Month: October 2019

The past few days I’ve been seeing 10-15 degree swings with an analog house thermometer and a digital house thermometer hooked up to my gas Williams vented heater in my greenhouse. This seemed pretty large. Part of the problem seems to be the thermostats don’t adjust quickly to temperature changes, e.g. are too low-pass. Could I somehow hook up something that responded more quickly to temperature swings?

So time to experiment! I did have a heat mat thermostat controller that turns a 120 V electrical plug on and off (designed to warm soil to a set temperature for germination). It has an external thermostat probe that seemed quite responsive. However, the gas furnace doesn’t turn on and off with 120 V, it’s instead a millivolt system that simply closes and opens a circuit to turn the furnace on and off. So I got an AC/DC power relay that converted a 120V electrical socket turning and off to turning my furnace on and off.

This seems to be working a lot better! Less than 5 degrees of temperature swing. This is a pretty simple thermostat so the next step is to try a programmable thermostat that has an external probe. Having it set a lower temperature at night automatically would be quite nice.

I got my new gas heater installed and it works with an external thermostat (instead of my ventless gas heater which has an internal thermostat and a knob at the top that you can make it warmer or cooler). This is cool except I’m finding out that most thermostats have a lowpass filter and only expect the temperature to fluctuate so quickly and no more. I first tried a Honeywell bi-metal completely mechanical thermostat (CT50K1028) which when I set it, has 10 degree swings:

I then tried a programable thermostat, the White Rogers 1F81-261 which came as our main house thermostat and we subsequently replaced with a wi-fi thermostat. I was hopeful a digital thermostat would have a much better response time, but when I set it to 70 F, the greenhouse hit 85 F before the thermostat registered 70 F and kicked off! That’s pretty bad! I read the manual and it doesn’t seem like there is a way to adjust that aside from setting a fast cycle and slow cycle (but I was already on the fast cycle).

I then found that I could adjust the thermostat anticipator on Honeywell mechanical thermostat and set it to the shortest setting and still saw a 10 degree swing, so that’s disappointing. While I expect this problem to become less as winter approaches because it will take longer and longer to reach the desired temperature as it gets colder and colder, I will have to continue to investigate to see if there are some better ways.