High Variance

Radiators, Thermostats, and Heat Momentum

We’re deep into our first winter in the new house and are just now figuring out how to properly control the indoor temperature. On the plus side, we’ve got the ingredients of a fabulous heating system. The brand-new gas-fired boiler is so much cheaper and cleaner than the oil-fueled relic that was here when we moved in. The house was built in the 1920’s and has one or two huge radiators in every room. We even have a programmable thermostat (a Lux TX500B which is nearly identical to the current Lux TX500E) so we can lower the temperature during the day when we’re at work and at night when we’re in bed.

For the last couple months, the thermostat would turn on the boiler at 6:00 am and keep it on until the house reached 68 F. The problem was that even though the boiler turned off, the radiators were still full of hot water and they would continue heating the house driving the temperature up to 71 or 72 and driving me kind of crazy. The same thing would happen in the afternoon when we got home from work.

My first thought was that there must be a setting on the thermostat to account for what I’m calling “radiator heat momentum”. There isn’t. The closest you can get is to lower the minimum amount of time the boiler is on and that doesn’t actually help in most situations. My second thought was that this was a great excuse to get a Nest–it’s the only thermostat anyone has ever described as sexy, but as far as I can tell, its AI smarts are all about saving you 10 minutes of programming. This is especially true if you have a pretty rigid schedule. And it has no special knowledge of radiators. Oh well.

The solution we’ve settled on is imperfect, but good and easy to implement with some semi-creative thermostat programming.

WEEKDAYS: In the morning, we simply set the thermostat to go to 66 and the temperature drifts up to where we want it after the boiler turns off. During the day, the house goes down to 62 and at 4:30 pm, the thermostat goes back to 67. The temperature drifts up a little higher than we’d like, but we don’t want it to drop below 67 before we go to bed and we can only have 4 different temperature settings per day. (Yes, I think this restriction is ridiculous.)

WEEKENDS: At 6:00 am, the thermostat jumps to 66 and drifts up to about 68 within the hour. At 7:00 am, the thermostat goes to 68 and that keeps the house from getting too cold during the day when we are around. We also set the Swing on the thermostat to 0.25 degrees; this means the boiler turns on fairly often for short periods of time but these little firings during the day don’t generate much momentum–it’s really just the big push in the morning and in the afternoon during the week that cause a problem. It’s possible we could raise the Swing and be a little more efficient and still not generate much heat momentum.

I’m don’t know why other folks aren’t complaining about this (since radiator heat isn’t that rare) and why the thermostats don’t handle it more transparently. My best guess is that less than mammoth radiators don’t hold their heat as well and so it’s just not an issue in houses with average radiators. Either that or the solution I’ve come up with is so obvious that no one feels the need to talk about it. My ego tells me it’s the former so I’m going with that.