Hybrid heat pump water heater

Loving this …

Like many, I suspect, I’m frustrated that I can’t dump the heat from the fridge directly into my HW tank, at least in summer. Short of some complicated refrigerant heat exchanger, pulling the warm air in that way is nice.

Remember those old alarm clock + kettle + toasters & “everything including the kitchen sink” kitchens that you get in small seaside hotels … well how about a MFHPHW?!

MF = Mini Fridge

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That would be nice. In the summer my HPWH was only costing me $10 a month pulling that hot attic air but that’s not an option in the winter here. I ran it in electric resistance mode for a week and the usage was much higher even pulling cooler air (70 degrees approximately) in my winter mode. Now not only is my HPWH using more electricity but my Mitsubishi mini splits are working harder to heat the house because the HPWH is stealing house heat. I’m curious if switching to pulling hot air from the attic is worth it in the summer as continuing to pull air from the house will help my mini splits when in air conditioning mode. So maybe the extra $10 to $15 a month spent on hot water will be mostly offset by easing the ACs load?

The trick of course is to switch the air intake (and outflow) in summer and winter.

Here in the northeast in a small apartment an as-is HPWH would be great in summer drawing heat from my apartment and dumping cool air. In winter it’s exactly what I don’t want … especially given that my heating is 100% resistive … and so in winter (without switching the air IO around) my place would probably run more efficiently with the HPWH in resistive mode.

Well I agree with 100% resistive heat You’re not saving anything in heat pump mode but from what I’ve read on various websites my Mitsubishis are efficient to the point where even having my HPWH steal heat from them I’m better off than having my water heater in resistive mode. I can’t pull air from my attic in the winter here in the suburbs of Philly because it’s too cold up there and I wouldn’t want to anyway so I’m pulling heat from the house. My question is in summer time I know that the really hot attic air makes my water heater more efficient but if I keep it in winter mode where it’s pulling warm air from behind my refrigerator and dumping cool dehumidified air out even though it is working harder than using the attic air is the fact that it’s helping my mini split air conditioner going to offset or maybe at least mostly offset the increased HPWH electrical usage from not pulling from the attic? It’s probably close to a wash in my opinion. That’s my winter mode duct…

It’s probably, as you say, “a wash”, but …

More efficiency in your case, if I understand your setup, could come from un-insulating the HPWH room in summer … i.e. the cool air output by the heat pump could cycle into a conditioned (by your mini-split) space vs being trapped in the insulated room and effectively working against the heating aspect of the tank. Leave the door open?

In winter, yes you want to draw the warm air (pre-conditioned by your fridge and more efficient mini-split) from the living space but you also, ideally, want to vent the cool air to the outside away from the “warm space” insulating the tank.

OK, I’m adding that the MFHPWH should also have an ice maker or can chiller :beers:

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I floated the idea of keeping the central air vent ( maybe adding another?) behind my fridge open in the summer and letting the cool air from the HPWH enter the house but I wasn’t sure if having basically an 8" hole between my conditioned space and my attic was a good idea considering the HPWH doesn’t run most of the day. I was thinking when someone showers or runs the range exhaust fan the vacuum created would pull air from the 8" duct to the attic as that would be the easiest source as the attic has ridge and soffit vents. So my storage room door ,which leads outside, has a large vent in it from when there was an oil boiler in there and it needed the intake air during the summer. I left that door vent open in the summer so when the HPWH was running the room wouldn’t become pressurized. When it got cold(ish) I sealed the door vent with poly board and HVAC tape, opened the central air vent behind my fridge and swapped the hpwh’s intake vent from the attic to behind the fridge… phew

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The interplay between hot-water-on and bathroom exhaust (shower mode) starts to get complicated. Eventually the only real solve is a heat exchanger (HRV/ERV) and ductwork. Seems like we have similarly obsessive concern … I start to wonder whether the excess heat my thoughts are generating could likewise be a factor! At least it’s cold right now.

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I thought about a (HRV/ERV) but probably not doing it at this point. That’s funny I did think that in my summer setup when someone showers ( exhaust fan on ) the HPWH would be on hence the air would be cool coming out plus the opening to the vented attic would make life easier on the bathroom exhaust fan. Problem is showers aren’t the only time the fan runs and then there’s the dryer…with the dryer running I’d be exhausting cooled conditioned air and drawing in really hot attic air.
It’s part about the money but I think it’s just as much a sort of hobby. Only certain people can listen to all the different permutations regarding ducts run where, vents opened or closed, air flow and temperature etc…

I’ve labeled my Rheem Hybrid water heater in my Sense inventory. Given Rheem is apparently having issues with the reliability of their monitoring, do you have any ideas about ways I can improve detection by Sense?

Yeah I was thinking of possibly running an exhaust duct to actively pump the cool air back into the condition living space. The storage room is well insulated now, before I put the heat pump water heater in I spray foam the walls and the ceiling as it doesn’t have any direct heat. I have that central air vent cut in the wall to help with that. But I get what you’re saying having it dumped the cold air into the storage room is nice for their small amount of time I’m in this storage room and indirectly it comes back into the house through the vent but it would be better to pump it directly into the house and let the storage room where the tank is be warmer

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I’ve had Sense installed for 2 weeks now today and this afternoon it detected my AO Smith Heat Pump Hot Water Heater. Its a 60 gallon unit I installed in 2013/2014. Sense picked it up as Motor 1. Model# PHPT60102. I believe it picked up the compressor only which is currently drawing 641 watts while running. Typical voltage spike on startup of an AC compressor of around 1000 watts, then within seconds levels out at 641 watts. The evaporator fan motor is rated for 80 watts output it is a ebm papst m4q045-da05-01.


