Heating system pressure going up

fafonso · 19-02-2020 12:19am #1

Hello everyone,

So I have an interesting behaviour going on with my central heating system... My boiler pressure goes up, I have no idea why, and I can't find an expert that can explain me what is going on.

This is what is happening:
1) I vent my radiators and pressure goes down to 1.5 bars at cold, they usually have a very small amount of air inside.
2) Boiler works for 2 - 3 days, and pressure starts to go up to 1.6, 1.7 bars at cold
3) I've noticed that sometimes, when working for 1 hour, the boiler display shows a pressure warning, blinking at 3.1bars
4) If the boiler doesn't work, pressure doesn't go up, and I've double-checked my filling loop valve, to make sure that water is not passing through.

Any help, idea or suggestion would be much appreciated

Thank you!

John.G · 19-02-2020 12:27am

Is the filling loop disconnected.??
Have you a unvented hot water cylinder?.

fafonso · 19-02-2020 12:38am

Hey John,

Thank you so much for your quick reply!

The filling loop is not disconnected, but I've checked that the tap is closed, and I observed that if I stop using the boiler for a few days, the pressure doesn't go up.

To be honest, I'm not sure about your second question. How can I check that?

John.G · 19-02-2020 12:48am

If the filling loop isn't passing and you havn't a hole in the (unvented) hot water cylinder coil then you must have a defective expansion vessel which has either lost its pre charge or is full of water due to a ruptured diaphragm and will have to checked out.

If your house does not have a cold water storage tank in the attic then you will have a unvented hot water cylinder pressurised normally by a pump.
You might also of course have a combi boiler with no hot water cylinder.

fafonso · 19-02-2020 1:13am

Hum ok, that's really helpful! Let me just try to make sure I'm getting it right.

I have a cold water tank in the attic and a hot water cylinder in the hot press. My pump is in the attic as well, and it works for both hot and cold water. This means that I don't have a unvented hot water cylinder, right?

In that case, it leaves us with either "a defective expansion vessel which has either lost its pre charge or is full of water due to a ruptured diaphragm and will have to checked out."

Is this correct?

Thank you so much for all your help John!

John.G · 19-02-2020 8:45am

Because you have a pump then you could have a unvented hot water cylinder and IF the coil is split/holed (unlikely) then because the water in the cylinder is at a higher pressure than the boiler system pressure; the boiler pressure should rise but this should happen with a cold boiler as well so probably unlikely it's the coil. If you do have a unvented cylinder you should see a (normally) white E.vessel close to the hot water cylinder. This has nothing to do with the boiler E.vessel.
A defective E.vessel should result in the pressure rising to > 3.0 bar each time the boiler heats and it should eject ~ 1.5/2 litres of water through its pressure reducing valve (PRV) each time resulting in the necessity to recharge the system each time. This is not happening so I'm still a bit suspicious of the filling loop, if installed correctly, there should be a isol valve at each end of the loop, if so, ensure both closed and remove one end of the filling loop, normally a flexible pipe.
Assuming a gas boiler then once you have ruled out the filling loop, you will have to get a registered gas installer (RGI) to check out the E.vessel which is inside the boiler casing if you have a system gas boiler.

fafonso · 19-02-2020 11:39pm

Hey John,

Thank you so much for all this info, loads to process

I'm only aware of 1 isolation valve. I will double-check again that it's not letting water go through and take it from there.

Once again, thank you so much for your help!

John.G · 20-02-2020 2:13am

You or the expert will have to start somewhere, I would start at the above, by removing it.

froshtyv · 21-02-2020 11:19pm

Here is another theory for you to test,

If you type in "Bleeding a Radiator of Hydrogen Gas" into youtube you will see a build up of Hydrogen gas in a heating system.

I have seen it on several jobs,

Happens when you don't have inhibitor in your system and the water reacts with pipe work rads etc..

Do you find that you are constantly bleeding air from your rads?

