Combi boiler pressure gauge question

JayneG

Hello,

Wonder if anyone can help me?

Is it normal for the pressure gauge on our central heating system to go up and down?

Sometimes, usually when the heating is not on, it goes down as low as zero, but increases up to around 0.5-1.5 of it's own accord when the heating is on. Am I right in thinking that if the pressure is able to increase without us manually intevening, then there is unlikely to be a problem/leak/etc, and is this normal boiler behaviour?

Thanks in advance :beer:

robowen

should be 1.25-1.5 when cold depending on the boilers own specs. could rise to about 2-2.5 when hot..again depending on boiler specs.
you probably need to put some more water in the system then go bleed the radiators of air.

ormus

they all vary slightly in the specs. mine has the typical minimum of 1 bar.
it cuts out if it falls below that.

roger196

When water is heated it expands, hence the pressure increases. If you want to know more about this, google "coefficient of expansion of water". When the boiler is cold, the pressure should be 1.5 bar ( some models will be 1.0 bar you need to check the instructions which can be obtained from the manufacurers website). If the pressure falls below 0.8 bar ( some models 0.5 bar), the boiler will cease to function until the pressure is topped up. This is normally done by a filling loop (a flexible braided pipe) which should be disconneted after topping up to prevent the possibility of the CH water contaminating the drinking water. A few models use a detachable key, check the instructions again.

fred7777

Jayne
It's perfectly normal for the water pressure to go up & down. When the heating is on the hot water will expand which raises the pressure. On a modern sealed system there is an expansion vessel which allows the water to expand as the pressure increase because it is hotter without causing leaks.
If there was a leak pressure would decrease. You can use the pressure gauge to check if there is a leak over time. If there is a leak the gauge will read lower each time the boiler is on and this will show up over time.

JayneG

Thanks for the replies - the thing that is confusing me is that the boiler is not ceasing to operate even when it falls to zero, as the heating still comes on later in the day in accordance with the preset settings.

roger196

Could be faulty pressure gauge. Would suggest replacement at the next annual service. This would be as cheap as removing gauge, setting up some test rig to check it and then having to order replacement gauge if test failed.

Yogibear

I find this very interesting we got our boiler serviced 4 weeks ago and the guy left the pressure on the red,it gradually fell to zero where it is now it didnt come on this morning and we are sitting freezing with no hot water either the council said we will put you down as an emergency as you are pensioners I thought good,but an emergency is 24 hours and nobody after 1700 we rang at 0900 and it`s now 1530:eek:

gas4you

Not an Ideal combi by any chance? They don't have a safety cut out for low pressure.

Yogibear, look for a filling loop under the boiler. As the chap left it in the red, the PRV has obviously operated when the system got hot and let the water out, dropping the pressure.

JayneG

gas4you wrote: »

Not an Ideal combi by any chance? They don't have a safety cut out for low pressure.

No it's a Gloworm Ultracom 18hxi - seeing as the heating's working in all other respects, I'm hoping it's just the pressure gauge that's faulty and nothing more serious (we've had weeks of trouble with the heating in the recent past and thought it was all behind us!)

gas4you

The pressure gauge must be somewhere other than the boiler then, perhaps the airing cupboard? Look in there for a filling loop.

These boilers are primarily designed for an open vent system, hence why they do not know when the system pressure has dropped.