Well it only took me two years to pull the plug but I just bought a Rheem 50 heatpump water heater last week as my old electric started to have a small leak. I got a sweat deal where I found if I entered some zip code form Oregon and paid the flat $75 shipping that HomeDepot gave me a $500 discount at checkout.

I have to admit that I am confused between the energy saving mode and the heatpump only mode. The app seems to tell me that energy saving mode is the most efficient but I don’t understand how that can be as it runs the resistance coils at times. I am currently having it run in heatpump only mode. Seems to work fine in my hot Florida garage. Can you lend any info on the two modes and help me understand why they claim the energy saver mode is more efficient - it does not make sense. Speaking of Sense has Sense picked up the unit?

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Heat pump only mode is just that, only the heat pump. But what you’ll notice is that when the ambient temperature is pretty low, it takes the heat pump a fairly long time to recover the water temp. Perhaps it’s an equipment protection, or perhaps they’re thinking it will run so long that it takes more energy, but “energy saver mode” uses the resistance elements plus the heat pump. I’m not so sure how “smart” it is about using resistive elements only when ambient temperature is low, I haven’t paid it much attention. I set mine to heat pump only mode as well (also in Florida), and only go to energy saver when I am expecting complaints from the family (i.e. teenagers just took showers and wife wants to take a hot bath).

Sense did eventually discover the HPWH, it was many months, might have been almost a year. I can’t remember, I think it guessed it as a dehumidifier or air conditioner. But once I confirmed that’s what it was, it correctly identifies it almost every time. It uses about 300 watts while running.

I have the same HPWH, had it since April 2019. Heat pump mode is most efficient, I think they’re encouraging “energy saver mode” as it’s more efficient than resistive mode AND they can avoid complaints fro heat pump only users who may run short if hot water. Having said that I only run in heat pump mode , live in the suburbs of Philly with my wife and son, and I’ve never been without hot water. I put mine in my unheated storage room which I insulated with closed cell spray foam and put a central air vent behind my fridge ( shared wall with storage room) so it can be indirectly heated , this also depressurizes the storage room as the HPWH is ducted to pull warm air from behind the fridge and dumps the exhaust into the storage room, phew! It’s basically stealing heat from my Mitsubishi mini splits. Sense found my HPWH but not all it. According to the Econet app it uses around 4 times what Sense thinks it does . I also had it ducted to pull hot air from my attic in the summer of 2019 which was real efficient but this summer I pulled air from the house and exhausted the cold air back into the kitchen. I figured the HPWH would be less efficient but it would ease the load on my AC.

Sense found the heatpump water heater in only one day, but it was only showing around half of what the unit was using. After about a month a got an alert that Sense determined it was not accurate and deleted the model. It has now been about two months and it has not found it again.

I have it in heatpump mode only and have not issues. I did call tech support and they told me that yes I am correct (I knew that) that heat pump only mode is the most efficient. The warning in the app to switch to Energy Savings mode is a mistake and they need to get that corrected if they update the app.

I have mine in the garage in North Florida, and even with it cold now (40-50) at night it is not really using much more energy than when it was put in when it was going down to 65-70 at night. It uses about 3kwh per day which is right in line with the DOE numbers. I imagine in the summer when the garage can get well over 100 degrees it will be super efficient with much lower energy usage.


Check out this post for reasons sense might be underreporting or misreporting HPWH usage:

Not sure why that would be an issue. The unit does not “know” what is going on when it starts up so the startup of the compressor should be the same. Doesn’t the unit automatically kick on when temp drops, so shouldn’t that be a constant condition anyway?

I have a Stiebel Eltron 300 heat pump water heater. Sense often detects it turning on but because it is a complex machine it has trouble differentiating between the condenser/compressor and the fan. So sometimes sense will say it was on for 3 seconds but when I look at the graph I can see it was on for 4-6 hours. I have been using sense for about 2 years. It is helpful but not as precise as I had hoped. We live off grid here and use only solar power and batteries with a backup generator to charge the batteries if no sun. It is very helpful for us to be able to see exactly where all of our energy consumption is.

One of the reasons that sense under reports is because realistically it only detects the on sign (large draw) then flatlines for an average consumption during that period and the off sign (drop in watts eq or greater than the average consumption probably +/- some threshold).

Realistically the compressor slowly builds over the entire runtime. And the “off” sign is usually confused with other items.

Consider the following. This device is taken from a shelly em reporting to senselink then to sense. This will likely be the best representation of the water heaters actual use. Shelly EM numbers are nearly identical to the EcoNet app.

Compare that to the sense detected device. There is obviously a large portion not detected due to the ramp.


Well for me I think that explanation works a lot better for my refrigerator. For the last few days I’ve had it reporting through a smart plug and through the native detection and they are pretty similar although the refrigerators native detection has a lot more flatlining and is much less erratic. In regards to my Rheem HPWH Sense isn’t even remotely close. It under reports by at least 70%. Maybe I’ll try deleting it and see if it detects it again because I don’t really pay any attention to it regarding the water heater anyway