If its not the above, then you will most likely need to replace your heating expansion vessel.

fafonso · 22-02-2020 10:25am

Hey Froshtyv,

Thank you so much for your message, I was not aware of that one! it's impressive!

Is that supposed to happen in new houses as well? I'm bleeding them once in a while, but the amount of air inside the system is not that much. I will keep that in mind as well!

I still feal it's something with the boiler. I saw it again yesterday, boiler at cold was 1.6bars, after heating the house for 1 hour, the boiler was flashing at 3.2bars. I think this was not the case before.

As the boiler is still under warranty, I may ask the guys to check the expansion vessel first.

Once again, thank you so much for your help guys! It's very much appreciated.

John.G · 22-02-2020 10:55am

Can you just drop the pressure, preferably when cold, by say 0.3 bar (open a rad vent and watch the pressure falling) to give 1.2/1.3 bar when cold, if it stays consistently at this pressure each time it cools down and is < 2.8 bar when hot, then I wouldn't be too worried overall, it might just mean that your E.vessel is a bit on the small side but if the pressure was ~ only 2.0 bar when hot previously then it would point to the E.vessel pre charge pressure dropping over time.
I am a bit surprised though that the boiler PRV isn't discharging with a pressure of > 3.0 bar.

froshtyv · 22-02-2020 1:36pm

fafonso wrote: »

Hey Froshtyv,

Thank you so much for your message, I was not aware of that one! it's impressive!

Is that supposed to happen in new houses as well? I'm bleeding them once in a while, but the amount of air inside the system is not that much. I will keep that in mind as well!

I still feal it's something with the boiler. I saw it again yesterday, boiler at cold was 1.6bars, after heating the house for 1 hour, the boiler was flashing at 3.2bars. I think this was not the case before.

As the boiler is still under warranty, I may ask the guys to check the expansion vessel first.

Once again, thank you so much for your help guys! It's very much appreciated.

I have seen it in new houses, mostly with radiators because the metal in radiator reacts with the water, not as much of a problem with underfloor heating pipes because they are a multilayer pipe with plastic inside.

fafonso · 03-03-2020 1:46am

Hey guys,

Sorry, I was travelling last week, but I've done again the test. Dropped the pressure to 1.4bars at cold, and now, after 1 week, the pressure is back up again to 1.7bars at cold, and when working, it's getting up to 3.2 bars.

I am a bit surprised though that the boiler PRV isn't discharging with a pressure of > 3.0 bar.

This is a really good point, no idea about what is going on!

John.G · 03-03-2020 9:18am

You will have to remove that filling loop to ensure that it's not passing.
I know you need to post a minimum number of times before you can put a photo up but can you take a few shots of your hot water cylinder and your pump, someone on here will tell you how to post them despite the above.

Wearb · 03-03-2020 10:01am

John.G wrote: »

You will have to remove that filling loop to ensure that it's not passing.
I know you need to post a minimum number of times before you can put a photo up but can you take a few shots of your hot water cylinder and your pump, someone on here will tell you how to post them despite the above.

Re above: Upload pics to somewhere like dropbox etc. and pm the links to me.

fafonso · 05-03-2020 4:48pm

Images on the way

Here they are. https://drive.google.com/drive/folders/1vPe8v05iXH5d_o_zY0viyvxl7MnFtOJv

John.G · 05-03-2020 7:53pm

If you look at the stainless flexible filling hose you should see another black isolation valve (hidden but at the bottom of the picture, shut this (as well as the one at the end of the filling loop, at the top, they will either be a 1/4 turn clockwise to shut valve or may take 3 or 4 turns to close but this will be self evident, just remove one end of the flexible filling hose but just slacken it off first to make sure there is no pressure, drop the pressure back down by 0.2/0.4 bar to give you ~ 1.3/1.4 bar when cold and watch the pressure then for a day or so and post back.
It would also be helpful if you can count the number of double/single rads and the details of your boiler as we can then probably find the capacity of its internal E.vessel. I presume there isn't a red one around somewhere, externally?.

fafonso · 06-03-2020 12:06am

Hey John,

Ok, I will try to do that test as well. But on that, and just for my understanding, in the picture that I have there with two red arrows, those two valves are closed, do you think that it's still possible that water is passing through, even having two closed valves?

