Regulating Indoor Air Temperatures with a Heat Pump
We live in zone 5A, and our winters sometimes get to -10F. With our natural gas furnace, it seems efficient to heat our leaky old (1921) house to 60F during the evenings, 50F at night, and 45F while we’re away. We sleep well under a mountain of blankets.
I’m planning to DIY install a mini split system now to condition a previously unconditioned space, and we were thinking of also putting an interior unit in each bedroom. That would let us set the baby’s room to a much higher temperature, and also have air conditioning for the few hot summer nights that we get. But I have two issues:
1. People love to tell you that heat pumps can’t handle step changes in temperature efficiently. Why is that? Is this a controller issue, where the thermostat declares a comfort emergency and the compressor goes into overdrive? Is it caused by the rate at which heat is removed from the refrigerant lines? Could I avoid overtaxing the system by scheduling the temperature step in one zone at a time, so the first bedroom would request 7kBTU from the 30k outdoor unit, then once it’s warm the second bedroom requests heat, etc?
2. LG’s thermostats are supposedly programmable for between 60F-80F. What’s a man to do when his wife wants it 50F?
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Heat pump efficiency varies pretty dramatically depending on the difference between the outside temperature and the indoor temperature. So the most efficient thing to do if you're only heating a few hours a day is to run the heat pump during the part of the day when it's warmest outside. And the least efficient thing is to run it when it's coldest outside. It may be more efficient to run more hours during the warmer part of the day than fewer hours during the coldest part of the day. Often times when people run on a schedule they ask for the most heat at the least efficient times -- for instance, asking the HP to warm the house up at 6AM.
> That would let us set the baby’s room to a much higher temperature,
Elevated sleeping temperature is a risk factor for SIDS, which is most common in babies born in the winter. Check with your pediatrician but my recollection is that babies should sleep in a room in the 60's.
Do you have a source on this? It seems extremely cold for a baby to me. I know SIDs is possibly one of the most delicate topics that exists, but my understanding of SIDs and babies born in the winter is more related to blankets and babies suffocating in them. If the air temperature was 72, this would be less of a concern.
From https://www.cuh.nhs.uk/maternity/going-home/caring-for-your-baby/reducing-risk-cot-death (N.B. 16 degrees Celsius is about 61 F, 20 is about 68 F. Nappy=diaper, vest=undershirt, babygro=one-piece pajamas, probably with feet)
In my personal experience, an undershirt in addition to pajamas almost always resulted in the baby being too warm (waking up sweaty and miserable), even on nights that seemed pretty cold to me. Baby blankets are typically very light, so if you have a heavier quilt or something, adjust.
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Do not let your baby get too hot or too cold. Ideally room temperature should be between 16 and 20 degrees centigrade. (See table below).
Room temperature Amount of bedding
14°C sheet plus three or four layers of blankets
16°C sheet plus three layers of blankets
18°C sheet plus two layers of blankets
20°C sheet plus one or two layers of blankets
22°C sheet plus one layer of blankets or sheet only
24°C sheet only
Bedding guidelines for babies wearing a nappy, vest and babygro.
This is interesting to me. It’s very different than what’s recommended in the US. Pretty much everything here recommends 68-72 F and not putting any blankets in with the baby.
Occupant health aside, there aren't too many climates where keeping an interior that cold isn't potentially a cause of moisture problems, both inside and in the building assemblies.
I would think in an old leaky building that would be less of an issue.
That's true.
"1. People love to tell you that heat pumps can’t handle step changes in temperature efficiently. Why is that? "
Most heat pumps have a resistance heating element that will come on if the thermostat setting is more than 2° above the current air temp. The resistance element costs 3 times as much to operate.
Heat pumps do heat air more slowly than furnaces. The air coming out of a heat pump will be 15-25° warmer than the air going in. So if your house is 50° your heat pump will blow 75° air at best and most furnaces will blow 130° air.
I believe heat pumps work best when you set the thermostat to one temperature 24 /7.
