Heating pressure fluctuating and losing pressure

[archieb13]

Hello,
The pressure in my heating system rises from 0.5 bar to over 2 bar when the system is hot. And can drop from 1 bar to 0.5 and down to zilch in half a day!
I had initially noticed that the system was losing pressure and had to top up frequently (every few days). I initially changed the PRV and subsequently the expansion vessel as it would appear the diaphragm had gone in the old one. My assumption was that the old one had gone and subsequently the system was over pressurising when hot thus releasing through the PRV etc.
However, since changing the expansion vessel the system pressure has still been dropping which would imply a leak - I have found a small leak on the valve just before the pump (expansion vessel is t'eed into system -> pump -> boiler) and plastic bag has caught about 30ml in 24 hours so definately needs sorting but why would the system be gaining so much pressure when on?
Could air be being sucked into the system and causing issues?
Would sludge in the system cause these issues?

Today I topped up this morning to 1 bar. System got hot and rose to over 2 bar. When system sw cold pressure as 0.5 bar (lost half a bar) and when switched on again rose back to over 2 bar??

I think that one question at the minute is even with low starting pressure why would the operating pressure still rise to over 2 bar?

I can't find any other leaks but by my calculations I could be losing up to half a litre per day which is a little worrying! The boiler is a potterton profile 80. The is no obvious water in the boiler and the system does appear to lose some pressure when cold which wouldn't get instantly evaporated away.

Any thoughts before I get the professionals in?

Any help much appreciated.

Thanks,
David

 

[MANDATE]

The amount of rise in pressure depends on the volume of air in the expansion vessel when heating is off.
The water side needs zero pressure and needs to be open to atmosphere when the expansion vessels 'air' charge is checked and corrected.
The 'air' pressure needs to be about 0.8 bar, this pushes the diaphragm towards the water side, so volume of air available for compression is the same as expansion vessel volume.
If this is not checked and corrected and air leaks from the air side it results in the water pushing the diaphragm back towards the air side.
You then have a bit more water in the system with some of it in the expansion vessel. You also have less air to compress and this is where most of these problems commence. If the initial volume of 'air' is halved the resulting pressure is doubled.
Under normal circumstances the pressure rise would be in the 1.0 bar or 1.5 bar, so when the PRV is discharging at 3.0 bar it's telling you there is a expansion vessel fault.
eg air pressure too low, air pressure too high, punctured diaphragm or pipe to expansion vessel blockage.
Once the PRV has been opened there is the possibility it does not re-seat correctly and you end up with small leakage.
Likewise a small leak at radiator valves will result in a pressure drop.

There are other possibilities to consider. If filling loops are not removed can mains pressure pass across to the heating side. Heat exchangers are another example where mains pressure could pass across. Pressure increases of this nature would occur without the heating being on.

 

[crashndie]

how big is your heating system ? check the prv isnt passing and the expansion vessel has the correct charge in it

 

[archieb13]

Thanks for the replies.

Just to confirm I have replaced the expansion vessel as the old one was full of water.

There are 14 radiators in the system. The replacement expansion vessel was a 18 litre charged to 1.5bar. (Which I checked)


Thanks

 

[MANDATE]

Thanks for the replies.

Just to confirm I have replaced the expansion vessel as the old one was full of water.

There are 14 radiators in the system. The replacement expansion vessel was a 18 litre charged to 1.5bar. (Which I checked)


Thanks

It would be interesting to know what 'air' charge the instructions specify? and also what water pressure when cold is specified.
With 14 radiators it could be the vessel is not big enough. You need to know the volume of water in the system, then how much extra volume is created and match to a suitable vessel.

 

[archieb13]

Thanks for the replies.

Just to confirm I have replaced the expansion vessel as the old one was full of water.

There are 14 radiators in the system. The replacement expansion vessel was a 18 litre charged to 1.5bar. (Which I checked)


Thanks

It would be interesting to know what 'air' charge the instructions specify? and also what water pressure when cold is specified.
With 14 radiators it could be the vessel is not big enough. You need to know the volume of water in the system, then how much extra volume is created and match to a suitable vessel.

I have done some sums based on a number of assumptions:

With a head of 4m (expansion vessel to tallest radiator)
System capacity 280 litres (14 radiators x 2 (2 panel) x 10 litre)
Expansion factor based on 70 degrees 0.023

This gives a sizing of 18 litres (which is what was fitted originally & replaced like for like)
And a pre charge of 0.9 bar

I never adjusted the pressure of the replacement vessel from the supplied 1.5 bar - I assumed plug n play. Can somebody confirm that I should relieve pressure to the calulated 0.9bar within the expansion vessel i.e. it is normal that they need 'tuned' to the system?

If this is a sensible thing to do I will give it a go tonight and let you know...

Thanks again,
David

 

[MANDATE]

My thoughts are, the lower the initial pressure then the lower the final pressure.
With a pressure rise of say 1 bar which I believe is to be expected, I'd prefer 1 bar rising to 2 bar than, 1.5 bar rising to 2.5 bar.
If you reduce the air charge to 0.9 bar without opening the water side to atmosphere and having zero water pressure, it will not guarantee the vessel is empty of water.
So I'd go for 0.9 bar 'air', then water at 1 bar, then observe the rise in pressure with heating on and where the pressure returns to when heating is off.