space, it works by taking heat from within the building envelope that the heating system has to replace. Typically the HPWH will have a COP between 3 and 4, let's call it 3.3 to make the math easy. So to produce 1 BTU worth of water heating the HPWH consumes 0.3 BTU worth of electricity, removes 0.7 BTU of heat from the house that needs to be replaced, and adds back the heat of that 0.3 BTU worth of electricity. So in effect the HPWH is heating the water using 30% resistance electric heating and 70% whatever the heat source of the house is.
Whether this is a good deal depends on what the heat source for the house is and what the alternative is for heating water. That's just in heating season. Outside of heating season the heat pump cools and dehumidifies the house , if you're otherwise paying to do that then you're getting it for free.
The practical issues of having the water heater in your living space are first the noise, and second diffusing the concentrated cold air. I wouldn't recommend trying to trap the cold air, that limits the efficiency of the HPWH and also creates a condensation risk. How best to diffuse the air depends on the layout of the house. In my house I vent it into a conditioned crawlspace where it is picked up by an HVAC return.
During the heating season, the HPWH "scavenges" heat from the living space to heat the water, the same way a "regular" heat pump scavenges heat from the outdoors to heat that living space. COP of the HPWH and all that fun stuff doesn't really matter during the heating season if you only care about the load on your primary heating system due to the operation of the HPWH. The HPWH will "steal" BTUs from your primary heating system to heat the water in the tank, with the mount of BTUs it "steals" based on a combination of how much hot water you use, and how high the temperature setpoint of the water heater is. You don't "lose" any more BTUs than what goes down the drain though -- any BTU losses through the insulation of the tank just go back into your living space, reducing how many "new" BTUs your primary heating system has to produce to keep your house comfy.
You can think of the HPWH as losing BTUs down the drain in the form of warm water. There are drain water heat recovery systems that can help reduce those losses a little, but they aren't always economical in every situation. Aside from those BTU losses down the drain, the HPWH does not do anything to load your primary heating system -- all the heat "losses" from the HPWH just go back into your living space, so they aren't lost. During the cooling season is where you win in terms of energy savings, since the HPWH helps to cool your living space, and the "removed" BTUs are now what is going down the drain with the warm waste water.
So to directly answer your questions:
1- No, not really, but it depends how you look at it. During the heating season is the only real concern here, and since you have a wood stove, the HPWH essentially lets you heat your water with wood, in a sense. As long as you have extra capacity with your wood stove and/or heat pump, then the HPWH doesn't really hurt you. You don't want to insulate it from the rest of the space though, since the HPWH needs a source of heat to scavenge, and won't work well if you put it in a small, confined space where it will cool it's surroundings excessively.
2- I REALLY LIKE Michael's idea of using waste heat from the kitchen's fridge to help out the HPWH. I *love* systems that make use of otherwise wasted energy like that. Whenever you can do something like this "for free", it makes sense to do. You can help to minimize the sound transfer with some baffles in the transfer duct (and there are duct sections specifically made to help with this). Baffles can be made with 1/2" drywall or plywood wrapped with MLV sheet, then line the duct with MLV sheet or other sound absorbing material. The baffles should force the air to make a zig-zag pattern so there isn't a straight-through path through the vents (you should only see baffles when you look into the vent).
3- Some people have reported noise issues with HPWH units. You can minimize this issue by setting the HPWH on vibration isolation pads (the rubber/cork pads supporting a platform of 3/4" plywood to support the HPWH will work here), then using 5/8" drywall around the room the HPWH is located in. Be sure to use flexible connections for the water lines to and from the HPWH too, to make sure sound isn't transfered through the water lines. If you're using PEX everywhere, the PEX itself acts as the flexible connection, just try to make a curve in the line so that you don't have straight shot of pipe from the HPWH to something like a pipe clamp.
Bill
Bill --
I have an AO Smith HPWH, it has been somewhat loud (61dB on my phone app). It's not the classic compressor noise where it sets up sympathetic vibration in the surrounding environment, it's on a concrete pad. Rather, the noise is emanating from the compressor itself. I found the most effective way to limit the noise was to box in the exhaust vent and connect it to about 8' of 8" insulated flexible duct. It's not noiseless, it's not as quiet as a refrigerator, but it is quieter.
So you're seeing noise carried through the air more than conducted sound? I usually call that "blower noise" when dealing with generators (where I have lots more experience doing things to quiet stuff down). Flex duct is a good way to go to quiet it down, make a non-direct (not straight) path, and use insulated duct with some more mass to it. You basically want a combination of "squishy surface" to limit sound reflecting around the bend, and mass, ideally "squishy mass" here, to limit sound going through the side of the duct. You could build a box with baffles the way I described earlier. You need to make the air turn a corner or two, and the baffles forming the corners (or bends in flex duct) should be lined with "squishy mass" like MLV (Mass Loaded Vinyl) sheet.
You can get butyl rubber tape that is squishy and sticky that you can wrap around things to help dampen sounds too. I wouldn't use this stuff on something like a compressor that can get hot (the rubber can get smelly then), but it could potentially be used on the flex duct to help quiet things down more. I've used this stuff on pipe traps on drains to quiet down trickly water sounds from noisy pipes.
Bill