No heat on a cold night is the call we get most between December and March. A fair number of them turn out to be something the homeowner could have sorted in five minutes, and we would honestly rather tell you that on the phone than charge you to come and press a button.
So here is the order worth going through. None of it requires tools and none of it involves opening anything up.
First, the thing that is not the boiler
Check the thermostat. Obvious, and it is still the answer often enough to be worth saying. Is it actually calling for heat — set above the current room temperature, in the right mode? If it takes batteries, are they dead? A thermostat with flat batteries does not always announce it; some just go blank, and a blank thermostat is easily read as a dead boiler.
If your thermostat is in a hallway, near a draught, in direct sun or above a heat source, it may also be reading a temperature no one in the house is experiencing. That is not a fault so much as a placement problem, and it is fixable.
Second, the power and the switch
Most heating systems have a switch that looks exactly like a light switch, usually at the top of the basement stairs or beside the boiler itself. It is very often red. It is also very often turned off by somebody who assumed it was a light.
Check that, and check the breaker for the boiler in the panel while you are down there.
Third, the pressure gauge
If you have a hot water system — radiators or baseboard that run warm rather than scalding, no hissing — there will be a pressure gauge on or near the boiler.
Most residential systems sit somewhere around the low end of the gauge when cold. If yours is reading at or near zero, the system has lost water, and most boilers will refuse to fire rather than run dry.
That is worth knowing but it is not worth just topping up and forgetting. Water does not leave a closed heating system on its own. If the pressure has dropped, it went somewhere, and finding where matters more than refilling it. A system that needs topping up regularly has a leak, and a small leak inside a system does considerably more damage over a winter than one that shows itself on a floor.
Fourth, if it is steam, look at the sight glass
A steam system has a small glass tube on the side of the boiler with a water level in it. If that level is very low or you cannot see one at all, the low-water cutoff has almost certainly shut the boiler down deliberately — which is the safety device doing precisely its job.
Do not pour water into a hot steam boiler to try to fix this. Adding cold water to a hot, low boiler is one of the few genuinely dangerous things a homeowner can do to a heating system. Let it cool, and call somebody.
If none of that does it
Then it is a real fault, and the common candidates are a failed circulator, a stuck zone valve, an ignition or thermocouple problem, or a control that has given up. Those all need somebody with a meter and a bit of experience.
When you do call, the things that help us most:
- Whether you have radiators, baseboard or warm air out of vents
- Whether it is the whole house or one zone or one room
- What the pressure gauge reads, if there is one
- Any code or blinking light on the boiler itself
- The make and model off the label, which is usually on the front or the side
A photo of the boiler and its label, sent by text, is worth several minutes of description. It very often lets us turn up with the right part in the van instead of looking at it, going away, and coming back.
The one to stop reading and act on
If you can smell gas, do not check anything else, do not switch anything on or off, and do not call us first. Leave the building and call your gas utility from outside. They come out for this and they do not charge for it. That is not a plumbing call — it is a gas call, and the order matters.
Worth doing before next winter
The cheapest heating money anyone spends is on a service before the season rather than a repair during it. A system that has been looked at in the autumn is a system that is still running in February, and everything on the list above is easier to notice when nobody is cold and cross.