Would you expect a central heating system to lose pressure in the cold?

[Eccentrica Gallumbits]

My central heating system and boiler were installed early 2002. I had to top the pressure up about 2 years after it went in, and again last October. And again today. I'm concerned that the intervals between top ups are decreasing from years to just a couple of months, but if it's just to do with the extreme prolonged cold, that's fair enough.

[pcolbeck]

Your lucky, I'm topping mine up every hour at the moment.

[clifftaylor]

Has it been serviced lately?? It may have a minor leak (probably in the boiler), and as there isn't much water in a flat-sized sealed system, the pressure will drop visibly if you lose a cup full.

[Eccentrica Gallumbits]

It's serviced annually, and the last service was just after I topped it up last time.

[toekneep]

I have first hand experience of two systems losing pressure in the last six months. In both cases a new pressure/expansion device (no I don't have any idea what I'm talking about) has been fitted and has cured the problem. I think the extremes in temperature just expose a developing problem.

[Diver300]

Most expansion vessels have a diaphragm with compressed air on one side and boiler water on the other.

If the compressed air leaks away (I don't know how) then your expansion vessel becomes a waste of space.

What you need to do is depressurise the system, often by opening a radiator bleed valve. Then find the filling valve on the expansion vessel, it's usually a Schrader valve. Pump the expansion vessel to 1 bar with air. Then close the bleed valve and pressurise the system with water as normal.

If there is no air in the expansion vessel, the pressure will vary more as the temperature changes. That can cause the pressure relief to let some water out when the water is hot, and the pressure is then too low when the system cools down.

[Kim]

If the compressed air leaks away (I don't know how) then your expansion vessel becomes a waste of space.

P*nct*re fairy, innit.

Need a Marathon Plus expansion vessel. 

[Greenbank]

Mine seems random. Gaps between fills have been something like:-

18 months
2 months
12 months
6 months
2 weeks
a month since last fill and pressure still ok.

Boiler serviced every 12 months.

[Gandalf]

Mine seems to exhibit similar behaviour to Greenbank's and mine is service annually.  Only takes a minute to do anyway.

[The Mechanic]

I had a similar problem with mine at the weekend.  The boiler went off and I eventually discovered that the pressure was down to almost mothing (it is an unvented system).  I topped the pressure up and it worked again.  The pressure seems to be holding up now but I do have a small intermittent leak from a radiator.  I guess that could have been the reason for the pressure loss.

[Notsototalnewbie]

Mine also seems random like Greenbank's. It is pretty new, not even two years old yet, and the guy who installed it said to expect to top it up 3 to 4 times a year which is about right. It also has an annual service.

It is only a very small system because I live in a studio flat - two full-size rads and a very tiny one in the bathroom.

[Paul]

Timely thread. Mine has suddenly needed extra topping up (3 times in a month, compared to about once a year for the last three years).

Found what I think is the problem last night: a small leak in one of the pipes from one of the radiators. Don't know why it would suddenly start to leak (extremes of temperatures maybe?), but useful to know that dropping pressure can be a symptom of something going wrong somewhere.

[Greenbank]

I've checked all of the pipes that are visible, and can't find even the most minor of leaks, but the vast majority of the pipes are below the carpets and floorboards.

[tonycollinet]

I've lived here for nearly two decades and didn't even know central heating needed topping up. It just works.

Or are the different types of system - you've got me worried now.

Do you have header tanks in the loft (vented system). If you do, then the system will top up automatically (Float valve). Sealed (unvented systems) must be repressurised periodically. They have a pipe between the mains water supply, and an inlet to the boiler pipework. Opening a couple of valves allows water from the mains into the heating system to repressurise.