Boiler Pressure Increasing Mystery

andrewsfoster67

Hi There,

I was hoping someone could advise on the below.

- I have a Combi Boiler (Vailant EcoFit Pure 830), roughly 2 years old
- Recently had 2 radiators replaced, pressure was fine before this
- When system is cold pressure is 1.3
- When Central Heating is on at peak temp, the system pressure goes to 2.5 to 2.6 (irrespective of whatever the cold pressure is, i.e. if i topped the system to 1.7 it still goes to 2.5, and if i bled the system to 1.1 the CH would still go to 2.5 when hot)
- If i use hot water only, then the pressure only increases by 0.1 to 0.2 bar
- If i use CH AND Hot Water the overall pressure spikes to 2.8 to 2.9, (has never gone above this so far)

-Aside from the above observation,  heating and hot water work perfectly fine 

Diagnostic performed so far:
- Had a boiler engineer perform a servicing last week
- The Expansion vessel was checked and is correctly pressurised
- The PRV is seated correctly and shows no issues
- No water leaks inside or outside the boiler
- As far as he could see the boiler was working fine, however he mentioned the increase in pressure was a bit higher than he would expect
- Filling loops are closed and show no leakage
- ALL radiators have been bled and contain no air at all and heat up fully from top to bottom

My question is whether the above pressure spike an issue? and any ideas what could be causing it?

grumbler

IMO, the expansion vessel is the only possible explanation.

ETA: is there one radiator without TVR in your house? There must be one.

Jeepers_Creepers

Hi AF.

The EV is the obvious candidate as Gumbler says. Just because one guy says he's tested it, doesn't mean it's been done properly. This needs doing with the water side of the system depressurised, or else he'll get a false reading as he'll be checking/pumping the air side against the water pressure.

No reason I can think of why swapping 2 rads should have caused this, tho'. Before these rads were swapped, what did the pressure do - just how steady was it?

It's not unusual for some increase in pressure when the system heats up, but a well set up system should keep this within, I dunno, 0.3 or so bar?

2.5 to 2.6 bar is high, far too high. It won't necessarily cause issues as long as your system is secure, but it is approaching the 'safety blow-off' pressure of ~3bar. (Your system will never show more than 3 bar, because it'll release any pressure above this...)

The filling loop  - is that disconnected at one end to be sure to be sure?

All rads are hot top to bottom and release no air, so that's pretty clear.

Struggling here - can't think of anything other than a faulty/incorrectly set up EV. The only other ways for pressure to increase (ie other than the water expanding due to heat) is (a) a let-by in the filling loop, or (b) a leaking PtoP exchanger which could let mains water get in to the system water, but this would tend to be letting-by all the time, and not just when hot.

Time to get a different plumber out, I guess...

grumbler

If it's the filling loop, the pressure would rise regardless of heating.

If it rises slowly with the temperature in the CH system, then it's the EV.

If it rises fast when the pump starts, then the pipework resistance is very high, possibly because all radiators have TVRs (not sure as I don't know at what side of the pump the gauge is)

neilmcl

grumbler said:

IMO, the expansion vessel is the only possible explanation.

ETA: is there one radiator without TVR in your house? There must be one.

Not the "only" possible explanation. My dad's Worcester Bosch combi's pressure was constantly on the rise when the heating was on and he was constantly having to drain off the system to keep it down. We too thought it was the expansion valve and had that checked as well as forking out for a new PRV, but after the third visit from a different heating guy it turned out to be a faulty filling loop which wasn't closing properly.

neilmcl

grumbler said:

If it's the filling loop, the pressure would rise regardless of heating.

If it rises slowly with the temperature in the CH system, then it's the EV.

If it rises fast when the pump starts, then the pipework resistance is very high, possibly because all radiators have TVRs (not sure as I don't know at what side of the pump the gauge is)

Not necessarily. As my post above explained my dad had a faulty filling loop but the pressure stayed fairly stable unless the CH was on.

Jeepers_Creepers

I understand that modern boilers have built in by-passes, so shouldn't have an issue even if all rads are closed.

Tbh, I cannot see the boiler's pump causing - being capable of - such a pressure rise anyway; they tend to be fast flow but low pressure devices afaIk; I doubt they are capable of a near 2bar pressure increase, but I could be wrong.

It surely has to be the EV?

Jeepers_Creepers

Andrew, time to email Vaillant - these sorts of companies are usually very good at replying helpfully. Your first post is very clear at describing the issue.

grumbler

neilmcl said:

grumbler said:

IMO, the expansion vessel is the only possible explanation.

ETA: is there one radiator without TVR in your house? There must be one.

Not the "only" possible explanation. My dad's Worcester Bosch combi's pressure was constantly on the rise when the heating was on and he was constantly having to drain off the system to keep it down. We too thought it was the expansion valve and had that checked as well as forking out for a new PRV, but after the third visit from a different heating guy it turned out to be a faulty filling loop which wasn't closing properly.

expansion vessel

The essential difference I see is that in your case the pressure "was constantly on the rise"

andrewsfoster67

Many thanks all for your quick replies! below are responses to some of the questions you asked

• I have 5 radiators with TRV's in the living room/bedrooms and 2 bathroom towel radiators (which were the ones replaced) with what look like simple angled radiator valves.

• The pressure doesn't increase when the system is cold so I'm confident it isn't a faulty filling loop.

• The pressure gradually rises when the CH is running, and reaches peak pressure of 2.5/2.6

Is there an obvious way I could tell if my boiler is releasing excess pressure? I don't hear any odd or irregular sounds when the boiler is on, and when the engineer did the servicing the PRV was fine, so not sure if the system reaching 2.8 to 2.9 when CH and Hot Water is on is just a coincidence.
I will try also shoot Vaillant an e-mail as well.

Jeepers_Creepers

To check if it's releasing excess pressure, find out where the 15mm copper discharge pipe exits your house and rubber-band a clear bag over the end.

If it was losing water through this, the pressure would drop to below what it started from when it cooled down again as there would now be less water in the sealed system.

Money is still on EV :-)