Boiler problem
Boiler pressure too high or too low?
A gauge in the red, pressure that keeps dropping, or a relief valve going off. What it usually means on a hot water boiler, and what to do.
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Low pressure
Pressure that keeps falling usually means water is leaving the system: a leak, a relief valve weeping or a faulty fill valve. Angi puts restoring boiler pressure at about $200 to $1,000 depending on the cause.[source]
High pressure
Pressure that climbs as the boiler heats often points to a waterlogged or failed expansion tank or a fill valve letting in too much water. Angi lists expansion tank replacement at $200 to $550 including labor.[source] If the gauge is in the red or the relief valve is discharging, turn the boiler off and call.
Steam boilers are different
Steam systems run at very low pressure and read on a different scale. If you have steam heat, the water level in the sight glass matters more day to day.
What normal looks like on a hot water boiler
A hot water boiler has a combination gauge, sometimes called a tridicator, that shows pressure in psi and temperature. Pressure rises as the water heats and expands, and the expansion tank absorbs that rise. Most residential relief valves are set to open at 30 psi. A reading that stays well below that and changes a little between cold and hot is what you want.
Write down two readings: one cold, after the boiler has been off for a while, and one hot, after it has been running. A big jump between them points to the expansion tank. A slow drop over days points to a leak.
The expansion tank
Most newer systems use a diaphragm expansion tank, a small tank (often blue, gray or white) with an air side and a water side separated by a rubber diaphragm. When the diaphragm fails or the air charge leaks out, the tank fills with water and can no longer absorb expansion. Pressure then spikes when the boiler heats, and the relief valve drips.
Older homes may have a plain steel tank hung from the basement ceiling. Those tanks are supposed to hold an air cushion at the top. Over time they fill with water, with the same result. Draining and recharging them is a technician job on most systems.
The fill valve
The pressure reducing or fill valve feeds city water into the system to keep it at the set pressure. If it fails open, it overfills the system and pushes pressure up. If it fails closed, a system that is slowly losing water drops toward zero and stops heating properly. On a boiler with a backflow preventer next to the fill valve, water dripping from the backflow device is another sign of trouble.
Steam pressure is a different scale
Steam boilers usually have a low pressure gauge reading 0 to about 30 psi, but they are meant to run at a small fraction of that. Many residential steam systems are designed to work at under 2 psi. A pressuretrol on the boiler shuts the burner off when pressure reaches its setting.
If a steam boiler reaches high pressure, the system often gets noisy and uneven. Clogged pressuretrol pigtails (the looped pipe under the control) are a common cause of false readings. If your steam boiler's safety valve ever opens, turn the boiler off and call. That valve opening means something is seriously wrong.
Low pressure and no heat upstairs
On a hot water system, low pressure means there may not be enough pressure to push water to the top floor. Second floor radiators go cold first. Before you call, check for wet spots near the boiler, at radiators and along pipes. Bleeding a radiator on a system with low pressure can make things worse by letting more water out.
Older two and three story houses in Utica, Clinton and Herkimer, many from before 1940, are the ones where upper floors go cold first when pressure drops.[source][source][source]
What to tell the company
Give the cold and hot pressure readings, whether the relief valve has dripped, whether you see water anywhere, and whether upper floors are cold. Say whether you have added water to the system yourself and when. For steam, give the sight glass level and the gauge reading while running.
Things not to do
Do not cap, plug or remove the relief valve or its discharge pipe. Do not keep opening the fill valve to bring pressure up if it keeps dropping; that adds fresh water and oxygen and hides a leak. Do not drain a steam boiler while it is hot.
Frequently asked
Why does pressure rise when the boiler is on? Heated water expands. A working expansion tank keeps the rise small. A big rise points to the tank.Why does my relief valve drip only sometimes? It opens when pressure reaches its setting, often during long runs on cold days. Note when it drips.Leaks that lower pressure
If a hot water system keeps losing pressure, water is escaping. Check the relief valve discharge pipe for drips or a wet floor below it, the base of the boiler, circulator flanges, radiator valves and any baseboard on outside walls. Look for white crust or green stains on fittings, which show slow leaks that evaporate.
Buried or hidden pipes, such as those under a slab or inside walls, can leak with no visible sign. A technician can isolate zones and watch the gauge to narrow it down. Finding a slow leak early protects the boiler, because each refill brings in fresh water and oxygen that corrode the inside of the system.
What normal looks like
Write down your gauge reading when the system is cold and again after an hour of running on a cold day. Keep those numbers near the boiler. Next time something seems wrong, you and the technician have a baseline to compare against, which makes diagnosis faster.
What to have ready when you call
- Your address, so the company can tell you if you are in their area.
- Boiler type and fuel if you know it: steam or hot water, gas or oil. A sight glass with water in it usually means steam.
- What it is doing, plus any code on the display or the reading on the pressure or temperature gauge.
Not sure about any of it? Call anyway. The company will ask the right questions.
Common questions
What should my boiler pressure be?
Check the label or manual for your boiler. The company you reach can tell you the right range for your system.
Can I add water myself?
On some hot water systems, yes, through the fill valve. If you are not sure, call. Never add cold water to a hot, empty steam boiler.
Still not sure what is wrong?
Call and describe it. A local heating company will take it from there.
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