Author Topic: Combi boilers - pressure and how do they work  (Read 567 times)

Combi boilers - pressure and how do they work
« on: February 23, 2021, 05:20:54 pm »
I have an Independent C30 Combi gas boiler. Had a plumber in a few weeks ago as no hot water.
First off he was unimpressed that I had the pressure up at 3 bar - he drained the system. Said that the expansion vessel was full of water. He repressurised it using a schrader valve footpump.
I was advised to keep the pressure between 1 and 1.5 bar.  Every time I drain water off using the relief valve the pressure creeps back up to 2 bar, and I have drained a lot off. Wha tis the function of that expansion vessel?

The heat exchanger plate was also replaced, I gather the old one was sludged up and that was the cause of the lack of hot water.

Re: Combi boilers - pressure and how do they work
« Reply #1 on: February 23, 2021, 05:28:50 pm »
That sounds like the filling tap is leaking into the system.

The content of the heating loop, a closed system, should be a fixed volume of water and the expansion vessel should have air in it. When the water in the heating loop is heated, it expands and that in turn compresses the air in the expansion vessel. When it cools, the water compresses again, the air in the expansion vessel expands and the pressure in the whole system should remain a fairly constant pressure. If the expansion vessel is full of water, when the heated water expands, there’s nothing to compress and if you are lucky the overpressure valve release the excess water into the wild (hopefull outside via the overflow)
Sorting my life out, one shed at a time.

Feanor

  • It's mostly downhill from here.
Re: Combi boilers - pressure and how do they work
« Reply #2 on: February 23, 2021, 05:38:09 pm »
Your expansion vessel may be goosed.
Depends what he meant by 'full of water'.
Was that the air volume was lost, but the bladder was still dry internally, in which case yes it can be pumped back up;
Or was there water coming out the Schrader valve? In which case it's goosed: water on the air side means the internal bladder is ruptured, and it must be replaced.

If the pressure is steadily rising over time even without head, then water's getting in somewhere.
As Beardy says, it could be passing at the fill loop: these are usually disconnectable, so you should be able to see any flow through the closed valves.
The only other source of water into the primary loop I can think of would be a pinhole in the secondary heat exchanger, where the Hot Water circuit gets heated from the primary circuit.
This would allow mains pressure cold water to leak into the primary circuit.
But I think that's what he has replaced.
<ETA: Like wot Diver300 said below.>

What's it for?

The heating primary loop ( the radiator circuit ) is a closed system, unlike older systems which were open vented with a header tank.
It is pressurised to 1 bar when cold.
As you heat the water, it expands.
Water is very incompressible, so this results in the pressure rising dramatically.
There is a safety pressure relief valve which will open at 3 bar, but the system should not be getting to that.

The expansion vessel is a small tank attached to the loop, containing a rubber bladder which is filled with air to around 1 bar. This will occupy most of the volume of the tank with the system cold.
As the system heats up, the water expands into the expansion vessel, compressing the air in the bladder.
Air is much more compressible that just water, so the compressibility of the whole system increases.
This means that although the pressure will rise at it heats up, it can expand into the expansion vessel; the expansion vessel limits this pressure rise to something like 1.5 to 2 bar.

Sometimes, the air volume just gets lost due to leaky valves, and all it needs is pumping up.
This should be done on a de-pressurised and cold system.
They are normally pumped up to around 1 to 1.5 bar, it will usually say on them.

However, if there's water coming out the inflation valve, the bladder has failed, the air volume is lost and the whole thing fills up with water.
At this point, it can no longer serve any useful function: there's no expansion air volume.
The whole system now has the very low compressibility of water, and your system will shoot up to 3 bar and then the relief valve will open venting pressure to outside.
When it cools, it will drop below 1 bar due to the vented water, and will need topping up again.





Re: Combi boilers - pressure and how do they work
« Reply #3 on: February 23, 2021, 06:05:40 pm »
You are supposed to disconnect the filling loop so that if the taps leak, you just get water leak out.

If the filling loop is disconnected and the pressure is still going up, it is most likely the heat exchanger within the boiler.

A boiler can't heat the domestic hot water directly, or hard water will cause limescale problems. So the boiler heats the pressurised water, and the pressurised water heats the domestic hot water via a heat exchanger. If the heat exchanger leaks, the mains water in the heat exchanger can leak into the pressurised water and raise the pressure too much.
Quote from: Kim
Paging Diver300.  Diver300 to the GSM Trimphone, please...

Re: Combi boilers - pressure and how do they work
« Reply #4 on: February 24, 2021, 11:57:57 am »
The filling loop could be the cause here. I have a metal braided pipe for the filling loop with a valve in line.
The valve has a square key with a slot in it which is horribly chewed up. I think it is sealed off firmly - I will double check.
In my system it is not possible to remove the filler loop.


