Mini-split for winter heating
MainePaul
| Posted in Mechanicals on
My family has an insulated vacation cabin that is used in he winter ( zone 6
A). Normally it is heated via wood stove. Would it be possible (and smart ) to maintain the temperature of the cabin a 40F to 50F when not in use using a mini split heat pump. If the set pt is 50F what would be an estimate of the COP at outside temperatures of 40F, 30, 20, 10, 0, -10.
The cabin requires 9000 btus/Hr with an inside temp of 70F and outside temp of 0F. Would a floor mounted inside exchanger be better than a ceiling mounted unit? Currently when not in use the cabin is unheated.
Thanks, MainePaul
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Replies
Paul,
Most ductless minisplits have minimum thermostat settings that are higher than your goal of 40°F or 50°F. You may want to install a propane space heater instead for that reason.
That said, in many regions of the country, a ductless minisplit will cost less to operate than a propane space heater. (Cost comparisons depend on the local cost of electricity and propane, of course.)
Here are links to two relevant articles:
How To Buy a Ductless Minisplit
Rules of Thumb for Ductless Minisplits
As I wrote in one of the articles, a cold-climate Mitsubishi or Fujitsu ductless minisplit should perform at the following COPs:
a COP of about 3.2 to 3.7 at 30°F;
a COP of about 1.8 to 2.8 at 5°F;
a COP of about 1.4 at -10°F;
a COP of “a little above 1.0” at -15°F.
When this has come up before, we've come up with ways to "hack" a thermostat to make it think that it's keeping the room at 60 when it's really keeping it at 50. The way I'd do it would be to put a remote thermostat in a small low-thermal-mass box, and put a tiny electrical heater in the box too. I'd adjust the heater until the box was 10 F hotter than the room. I don't know that anybody has tried this. I'm guessing that something like a 1 W heater would do it. Tips on how to set that up safely available upon request.
Very roughly speaking, the COP of a heat pump depends on the temperature difference between the outdoors and the space being heated. So if you use it to keep the room at 40-60 F instead of 70 F, you should get significantly better COP than what Martin's table lists. This also means that the benefit of finding a way to run at 50 F instead of 60 F is not only the decreased heating load that results, but also a higher COP.
Unlike most vendors, Fujitsu's cold climate mini-splits have a "Minimum Heat" mode, which keeps it about 50F. The lowest setting in normal heating mode is 60F. See the description of Min. Heat mode in item 6, p4 of the manual.
http://www.fujitsugeneral.com/us/resources/pdf/support/downloads/(OM)9-12-15RLS3_English.pdf
Dana, that's a much better solution than my hack! I'm pleased to hear about that.
To "hack" a thermostat you could go with an industrial temperature control module http://www.alliedelec.com/omron-automation-e5cc-rx3a5m-000/70235557/ this would allow you to set your own parameters. Or use a Siemens LOGO! PLC to make a custom thermostat. Or more in the modern hacker movement an arduino could be made into a thermostat.
I very recently was informed that Mitsubishi units CAN in fact be set to heat to a 50F setpoint with the remote (no hacks required!), and that this has been true since at least the GExxNA series mini-splits!
The reason it's not obvious is that it's only possible to set it that low under the "SMART SET" mode. See page EN-8 (p.9 in PDF pagination) in the FH series user manual:
http://www.ecomfort.com/manuals/mszfhoperation.pdf
"Normally, the minimum temperature setting in HEAT mode is 61°F
(16°C). However, during SMART SET operation only, the minimum
temperature setting is 50°F (10°C)"