Heat Pump Hot Water Heater Spiking Electricity
I have a 60 gallon State brand hybrid heat pump hot water heater model number HPX-66DHPTNE. I’ve used an Emporia Vue energy monitor to track electricity usage since mid December. The water heater is set to efficiency setting, supposedly will only use the heat pump to heat the water. However, I have noticed by tracking the energy consumption, that the usage will spike from about 0.4 kWh to about 4.2 kWh, and stay there for about 15-20 minutes, if members of the house take a shower within let’s say a 2 hour time period of each other. I’m assuming what’s happening is the heat pump can’t keep up with the demand and it’s kicking on the electric heating element even though it’s not supposed to do that with the efficiency setting. Throwing out some facts to give full information. Showers are not excessive, probably 10 minutes or less for each shower. Have a 2.5 gpm shower head. If showers are spaced out more than a few hours then I won’t see the spike. Basement gets pretty cold where it is winter in New England, as well as the air from the heat pump further chilling the space, and I have no other heat source in the basement (oil furnace turned off because I’ve been using air source heat pumps to heat the house with no head in the basement). I have only been monitoring since December so I’m not sure if this also occurs in the summer when the air temps are warmer. Does anyone else have any experience with this, if this is normal? I’d much rather eliminate the electric heating from coming on at all and deal with a longer heating recovery time. Is it possible to just disable the electric heat altogether without damaging the unit to insure it won’t kick on?
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What is the actual “pretty cold” temperature of your basement. The installation manual states, “The ambient air temperature must also be considered when installing this unit. In Efficiency Mode the air temperature needs to be above 45°F/7°C and below 120°F/48°C for heat pump operation. If the air temperature falls outside these upper and lower limits, the electrical elements will activate to meet the hot water demand and the heat pump does not operate in either Efficiency or Hybrid Mode.”. There are also rules about the size of the room, whether louvers doors are required, etc, but I’m assuming that this is installed in an adequately sized open basement with no walls. I have an inexpensive humidistat that also provides the current temperature as well as the high/low temp over the last 24 hours. It has a magnet so I stuck it to a lally column. Something like this might be useful for you.
The basement is about 25 feet by 25 feet and fully open. Has a doorway to the garage where new air can periodically enter. Just checked the basement temp and it’s around 48-50 degrees F with the heat pump running. I haven’t ever noticed the line in the manual that mentions it won’t operate the heat pump below 45 deg, but I’m assuming that’s what’s happening. There’s just not enough warm air for the heat pump to meet the increase in demand. It’s sort of an annoying issue because there’s 60 gallons of hot water. Taking 2 showers of 10 mins with 2.5 gpm shower head is 50 gallons and then the peak water use is done for the day. If there was a way to disable this without causing damage then the tank could take the next 6-8 to recover and no one would have an issue. But instead the tank is thinking oh crap the water temp is falling, time to kick on the electric heat!
I agree that what's most likely is the HP is shutting off due to minimum temperature. The minimum is set so that ice doesn't form on the coil, usually what they actually sense is the coil temperature and shut off when it drops below 32F.
So is this space conditioned? Insulated? 45F sounds low for conditioned space.
45F sound too low to me as well. Bit further north, my uninsulated basement with stone rubble foundation (well air sealed and new windows) is sitting at 66F. I don't think I've ever seen less than 60F down there even.
If the ambient room temp is consistently 48-50F, I would imagine that it should work properly. You may want to invest in a more sophisticated room temperature monitor that can monitor the temperature continuously, and place it very near the water heater so that you can see the ambient temperature that the water heater “sees.”
Also, you’re not using 50 gallons of hot water for two 10 minutes showers because the hot water is tempered with cold water, so you’re using some amount of hot water less than 50 gallons (depending on your hot water temp, your cold water temp and your desired shower temp.)
On Rheem "efficiency" setting runs the heat pump but helps with resistive heating when demand is high. Rheem has "heat pump only" which will essentially lock out the resistive heating most of the time (but during episodes of very high demand will still let it run a bit).
Maybe your unit is similar.
I've been told to go with the 80 gallon heat pumps for just this reason.
All the HP water heater are controlled by the program in their computers.
Rheem’s patent says that the element will come on should the tank fail to reach the set point in the set number on minutes. Every mode setting had I time limit even the “heat pump” mode. Also should the tank temp ever fall below some undisclosed temp the heat pump will turn off and the element will warm tank above the programed minimum temp.
Unless State had a patent on their control program you are able to read about, all we can do is guess at what rules State’s programmers decided to invent. It is unlikely they infringed on any other patents but the controls are likely to be as similar as possible.
Walta