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My house has gas-powered hot-water radiators for heat. Ever since we moved in, I've noticed that if the temperature is just a bit chilly outside (40°F to 50°F), the house feels very cold and everyone needs to put on sweaters and sweatshirts.

But if it gets very cold (17°F or 30°F), then the house feels perfectly comfortable.

What could be going on here? I have a programmable thermostat, so I never change the settings. Depending on the time of day, it's set to betweeen 67°F and 70°F.

  • Do you also have a heat pump? – mikeazo Dec 8 '16 at 13:05
  • @mikeazo Not that I know of, but I'm not sure. What is that and how would I determine if I have it? – Bill Dec 8 '16 at 13:06
  • Do you have an air conditioner? My house has a heat pump that runs one way in the summer (to make the house cool, so it really is my AC). In the winter, it runs another way to act as a heater. It is only efficient down to a certain temperature, then my actual heater (electric in my case) kicks on. The heat from the heat pump isn't really very hot. So it can actually feel cool, but it is heating the house. When the real heater kicks on, it warms up a lot. – mikeazo Dec 8 '16 at 13:09
  • Another possibility is that when it is just a little chilly outside, the house doesn't cool as fast so the radiator doesn't turn on as often. Making it feel cooler. When it is really cold outside, the house cools off faster, meaning the radiator turns on more. – mikeazo Dec 8 '16 at 13:11
  • @mikeazo I do have an A/C system, but it's a mini-split system so it's completely separate from the heating system and not connected to the central thermostat. – Bill Dec 8 '16 at 13:23
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It probably has to do with two things: Perception and mixing.

When it's very cold the radiators are on more often. You feel the heat coming off them, which makes it feel warmer in the home even though the average room temperature is about the same.

Also, when the radiators run less often in warmer weather, parts of the house (or each room) cool off faster than others. This results in cool pockets that you notice, if only subconsciously. Because the radiators aren't in use, causing convection and mixing, more temperature variation may occur.

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The first thing you need to do is keep a thermometer near you. see what temperature the house actually is at in each of the scenarios you mention. Next, see if you can get a hygrometer (humidity measurement tool). The RH can strongly affect your sensation of cool or warmth.
And you didn't specify, so make sure you do these tests when the house temperature has stabilized. Otherwise you might be reporting comfort levels while the system(s) is still running to adjust the temperature.

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Is your house a 1 floor ranch style house or a multiple floor or 2 story house? Are there radiators in every room? Check the pressure gauge reading when the boiler water is cool/cold, record that pressure. Does the pressure rise when the boiler is on and the water temp is rising. The pressure should rise only a few pounds between cold and hot. If the pressure rises more than this you probably need a service tech to look at the system. Possible problems; expansion tank flooded, auto fill not working, system not balanced, outdoor reset control not working or not set properly if you have one. Does the system have a pump to move the water in the system or is a gravity flow system.

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I finally asked my plumber about this, and he explained the problem and how to solve it. Our house has cast-iron radiators, so when it's very cold out, the boiler runs for a while and the radiators get very hot and continue to radiate warmth into the room. But when it's just a little cold out, the boiler doesn't need to run as long, so the radiators don't build up a lot of heat.

He said that when this happens, just set the thermostat up a few degrees higher than what we actually want. This will force the boiler to run longer and make the radiators heat up so the cast iron continues to radiate heat out into the room. Once the temperature is reached, you can lower the thermostat back to its original temperature and the house should remain comfortable.

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