ERV Pre-heater thermostat
My wife and I live in a new house, heated by a Mitsubishi hyper-heat mini-split, with a Panasonic Intelli-balance 200 ERV. The climate is cold, climate zone 6, in central Montana. Our HVAC installer recommended putting in a Neptronic pre-heater for the ERV, to prevent it from icing up, thereby keeping it going during cold weather rather than having to go through multiple defrost cycles. This was all well and good, and we had them install the heater. However, the thermostat they installed on the heater is just a room temperature range thermostat, 55 to 80 degrees. If we have the heater turned on, even with the thermostat set at its lowest level, then effectively we’re largely heating the house with resistance electric heat, rather than the much more efficient heat pump. All we really need is to heat the outside air to a temperature that won’t ice up the ERV. Is there any reason they couldn’t replace this thermostat with one with a much lower temperature range? That is what we understood would be installed. When I questioned the technician on this, he stated that this is the correct configuration. Does anyone have recommendations for a low temperature range thermostat which would work for us?
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Replies
I know these are common out there but I never really saw the need for one. An ERV going into defrost only looses some ventilation airflow capacity when cold. There is nothing wrong with this as long as you can still get the overall ventilation capacity you need.
Also most of the BTUs from pre-heater are sent right back out so overall very little of it makes it into the house.
I'm in zone 5 and my ERV barely goes into defrost, a couple of times during a cold snap. Not something that is worth to spend energy to avoid. Modern ERV simply do not ice.
I would turn off the pre-heater and let it be.
If you have dedicated ducting for the supply, moving it to the supply side can help with wintertime comfort. Even a high efficiency ERV will supply air fair bit bellow room temperature during a cold snap, having an in-line heater there will help. If the supply is ducted to your air handler, this is not needed as your air handler can serve the function.
+ to Akos response. The Intelli-Balance 200 goes into the defrost mode at 14 degrees according to the manual.
Some of the operational charts I've seen have reductions of upwards of 66% during cold temps. Even if you had an ERV with 300% of your required capacity, none of them automatically adjust the flow rate to compensate for defrost cycles.
The room temp thermostat is definitely not correct. You want one that turns the heat on only when the incoming air is near the defrost threshold. In an ideal setup, it would only heat up enough to get the air temperature above the defrost threshold, but that's getting pretty complicated for an add-on.
I use the Johnson Controls A19ABC-24C for applications like this. It can be set to open only at temps below -10C , or 14F for that unit.
As far as energy use, the NRC found that at least in very cold temps, preheat like you are doing was actually more efficient than defrost cycles + post heat which carry a significant energy cost with respect to the building’s heating system. I have reviewed the full study..this is a synopsis: https://www.e3s-conferences.org/articles/e3sconf/abs/2021/22/e3sconf_hvac2021_11010/e3sconf_hvac2021_11010.html
So while the preheat strategy did increase direct energy use by the preheater itself, the overall building energy use was less than when compared with defrost + post heat strategies.