Heat Pump Water Heaters

[Jim Katen]

Is it just me, or are these things really stupid? 

Today's new-construction house had an AO Smith heat pump water heater in the garage. When I arrived on site, it was 42 degrees outside and about 39 degrees in the garage. The water heater was set to 120 degrees in the "hybrid" mode and its heat pump compressor was running non-stop while its indicator showed that it was also running one of the resistance heating elements. Even so, the hottest temperature that I could get *at the water heater's outlet pipe* was 110 degrees. By the time it got to the fixtures it was about 102 degrees. Even when I switched it over to pure resistance heating and checked back a few hours later, the best it could do was 114 degrees at the outlet pipe.

Most of the ones that I see are set to 130 or 135 in order to get 120-degree water at the fixtures. In our climate, these are always installed in a garage because otherwise they'd be fighting with the heating system in the house. In the winter, the garage is always going to be close to 40 degrees and the heat pumps generally stop working and switch to resistance heat at 37 degrees. 

So are these just really stupid or am I missing something? 

[Bill Kibbel]

On 10/26/2024 at 1:30 AM, Jim Katen said:

Is it just me, or are these things really stupid? 

When these first started to appear, I had the chance to discuss them with someone involved in the development and initial trials.  At that time, all data (efficiency, sizing guides etc.) were based on these systems having a continuous flow of air at just over 67° supplied to the intake.  They're all installed in basements here, without any ducting of exterior air - regardless of the manufacturers' recommendation.

[Jim Katen]

Those basement's must be freezing cold. 

Where was the 67-degree air supposed to come from? I can see having them in the southern states, but they just don't seem to make any sense up here. 

[hausdok]

Most basements back east, at least as far as I can remember, stay generally "warm" without heat, meaning the air in them is above freezing. They're built deep enough to go below frost level and gain a certain amount of ambient ground heat. Of course, in my memory they weren't placing panels of foam insulation on the soil before pouring the floors, so I don't know if that made a big difference. Ours was built from rubblestone and, with the furnace and water heater there, plus what heat radiated off the hot water pipes, which were uninsulated, I'd putter around there all day as a kid tinkering with stuff I was working on. The furnace didn't supply any direct heat to the basement but those uninsulated heating ducts, like the pipes, radiated heat.

I'm not smart enough to comprehend all of what engineers imagine. I only know that if it works, it works, and that's good, and when it doesn't I'll declare it shit. Well, used to, in the days I was working.

[Jim Katen]

1 hour ago, hausdok said:

Most basements back east, at least as far as I can remember, stay generally "warm" without heat, meaning the air in them is above freezing. They're built deep enough to go below frost level and gain a certain amount of ambient ground heat. Of course, in my memory they weren't placing panels of foam insulation on the soil before pouring the floors, so I don't know if that made a big difference. Ours was built from rubblestone and, with the furnace and water heater there, plus what heat radiated off the hot water pipes, which were uninsulated, I'd putter around there all day as a kid tinkering with stuff I was working on. The furnace didn't supply any direct heat to the basement but those uninsulated heating ducts, like the pipes, radiated heat.

I'm not smart enough to comprehend all of what engineers imagine. I only know that if it works, it works, and that's good, and when it doesn't I'll declare it shit. Well, used to, in the days I was working.

Yes, that's my experience too. But a heat pump water heater in one of those basements will work like an air conditioner. The 67-degree basement won't be 67 degrees after one hour, let alone one day. 

[Tom Raymond]

Yes, they're stupid. They cost five times as much as a gas water heater, and three times a conventional electric one. They only work under ideal conditions, and conditions are never ideal, so they are wildly inefficient.

Albany's reckless energy policies are incentivizing heat pumps as the technology of our all electric future. Our mechanical code requires resistance heat backup for every heat pump installation because they don't work in our severe climate.

Total cost of ownership, I'll stick with the gas heater. Even factoring for a six year service life, it's more hot water, faster, for a fraction of the price.

[Chad Fabry]

They're really stupid. In the winter, they burn natural gas because they're stealing heat from the conditioned spaces. In the summer, they make basements clammy. And as Jim stated, they're also bad at their primary purpose.