It's interesting that now it's kinda stable around 1.6 bar at cold, but it's still going up to 3.1 / 3.2 bar after working for 1 hour. Basically, it went from 1.4 to 1.6 in one week, and it's stable at 1.6 bars for almost 1 week now... I don't get this lol

It would also be helpful if you can count the number of double/single rads

12 rads

and the details of your boiler as we can then probably find the capacity of its internal E.vessel.

It's a Glow worm, home 25s. I saw in the docs that the pre-charge pressure is < 0.075 MPa (0.75 bar).

I presume there isn't a red one around somewhere, externally?.

I'm not aware of any other tap/valve connected to the filling loop.

John.G · 06-03-2020 9:45am

Just remove one end of the filling loop with both valves closed, then that rules that out definitely, its should be removed anyway even with all systems normal.

The very slight rise in cold pressure could be air ingress or gas due to corrosion (unlikely IMO) when you vent just the air/gas from the system if the pressure returns to normal then you will know that this is the problem.

Your boiler probably has a 8 litre E.vessel so under normal circumstances ie 0.75 bar pre charge pressure and fill pressure of 1.5 bar I would expect a hot pressure of ~ 2.5 bar with 12 rads but if the pre charge pressure has fallen, for whatever reason, to 0.3/0.4 bar then the final hot pressure will be ~ 2.8/3 bar.

Also, to check if the boiler pressure is false, shut off the pressure regulator by means of the black knob on the bottom (near the left hand red arrow) then open both filling isolating valves and see if the local pressure indication rises and falls between 1.6 and 3 bar with cold/hot boiler. When test complete re shut both isolating valves, re open regulator black knob and remove filling loop.

06-03-2020 10:56am

If it’s heat orientated then the system is not dealing with expansion

If the increase is as soon as the boiler comes on or in response to zones closing then it could be circulation issues.

For it to be the filling loop the increase will be seen constantly irrespective of the above.

If it is the expansion vessel then that’s usually a symptom not a cause and the quality of the system water should be checked but saying that if it is the expansion vessel then a recharge will most likely put it to bed for you.

It is possible to get incorrect system pressure readings which can at times be the impact of contamination on the boiler.

John.G · 06-03-2020 12:13pm

Also see my edit, above.

fafonso · 08-03-2020 11:35pm

Hey guys,

Thank you so much for your messages!

The very slight rise in cold pressure could be air ingress or gas due to corrosion (unlikely IMO) when you vent just the air/gas from the system if the pressure returns to normal then you will know that this is the problem.

This is usually what happens, I vent the rads, and the pressure at cold goes down to what it was before. What could lead to air ingress?

Your boiler probably has a 8 litre E.vessel so under normal circumstances ie 0.75 bar pre charge pressure and fill pressure of 1.5 bar I would expect a hot pressure of ~ 2.5 bar with 12 rads but if the pre charge pressure has fallen, for whatever reason, to 0.3/0.4 bar then the final hot pressure will be ~ 2.8/3 bar.

This is exactly what I'm observing. Before, it would go from 1.5 to 2.5 / 2.6, and now is going from 1.5 to 3.0 / 3.1

Thank you so much gary71, really good tips!

I'm not sure I got this one though: "If it’s heat orientated then the system is not dealing with expansion"

I'm doing some more tests and I'll also try to get a boiler warranty call to check the expansion vessel!

Thanks a million and I will update here as soon as I have news

09-03-2020 12:21am

fafonso wrote: »

Hey guys,

Thank you so much for your messages!

