We have an oil-fired boiler that does both the hot water and the forced hot water baseboard heating system.

It was unusually cold and windy last night here in north-central Massachusetts. I turned down the thermostat before going to bed as usual. Apparently one or more blockages froze in the main floor loop during the time it was cooling down before the thermostat called for heat again. By morning the circulation pump was constantly running, but nothing was flowing thru the main floor loop. The upstairs loop was working fine.

Using hair dryers and a heat gun, I think all the pipes were warmed to melt any ice in them. This also happened 25 years ago, and at that time I created small pinhole taps at a few points in the loop where it is accessible in the basement. These were for relieving pressure, and to see where the ice blockages were. I opened these taps, and initially just a short burst came out. After clearing the ice blockages, a steady stream came out as expected.

However, after a little while water stopped flowing and air was getting sucked into the one tap hole left open about midway thru the loop. Now both loops seem to not have water in them, and the top floor is no longer getting heat (the loop that never froze). I can see that the circulating pump running constantly, and no water comes out of the valves when opened just above the zone valves by the boiler in the basement.

Two questions:

  1. How do I get the heat working again?

  2. How is water supposed to replace any leaked out of the heating system, or get in there in the first place? I assumed there was a one-way valve someplace that lets normal house water into the heating system, but apparently not. I looked at the spaghetti plumbing by the boiler and think I figured out what each pipe does, but I see no obvious connection from the water system to the heating system. Perhaps this is hidden inside the boiler unit?

  • Pictures of your boiler spaghetti would be helpful in helping you find the fill valve.
    – Ecnerwal
    Commented Feb 14, 2016 at 22:00
  • If you can't sort this pronto, it's time for the call to your oil heat service provider's 24 hour service before things get REALLY expensive. The service call is cheap, relatively.
    – Ecnerwal
    Commented Feb 14, 2016 at 22:28
  • @Ecner: I already did that before posting this question, but they had a large backlog and weren't sure when someone could come. Other plumbers either don't answer at all or are also backlogged. My main objective in doing it myself is to get the system up and running again before something re-freezes. The main floor was up to 50F then, but I knew it would start going down again. Commented Feb 15, 2016 at 14:43

1 Answer 1


Step zero for the future (you've figured this out by now, but let's state it loud and clear) - extreme cold is when you don't want a setback at all - I shifted my thermostat from its usual automatic to "hold temp" last night (same cold front) for that reason. You can also have a circuit installed that runs the circulator every so often regardless of call for heat as another way to prevent this.

There is normally a valve to fill the system, it will be external to the boiler, and it may well not be automatic (or there may be an automatic and a manual in series, with the manual normally shut.) There is usually but not always an "anti-backflow preventer" as part of the fill system. There may also be something that looks like a pressure regulator, with a toggle on it that can be manually manipulated.

enter image description here

In front is the overflow pipe from the relief valve, behind is the "pressure-regulator-looking" fill valve (the part that's manipulatable is just off the picture, sorry - a toggle you can lift to force some flow) and below that is a manual valve that has to be open to fill the boiler. The entry location is typical (right at the bottom of the boiler) but that's not a given.

Circulator pumps don't do well with air in the system. You will need to replace the water that's leaked out AND remove air from the system at bleeder valves (typically found on elbows in high parts of the system)

There should be a temperature/pressure gauge on the boiler. You typically want 20-30PSI showing on that.

  • 1
    This turned out to be exactly the answer. I had seen the regulator-looking thing, but didn't know what it was so left it alone. Mine also has a normal manual valve in series, which was open. I had to flip the lever up to open the valve and let water into the heating part of the system. Mine was above the boiler, but otherwise identical to your picture and description. Commented Feb 15, 2016 at 14:38
  • Good, glad that helped get you up and running. A few years back I ended up lighting the "rather dubious, came with the house, but we didn't remove it" woodstove at a different house - because there was a 3+ day winter power outage. There was much about the setup that was "wrong" (shared flue with oil, etc.); Never used it OTHER than for that long power outage, and would have preferred a more correct setup. But; it was good to have the option when the need struck, and it kept the place from freezing.
    – Ecnerwal
    Commented Feb 15, 2016 at 16:12

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