Very few thermostats allow low set points for fear that if the thermostat is seeing 50° the pipes in an exterior wall could easily get frozen and a jury could be convinced the manufacture should pay for the damages.
I think you should spend some quality time with a calking gun and stop up the air leaks.
Walta
Heat pumps are most efficient when they can modulate to control the temperature in a space, which basically means they are running all the time, but at varying levels of heating. A typical furnace is full on or full off, and produces a LOT of heat while it's running. In engineering we call this "PWM", for pulse width modulation. The total energy delivered is the area under the "on" times, so longer on-times means more energy goes into the space. A high amount of energy is delivered over multiple brief periods to keep the AVERAGE temperature of the space at the target temperature even though the instantaneous temperature is always wandering a little above and a little below the target.
Heat pumps like to run continuously at varying capacity levels to match the needs of the space. This means they can deliver a low amount of energy ALL the time, to keep a relatively constant average temperature with very little wandering above and below the target.
If you try to get too big of a temperature change in a short period, you exceed the capacity of the heat pump and it kicks in the electrical resistance auxiliary heat to try to get to the target temperature within a reasonable period of time. This puts you outside of the optimal efficiency range of the unit.
Temperatures below 50F are risky because that thermostat is only seeing the temperature where it's located, and it's entirely possible, even probable, that there are areas that are lower than that temperature -- especially on or inside of exterior walls. If you really want to fake out a thermostat to run things cooler, you can try putting a small incandescent (not LED) lamp under the thermostat to fake the thermostat into seeing a warmer than actual temperature. The result will be the room temperature will be kept a little lower. This is something you have to play with experimentally to get right, so it's not an optimal solution.
I agree with Walta that doing some air sealing work will probably be a big help. Heat pumps work best in well sealed spaces where they can modulate to control the temperature without big swings from air leaks in wind and the like.
Bill
Heat pumps are sensitive to large changes in the thermostat setting or sudden large changes in outside air temperature, or even possibly room temperature changes because it forces the system to use what is termed emergency heat or auxiliary heat, which can be the same thing for most heat pump systems. It is like trying to heat your house with a large 15 Amp toaster...the type where you push the toast down and see the coils glow. It is very energy inefficient and it is really there to keep hot air coming into the house temporarily while the outside heat pump runs in defrost mode, meaning it is running like an air conditioner to reject heat to the outside coils to defrost them, so it is producing cold air that those toaster oven coils have to rapidly heat. So small changes minimize those toaster oven coils from kicking on. As far as you wife wanting 50 F from a 60 F thermostat, maybe you could recalibrate your thermostat to think 50F is 60F?
> Is this a controller issue, where the thermostat declares a comfort emergency and the compressor goes into overdrive?
Let's say full power mode (vs overdrive). Which is usually less efficient than partial power mode. You can usually avoid the issue by very slowing turning up the thermostat - so there is never enough error to cause it to "declare an emergency".
Another aspect to the use of a heat pump to recover from a large temperature drop is capacity of the unit. Frankly, I don't know if the electric resistance heating coils for supplemental heat are on a separate breaker (or can be disabled) in an air-source heat pump. My own system is GSHP, and I leave the breaker for resistance backup heat off (I've needed that once in ten years, when the water valve failed). But with any heat pump, capacity typically is determined by design heat load, with little or no excess capacity. When pumping heat into a house to recover from a large drop in temperature, part of that heat is leaving steadily through the exterior shell, and only the excess is available to raise the temperature of the interior air and structure mass. Thus a heat pump, lacking the typical huge oversize of a fired system, cannot pour heat into the house at any great rate to recover temperature drop quickly, the way a fired heat system could. Bringing the house back up from a large temperature drop could take hours, longer than desired. That's why you hear the thermostat advice for heat pumps: "set and forget."
Just to be “that guy”, technically heating elements are extremely energy efficient in terms of nearly 100% of the electricity is converted into heat, they’re just not economical haha.
I still like baseboard heating elements for shoulder seasons or bedrooms where you only need a LITTLE heat.