Mini Split - Maintain or Recover Heat

[Kris Cook]

I have a question re: heating the shop with a mini split. It hasn't gotten cold here yet (almost 50 degrees today and mid to low 30s at night). I have been turning the heat down to 60 at night (or when I am gone) and turning up to 66 when I am in the shop. The shop proper is well insulated with 6" walls and a second floor above (that isn't fully insulated/completed) yet. The garage is adjacent to the shop and separated by a 4' wide roll-up door which isn't insulated. The garage is currently not heated.

I plan to use the wood stove when it gets really cold but for now am solely using the mini split to heat the shop. The unit I have doesn't have the "superheat" feature but is rated down to 5 degrees F.

So, my question to other users is - for heating the shop when outside temps are say 20 degrees or higher does it make more sense from a cost standpoint to keep the shop at 66 degrees all the time (maintain heat) or let the temperature drop to 60 or less at night or when away and turn the heat up when I want to use the space (recover heat).

[Tom Bain]

I don’t know which would be more efficient, but personally if I know I’m going to be back in the shop the next morning I don’t adjust the heat. I only turn it down when I know I will be away for an extended period (multiple days).

[Jack Frederick]

When they rate a unit to a particular temp they are saying that at that temp it will deliver its rated capacity. So an 18 will deliver 18 and so on. Yes as the temp drops the unit losses some efficiency, but unless at its extreme temp it is better than electric resistance heat. Manuf do publish temp/output charts for their units, but you have to dig for that info for some reason.

[Keith Outten]

I keep my shop at 70 degrees year round. There is a major benefit to conditioning air in a big room full of machines made of steel and cast iron, they hold the heat and cooling temperatures keeping the shop at a pretty constant temperature. My shop time is variable though, I work at whatever hours suits my schedule.

[Jim Becker]

I don't do setbacks on HVAC systems including the shop mini split. For the latter, at my old shop which was reasonably well insulated, I used 65º F for the standard "cold season" temperature and 75º F for the "hot season" temperature. In the "cold season", I'd get a few more degrees from lights and machines and human efforts without changing anything. In both cases, the unit barely ran and the effect on the electric bill was barely noticeable.

[Rob Luter]

I just had the HVAC replaced in my home. It's a high efficiency unit that modulates airflow and heat cool versus just cycling on and off. The fan runs on low speed all the time to circulate the air. The sales Engineer claimed that night setback of more than a couple degrees was less efficient than just letting it ride. All the thermal mass in the house is like a flywheel and the energy required to heat it back up exceeds the energy it would have taken to keep it at the original setpoint. We knock ours back about 3 degrees at bedtime year round just to make it more comfortable to sleep at night.

[Bill Dufour]

Figure I have read is a standard 3 bedroom house weighs around 10-20 tons plus furnishings. That is a lot of thermal mass. Of course a lot of that is outside the condioned space.
Bill D

[Kris Cook]

Thanks for all the replies. I was leaning towards keeping the temp up for reasons stated above (mainly maintaining heat in the thermal mass) but started second-guessing when I considered the potential heat loss to the garage and to a lesser degree to the upstairs. Both of those conditions will improve as I get the rest of the work completed so I think I will keep the heat up to at least 60 at night.

Thanks again.

[Paul F Franklin]

There are numerous studies out there that show setting back the temperature for at least 8 hours a day will save energy regardless of the heating system type (conventional furnace, heat pump, or mini-split) and regardless of heating or cooling time of year. The savings tends to be less for heat pumps and mini-splits, but its is still a savings. The savings is also less if you have a super-insulated space. The basic driver here is the difference in temperature between the conditioned space and the outside; the greater the difference, the faster heat flows from warmer to cooler. Using setback on the temperature reduces the difference in temperature and lowers the rate of heat loss or heat gain. Insulation and air sealing also reduce that rate and that's why those improvements also save energy.

[Kris Cook]

That makes sense. Thanks Paul.