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Community and Q&A

Timer for Heat Pump-Powered Water Heater

DCcontrarian | Posted in Energy Efficiency and Durability on

I have a heat pump water heater (AO Smith) and while for the most part it’s great, the noise is still an issue. It can be heard in every room of a 4000 square foot house. After battling it for a while, my latest thinking is to put the water heater on a timer so that it only runs in the middle of the day and the middle of the night, times when either the house is likely to be empty or everyone is asleep.

The water heater has a port that says “For smart grid or home automation use only.” (I don’t know what other use they imagine!) Which seems like it would be perfect, except that a little bit of googling tells me that AO Smith has a proprietary commercial protocol and a simple timer isn’t going to be simple.

So now I’m thinking of a simple 220V timer, the kind that cheapskates used to install on resistance water heaters. The question is, do you think it would be bad for the water heater to turn the power on and off twice a day?

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  1. mr_reference_Hugh | | #1

    DCContrarian, before you install the simple timer, consider that the timer might shut off the heat pump while the heat pump is working.

    I would avoid the issue of installing a timer and ask AO Smith if you could damage the heat pump if you repeatedly (once or twice a day) turn off the heat pump while it is running.

    I had an HVAC installer tell me to never shut off the heat pump while it was running, but I have not been able to find information to confirm or explain this.

    1. Expert Member
      DCcontrarian | | #7

      So this is basically what I'm worried about. Electric motors draw a lot more current when they're starting than when they're running, high-load electric devices are designed to minimize the load until the motor gets up to speed. With a compressor that means emptying the compression chamber before starting the motor and not letting it fill until the motor is up to speed.

      I know that this heater has a power-on process that takes about seven minutes, and I can hear sounds that sound like valves opening and gas moving while it goes through that. What I don't know is whether there is a shut-down process at the end of a heating cycle, and whether routinely skipping that process would injure the compressor.

  2. Expert Member
    BILL WICHERS | | #2

    I also agree a timer might be a bit of a risk. See if the heat pump has an "inhibit" option, something that can take a "contact closure" (which means a dry contact, a relay contact) to signal it to either run or not run. Some equipment has this type of option, and if your unit does, you can just wire the timer's contacts into that input. If the input needs to be powered, you can probably use a 24v transformer like you'd use for a doorbell, but be sure that that's what the water heater wants.

    Power cycling refrigeration equipment can be an issue if liquid refrigerant gets into the compressor, since you can 'slug' the compressor and damage the compressor assembly.

    I would try damping the sound before trying to modify the electrical side of the unit. Use flexible jumper hoses ("connectors") between the water heater and the water lines. Use overly long hoses here, so that you can make a one-turn loop in each. This will help to 'decouple' the water heater from the water lines. Use the braided lines, not the corrugated copper lines (which are stiffer). This will help a lot if you have rigid water lines in the house, and will help less if you have PEX. If you can, clamp the flexible hose to the masonry wall. I usually do this by mounting a small block of 2x4 to the wall edgewise with a large 3/8" tap con, then using a two-hole pipe strap over the water line with a piece of 1/8" neoprene sheet over the pipe under the pipe strap. I used this method to greatly quiet down conducted noise from my well pump at home.

    Butyl rubber tape on the water lines can help too, but I wouldn't want to use this material on the hot water lines since it might smell.

    Assuming the water heater is in a mechanical room of some sort on a concrete floor, try lining that room with mineral wool for sound deadening. Sometimes that's enough. You can get rubber/cork/rubber layered vibration isolator pads to set the water heater on to decouple in from the floor (these are commonly used under mechanical equipment commercially).

    The last resort would be to really soundproof the mechancal room, either going with a staggered stud wall (best), or resilient/hat channel (next best) and a double layer of 5/8" drywall on the walls and ceiling. I would check how the noise is getting out though -- if it's mostly conducted sound coming through pipes (which is what I suspect you're probably hearing), then that's easier to deal with as I described above as opposed to soundproofing the mechanical room.


    1. Expert Member
      DCcontrarian | | #5

      Thanks Bill.

      I'm pretty sure the sound is coming through the air. It's a heat pump, it needs a lot of air flow. There are two sources of noise, the compressor itself and the fan. When it first turns on it's just the fan for a minute or so and then the compressor kicks on, and it sounds like the compressor is about 2/3 of the noise and the fan is 1/3. Putting my hand on the unit while it's running, there doesn't seem to be any vibration being transmitted to the equipment itself. There's a thread going on that's approaching 200 posts about this problem.

      It is in a utility room that currently has a louvered door. In the long run I may try to soundproof that room, before I do that I have to determine if the sealed room will have enough volume to allow for enough air flow for the unit to run. In the meantime, I thought switching it to running in the less objectionable times of the day would mitigate the problem for now.

      The unit has a digital thermostat so I don't think there is anything I can hack, if it had an old-school thermostat I agree that tapping into that would present the least risk.

    2. user-5946022 | | #9

      @ BillWilchers
      In regards to your statement "Power cycling refrigeration equipment can be an issue if liquid refrigerant gets into the compressor, since you can 'slug' the compressor and damage the compressor assembly"
      Would this also apply to a whole house dehumidifier?

  3. brianvarick | | #3

    Does it not have a schedule that can be utilized? My solution for my rheem was similar where I only let it run between 12-6am and it is off for the rest of the time. We have an 80 gallon and we have never really run out of hot water. It runs for about 2 hours to heat the water up every night. It works perfect for us and ends up being super efficient, I think we are on pace to spend less than $100 for the year.

  4. Expert Member
    Akos | | #4


    I would be curious to see what is causing the noise on the unit. Do you have a spectrum analyzer app on your phone that you can take some measurements with.

    Sound is one area that hydronic heated houses have a drawback. They are too quiet. This tends to mean that sounds that would not be heard in a forced air home get annoying pretty quick. Maybe time for a white noise generator or an ERV with louder ducting to add some background noise.

    1. Expert Member
      DCcontrarian | | #6

      Yep. This house has triple-glazed windows, thick walls and excellent sealing. I put a lot of attention into all of the mechanicals to get them as quiet as possible. It is super-quiet. As a result, any noise really stands out. When someone is emptying the dishwasher, from the third floor it sounds like a train is crashing into the house.

      Our old house had a power vent gas water heater that was objectively louder than this, but not nearly as annoying.

    2. Expert Member
      DCcontrarian | | #8

      You know, I actually tried running white noise to see if it would help. We had to turn it up so loud to cover the water heater that it was noticeable.

  5. mr_reference_Hugh | | #10

    What is the exact model number of your unit?

    If the noise is having such a drastic impact, have you called AO Smith to ask whether they have equipment to allow for adding a timer and how much the equipment would cost?

    Before sounds insulating the room, would you consider venting the HP to the exterior of building and installing a solid core door on the utility room? Could you buy a used solid core door (not too expensive) and install it to see if it would make any difference in the sound attention (before deciding to vent the unit to the exterior)?

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