Pressure Fluctuation in sealed CH system

15 Oct 2007
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United Kingdom
I'm hoping someone has some ideas about what is going on with my central heating system. Here is the basic setup:

Boiler = Worcester Greenstar 24i System. It's been installed and working for about 1 year. There is no by pass on it. Instead there is one fitted into the pipe work at the HW tank where the control valves are.

The heating system has an S plan valve system: One valve for the HW and another for the CH. Separate timers control each valve.

The radiator system is sealed. There are nine rads on the system, although three are off at the moment and the pipe work is capped due to work in the house. All the rads have TRVs fitted except for one where the room thermostat is.

The problem I am having is that when the boiler comes on for CH the pressure in the system increases fairly rapidly from 1 bar to about 3.5 bar. The red zone on the gauge starts at about 2.7 bar. Water in the system gets vented to the overflow port and drains away outside the house. When the system cools down there is usually no pressure shown on the gauge.

To troubleshoot I initially bled all the rads when the system was cold, all TRVs and lockshield valves fully open and the pump was off. Quite a bit of air was removed from one rad, but the others had no air.

Unfortunately this failed to solve the problem.

Next I ran the CH using only one rad at a time. This was to try and flush out any air pocket that might exist. Then bled the rads again. No air was expelled at this stage.

This also failed to solve the problem.

Today I tried just heating one or two rads on the system at a time to see what happened. In most cases the system pressure would rise from 1 to about 1.6 bar and then fluctuate a bit as the boiler cycled on and off. After two or three cycles the overall pressure would come down a bit.

The exception to this trend was seen in two rather large rads. When these were being heated the system pressure went up to about 2.5-2.7 bar. I noticed during this time that the flow pipe temp was quite hot but the return pipe temp was very cool. The boiler flame stayed on quite a while until the return pipe temp began to approach the same temp as the flow pipe. In any case, as these big rads heated up and the temps stabilised, the boiler flame came on for a shorter time during each cycle and the overall system pressure settled at about 1.8-2.0 bar.

Well, sorry to be so long winded, but I wanted to get as much info into this post as I could.

Any ideas would be appreciated.

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Have you checked if your expansion vessel needs recharging? Check the schraeder valve as well to make sure it isn't letting by
Good point. I was thinking about that. No I haven't checked it. Not sure exactly how but I'll do some research.

Forgot to mention this earlier but, should I be able to run the temperature control at any level? It's at 3 right now, but that results in not very hot HW.

Unlikely to be a faulty expansion vessel, so check the pressure.

Is this a new problem or done it since it was installed.

How big are the 9 rads, it could be the expansion vessel is undersized.
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This is not hard fact, but my recollection is that I have always had to top up the system from time to time since it was installed. Not a lot, though, like I have to do now. Like I say, I never paid attention to how much, just noticed that from time to time the starting cold pressure was less than 1 bar, so I'd add some in.

This summer, while it was only doing HW, it seemed to be fine.

Some further tests I did tonight:

I've had the cap off the valve on the expansion vessel and just quickly pushed the inner part of the valve to see what came out. It hissed briefly and no water came out.

Regarding what pressure it should have: I don't know what the resting (cold) pressure should be. And should it be measured after draining the water or can I leave water in the radiator system?

I turned down the boiler temp to 2 and it still over pressurises with all the rads on the system.

Interestingly: With only one or two of the smaller rads on then the maximum pressure only gets to about 2 bar. The two big rads are different, though. These rads are something like 10,000 or 11,000 BTU each. Having either one - or both - on the system, and none of the others, results in the pressure going up to 3 bar or more. This is with the boiler temp control set to either 2 or 3.

The four smaller rads are something like 2000 to 3000 BTU each.

There are only six on the system at the moment, the other three are not on due to work we are doing on the house.

If all six of the existing rads are on the system then it only takes a few minutes to go from slightly less than 1 bar to 3.5 bar. At this point the boiler begins to vent down the over flow pipe to the outside. The time for the pressure to rise is related to the setting on the boiler control. If it's set to 2 then it takes longer for the pressure to rise compared to if the control is set to 3.
24 i is the cowboys' favourite, combine that with the fact that you don't have a bypass on a s-plan, and you have a suspect install. Big chance your expansion vessel is either not charged or faulty.
Is the benchmark sheet filled out correctly? I would be surprised if it is.
How do you work that out, John? :confused: Or better said maybe, how do you deduct that? Out of 9 rads, only 5 are mentioned.
I can ususally guess the cowboy jobs, but don't recognise undersized installations.
Thought you meant the boiler was undersized for some reason.
It's been a long day.
doitall -

I've added up the output of the radiators currently on the system. There are six hooked up to it at the moment and they total about 38,000 Btu. The two big ones are a little over 10,000 Btu each. The three that aren't on the system right now, will add another 5000 Btu (approx - I haven't looked those up right now).

bengasman -

I'll try and find the benchmark sheet tomorrow. Too tired right now. As to the bypass - I assumed that all systems had one. Obviously not. Can you tell me where to look for what the correct charge on the expansion vessel should be?

Thanks for your help, every one.
You need to calculate the system water volume.

The rads are easy at 0.19ltr per section.

What size pipes, and what type of hot water cylinder.

Obviously you need to check the existing one first.
Bypass is normally near the pump/motorised valves.
Precharge not that critical, about one bar. I usually set mine a bit on the high side as I like to set up systems on maximum pressure (2.5 bar) Wouldn't recommend you try the same.

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