Proper use of multi-split heat pump
We recently had a multi-split heat pump system installed in our home in CT (5a). We have one head upstairs, one that feeds the kitchen and living room on the main level, one that feeds a back room on the main level and a mini-split wall unit in the lower level rec room. We were a little bummed to find our energy bills after the first month didn’t really improve – our oil bill went down, but our electricity bill went up so much that it canceled out any savings. I am wondering if we are not using our system as efficiently as possible. Since it has four “zones” effectively, we have been running them all at different temperatures. In fact, the rec room wall unit is only turned on when we are down there. I am wondering if this is not the best way to use this system during the winter. The main unit in the kitchen/living room is always set at the warmest temp and never seems to quite reach that (68 degrees). Are we actually wasting energy by making it work too hard while the rest of the units are at a lower temperature (or off)?
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Replies
Chris,
Unless you also did some insulating and/or air sealing, the amount of heat your house needs will have remained the same. And In my experience, heat pumps don't make sense in terms of operational energy use unless the heat demands of the building are low (or you have cheap electricity and relatively expensive gas or oil). While the various manufacturers are making increasingly efficient units, their current COPs (efficiency) of around 3 means that your electricity rate needs to be no more than 3 times the gas or oil rate just to break even on the energy delivered to the space.
That said, if running even just one you can't get that room to 68 degrees, it sounds like maybe the refrigerant levels are off, or something else is amiss. Then yes, it would be working too hard.
Chris, a couple of questions. What are the prices for electricity and oil in your market? And, are you keeping the house warmer now than you did with the oil, or is it not as warm?
I would certainly have the installer check the equipment. It should be easy for them to check refrigerant charge and a few other things to verify that it's working correctly at each head. I would talk to the installer or the manufacturer to find out how the unit should perform under part loads.
Cramer and David,
Thank you for your responses. When you factor in all of the delivery charges and other fees from the utility company, we are paying about 16 cents per killowatt/hour for electricity. Our oil costs vary, but for this past month it was $3.89 per gallon.
I spoke to LG who makes our unit and the technician told me that the split zones works better for cooling than heating. He thought it might work more efficiently if I kept all the zones at a similar temperature instead of trying to run some zones much lower as they are still getting the same refrigerant sent to the head even if the heat output is different. I did a little test this weekend where I ran them all at the same temperature (which happened to be the higher temperature of 68 degrees). First, I was now able to get the main room up to 68 so clearly that unit was being forced to cover the slack for the other areas that were set cooler before. Second, looking at my meter, my electricity use went from about 80 kWh per day down to 61 kWh per day. Of course, that is also likely affected by the fact that we had a pretty warm weekend here in CT. I'll keep monitoring it as the temps head back down again this week.
Finally, the guy at LG told me that although the unit will work at sub-freezing temperatures, the real efficiency gains from the system are maximized in the 35-60 degree range. I still think we'll come out ahead compared to the old oil burner, but the system just won't "pay for itself" quite so quickly. I guess I can console myself in knowing that at least we're reducing our dependence on fossil fuels (as my electrical bill is paying for the use of renewable energy sources).