This is usually what happens, I vent the rads, and the pressure at cold goes down to what it was before. What could lead to air ingress?

This is exactly what I'm observing. Before, it would go from 1.5 to 2.5 / 2.6, and now is going from 1.5 to 3.0 / 3.1

Thank you so much gary71, really good tips!

I'm not sure I got this one though: “If it’s heat orientated then the system is not dealing with expansion"

I'm doing some more tests and I'll also try to get a boiler warranty call to check the expansion vessel!

Thanks a million and I will update here as soon as I have news

Your system pressure reading can increase from the reading you have at cold as you put heat into the heating system.

The more heat the more potential movement, your expansion vessel is there to absorb the increase in pressure as the temperature increases, if the expansion vessel is defective or not sized correctly there is greater movement shown on the gauge directly related to the increase in temperature.

John.G · 09-03-2020 11:56am

fafonso wrote: »

Hey guys,

Thank you so much for your messages!

This is usually what happens, I vent the rads, and the pressure at cold goes down to what it was before. What could lead to air ingress?

This is exactly what I'm observing. Before, it would go from 1.5 to 2.5 / 2.6, and now is going from 1.5 to 3.0 / 3.1

Thank you so much gary71, really good tips!

I'm not sure I got this one though: "If it’s heat orientated then the system is not dealing with expansion"

I'm doing some more tests and I'll also try to get a boiler warranty call to check the expansion vessel!

Thanks a million and I will update here as soon as I have news

Apparently air can enter a pressurised system, if you just google this it will explain it far better than I can. As suggested above, it could also be gases released due to corrosion but assuming that the system has inhibitor added to the correct strength the one would think, unlikely, there are test kits available I think which also check for the PH value of water, also important.
You may end up just having to get a external E.vessel fitted, your plumber can do this, if this is ~ 12/18 litre capacity (the larger, the better) then it doesn't matter if the internal E.vessel is losing its pre charge pressure or is of marginal capacity for your system or whatever.

fafonso · 09-03-2020 10:21pm

Good stuff! Once again, thank you so much for your help

fafonso · 10-03-2020 9:24pm

So, while I wait for the plumber to come, I did some more tests...

In the last couple of days, the pressure didn't drop more than 1.7bars at cold and I decided to vent my rads again: Only air came out of the system and the pressure dropped to 1.1bars

My theory is that as the e. vessel is not working properly, the pressure increased above 3.0bars when working hot, and the security valve is discharging some water.

What I still don't know is why I'm getting this much air in the system... could it be the malfunction e. vessel?

John.G · 11-03-2020 9:00pm

It is very easy to establish if the PRV is discharging water, just tape or tie wrap a very small plastic bag to the end of the discharge pipe and if you find water in it or it has ruptured then you know the PRV has lifted. You might find this discharge pipe turned into the wall adjacent to and outside the boiler but you should have no problem getting the bag on it.
A defective E.vessel will not directly cause air ingress but indirectly will, if the system has to be topped up frequently due to the PRV discharging regularly/constantly, air will then be released from the raw water make up each time the boiler heats up.

Other reasons for air is to check for any automatic air vents (AAVs), if fitted, these are small brass bottle shaped devices and have a small screw cap on their top, just unscrew this cap(s) a few turns to release any air and then reclose when finished.

fafonso · 11-03-2020 11:14pm

Good idea! thanks a million John

Zane97 · 25-03-2020 9:43pm

If his expansion vessel is defective in the gas boiler, can it be left like that and just fit a new one on the line some where? Eg in the hot press a the cylinder?

John.G · 26-03-2020 12:57pm

Yes, external EV connected to the return, it would/will be interesting to see what the "plastic bag" test shows up.

Zane97 · 26-03-2020 1:17pm

John.G wrote: »

Yes, external EV connected to the return, it would/will be interesting to see what the "plastic bag" test shows up.

It can’t go on the Flow no on a sealed system?

Heating system pressure going up

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