The pressure of your solar water heater drops

A pressurised solar water heater is closed and under pressure: the fluid in its pipes, a mix of water and glycol, needs that pressure to circulate properly and not boil. When the pressure drops, the system works less well or stops working altogether.

If you have a drain-back system, this doesn't apply: it is filled with water and works without a set pressure. See How a solar water heater works to find out which type you have.

What is normal#

The pump station has a pressure gauge. When the system was installed, the installer set the pressure and often wrote it down, on the station or in the manual. How high it should be depends on how far the collector sits above the station. When the system is cold, it is often between 1.5 and 3 bar.

Some change is normal. When the fluid warms up, it expands and the pressure rises. On a sunny afternoon the pressure is higher than in the morning. So always compare at the same moment: preferably early in the morning, when everything is cold.

Why the pressure drops#

• Air escaping. After filling or maintenance, air keeps escaping for a while. The pressure then drops a little in the first weeks. That happens once.
• A small leak. At a fitting, on the roof or at the collector. Sometimes you see a stain or a crust around a joint.
• The safety valve. When the system gets too hot, this valve releases fluid to limit the pressure. There is often a container underneath. If there is fluid in it, the valve has opened.
• The expansion vessel. The round tank that absorbs changes in pressure can lose its pre-charge after some years. The pressure then jumps up and down with the temperature and drops gradually.

What you can check yourself#

1. Read the pressure on a cold morning and write it down, a few days in a row.
2. Look under the safety valve for fluid in the container.
3. Look along the pipes and fittings you can see for stains or drops.

Don't top it up yourself. A pressurised system must be filled with the right fluid and then bled. With plain water added, the glycol protects less against frost, and in a hard frost the collector can break. Don't open taps or bleed valves either: the fluid can be hot.

When to call an installer#

• The pressure has clearly dropped over a few weeks.
• There is fluid under the safety valve, or you see a leak.
• The pressure swings much more than it used to.

The installer finds the leak, checks the expansion vessel and tops up with the right fluid. Have the glycol tested at the same time: after some years it protects less against frost and can turn acidic, and then it must be replaced.

How to see it sooner#

A pressure drop is slow. That is why you often only notice it when the system has been yielding less for a while. With the BoilerBuddy Alpo you see per day whether the collector still delivers heat to the tank, so you see a problem coming.