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I installed a minisplit HVAC (well, HAC) system a few years ago. Very happy with it as housewide AC, mostly happy with it as heating (more efficient than the high-efficiency gas forced-hot-water system).

I've been keeping the gas FHW system "live" as backup, and because it supplies my hot water.

Question is, is there anything I should be doing to help protect that FHW system while it's mostly idle? There is some small risk of pipes freezing if not in use (happened to a friend's system even though that was in use, but that was thin copper rather than my cast-iron), but I worry that draining the radiator loops might increase corrosion, plus of course not having it available to immediately take over should the minisplit hiccup and house temp drop below 60.

Is there a recommended approach to setting a gas/FHW system to idle/backup mide, sould I just set the thermostat low and let it be, or are we all still making this up as we go?

keshlam
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  • This is not an answer but second hand information. A friend has a similar setup and worried about subzero freezing conditions. He put the system on a smart timer to run for an hour (guessing on the hour part) every night the outside temperature dropped below a certain level. I am not certain if that is smart to do, but it solved his worries about the system. The location is Seattle, so it is rare to have freezing days, just some evenings. – John Hanley Jul 17 '23 at 20:15
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    I've considered something like that. Trying to judge whether it's needed, advisable, overkill, or just plain stupid. – keshlam Jul 17 '23 at 21:04
  • Most of the heating pipes run through inner spaces, but... @MTA, I think that's an answer, or close enough to one that I'd accept it. – keshlam Jul 17 '23 at 23:29

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Without seeing your actual layout, who can guess as to freezing? You need to take temperature measurements on pipes and radiators when it's bitter cold. A non-contact IR thermometer is useful for checking cold spots that you can see, but may be hard to reach.

Meanwhile, cast iron pumps that don't run for years can seize up from rusty sediment. Same with check valves. I bought a home that had a basement heating zone that had not been run for several years, and although the pump worked, a brass Flo-Check that was soldered into the return line would not open to pass water. I ended up heating it with a torch until I got steam "bumps", and that blew the flapper open.

If this were my place, I'd use a clock thermostat and "exercise" the heating system for a few minutes once per week during heating season. It won't cost much in fuel and will provide assurance that the backup heating system will actually work when you need it.

MTA
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  • Just waiting to see if anyone else has an Even Better Idea before giving it the checkmark. – keshlam Jul 18 '23 at 01:16
  • If freezing pipes at the cold edges of the house is a concern, something more like 10-15 minutes every 3 hours is one option, and filling the loop with antifreeze is another (but you still want the weekly pump run...) – Ecnerwal Jul 18 '23 at 01:20