I just moved into a older house (1937) which has a boiler heating system (steam). When the boiler kicks on the gauge barely goes above 0 psi, and after heating the gauge will drop to about negative 5 -7 psi. It eventually settles back to 0 psi if the heat does not run for a while.

I will also add that most of the house heats up except for the room farthest from the boiler. And the water level appears correct on the sight tube.

Do I need a new gauge? And how can I get the far room warmer? Is there something else I should consider?


  • Hello, and welcome to Home Improvement. Some more info would be helpful (e.g. pictures of the boiler and gauges). And, you should probably take our tour so you'll know how best to participate here. Oct 24 '19 at 1:37
  • You said that you have a hot water system and not a steam system. Then you said that the water level appears correct in the sight tube. What do you have; steam or hot water? If you have a steam system you will have a sight tube and hot water usually wom't. Please show a picture or 2 so we will know what you have and how to answer your questions
    – d.george
    Oct 24 '19 at 13:30
  • If gauge is on "return" pipe probably it could be the effect of the pump's suction.
    – DDS
    Oct 24 '19 at 14:13
  • You are correct, I have steam. I was told hot water when I bought the house. But I just emailed the Slant/Fin company this morning with the exact model. They said steam. Oct 24 '19 at 15:22
  • Most steam boilers will run and heat using only 1 psi of steam. To give you a good reply more system information is needed.
    – d.george
    Oct 24 '19 at 19:36

Water cannot compress, only gasses like air and steam can. So if there is no steam an no air in the system, the expansion of the hot water as it pushes against the cooler pipes can be problematic. So most systems have an "expansion tank" that holds an amount of trapped air to allow for that change in pressure so that the pipes don't burst. Once everything is at the same temperature, no pressure shows. But the hot water should be flowing in the pipes, that's why you get the heat. When it turns off, the water cools faster than the pipes, so you get that negative pressure again for a while. In other words that part all sounds normal.

Heat given off into the room is based on the flow of hot water. So there is usually a circulating pump in the hot water piping system, often right next to the boiler. Do you hear that running? If not, it should be and that's your problem. Hot water could still be circulating a little bit due to natural convection, but you need the pump to make it work well. Even if it is running the pump may be bad, especially if it is old. The impellers are usually made of bronze and can corrode if your water chemistry is bad.

  • I just wrote the Slant/Fin company this morning, I have a steam boiler, not water. Sorry about the miss direction; I was told water when I bought the home. Oct 24 '19 at 15:23

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