Re: Combi boilers - pressure and how do they work
« Reply #5 on: February 25, 2021, 07:11:20 am »
Is the pressure high all the time or just when the heating is on?

If the latter, it could be air in the system; expanding when the water gets hot.
<i>Marmite slave</i>

Re: Combi boilers - pressure and how do they work
« Reply #6 on: February 25, 2021, 11:50:51 am »
Thanks all for the help here. Looks like the filling loop is the fault - the little slotted key on it is well chewed up and it was open slightly all the time.
I screwed it tightly shut and now have pressure of around 1 bar. I have been topping up with a little bit of water over the last 24 hours.

I guess the filling loop needs replaced, and that will need a plumber.
Seems a shame because a bit of soft metal has got chewed up - I though of welding on a short bit of rid such as a nail onto the slotted key.
I dont have a welder so maybe Araldite on a nail cross-ways to form a key.

Re: Combi boilers - pressure and how do they work
« Reply #7 on: February 25, 2021, 12:00:09 pm »
Our boiler needs a filling loop repair.  For years it wouldn't close fully and pressure would rise slowly over the week.  I would just bleed it off at the radiator in the kitchen.  Then suddenly it has stuck closed: the system has worked faultlessly for the past two winters like this but it is in the plan to get a full service and repair this year.  According to what we should expect it's only done about half the normal hours so although it's 15 years old this year we hope that there is enough life left in it to allow us to consider greener options at more competitive prices in the future.  It's pricey to go for a heat pump alternative costing at least £10k as we understand it given that a new boiler replacement fully nstalled is less than £2k.   What cost burning yet more gas ...  🤔  £8k is an awful lot of gas - well over a decades worth at current use and cost.

Re: Combi boilers - pressure and how do they work
« Reply #8 on: February 25, 2021, 01:31:31 pm »
Thanks all for the help here. Looks like the filling loop is the fault - the little slotted key on it is well chewed up and it was open slightly all the time.
I screwed it tightly shut and now have pressure of around 1 bar. I have been topping up with a little bit of water over the last 24 hours.

I guess the filling loop needs replaced, and that will need a plumber.
Seems a shame because a bit of soft metal has got chewed up - I though of welding on a short bit of rid such as a nail onto the slotted key.
I dont have a welder so maybe Araldite on a nail cross-ways to form a key.

The filling loop is a simple piece of flexible pipe with isolation valves at both ends, normally, not just one (as you should be able to remove the pipe) so actually both isolation valves have to fail - you seem to have only identified one. Pic?

Re: Combi boilers - pressure and how do they work
« Reply #9 on: February 25, 2021, 01:39:07 pm »
 Our combi boiler which is a Vaillant doesn't have a flexible pipe fill loop.

Feanor

  • It's mostly downhill from here.
Re: Combi boilers - pressure and how do they work
« Reply #10 on: February 25, 2021, 01:48:11 pm »
Some boilers don't have a traditional removable fill loop.
We have a boiler in the Edinburgh flat where the incoming mains cold-water goes to (IIRC) a 3-position valve: Off/On/Fill.

Re: Combi boilers - pressure and how do they work
« Reply #11 on: February 25, 2021, 01:49:31 pm »
Isn't the flexible pipe a legal fiction to make it not a fixed appliance or somesuch?

Feanor

  • It's mostly downhill from here.
Re: Combi boilers - pressure and how do they work
« Reply #12 on: February 25, 2021, 01:55:10 pm »
I think it was related to water regulations.
The concern being manky radiator water back-flowing into the mains, contaminating drinking water.
So the fill loop was to be detached, and it had 2 valves, and a non-return check-valve.

Presumably the regulations have changed, to allow for the boilers without a removable fill loop.

Re: Combi boilers - pressure and how do they work
« Reply #13 on: February 25, 2021, 02:59:32 pm »
Yes , it is a condition of the WRAS regulations to prevent the possibility of contaminated water getting back into the public water main.
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Re: Combi boilers - pressure and how do they work
« Reply #14 on: February 25, 2021, 04:58:20 pm »
The traditional YACF question... how do you post a picture here?

Kim

  • Timelord
Re: Combi boilers - pressure and how do they work
« Reply #15 on: February 25, 2021, 05:46:06 pm »
Put it on the web and place its URL in [img] tags.  Tedious details covered in the perennial how-to-post-a-picture threads in Office Reception.
Careful, Kim. Your sarcasm's showing...

hellymedic

  • Just do it!
Re: Combi boilers - pressure and how do they work
« Reply #16 on: February 25, 2021, 05:51:46 pm »
Put it on the web and place its URL in [img] tags.  Tedious details covered in the perennial how-to-post-a-picture threads in Office Reception.
Or in Phototalk...