Query on Expansion Vessel re-pressurising, replacing PRV

[gwiseman]

Boiler / Problem: Worcester Bosch 35CDi MK1 combi (4 1/2 years old) / pressure drop to zero, re-fills repeatedly drop to zero over a day or so.

I've spent several days reading through messages about pressure drops through leaky PRVs, checking EV pressures / filling loop valve leaks etc. and thank all the regulars for their comments - I now know what is going on with my system, and what to check for.

Two specific questions:

(1) I got the PRV to stop leaking to outside by twisting valve head repeatedly, presumably flushing some crud away from the seat, and no water leaks by now.

In your experience, will this fix likely last, or is it simply a question of stalling replacement for a few months perhaps ?

(2) I would like to check / repressurise the EV (presuming no water drips out of Schrader valve). Posters to this forum repeatedly advise 'release pressure to zero' before doing this.

Do I need to *completely* drain the rad circuits until they are empty before doing this, or is it OK to let out sufficient water that my pressure gauge reads zero, leaving water in my system, before I get busy with my bike pump ?

Thanks for any advice.

 

[kevplumb]

1 if its not leaking leave it alone


2 isolate the boiler at the taps underneath and just drain the boiler
vessel charge approx 1 bar

 

[ChrisR]

As you pump away, remember to let the (several) litres of water come out. You may get away with WB isolators not leaking after use. Using a drain cock on the pipes is safer - though they aren't perfect!

 

[gwiseman]

Thanks kevplumb and ChrisR.

The flushing out of the PRV was only a temporary fix: it began leaking again after a few days.

I have now acquired a spanking new (correct WB) PRV from www.mjtcontrols.co.uk. and am about to fit it and repressurise the EV.

Q: Should I use PTFE on the PRV threads or just tighten with a wrench ? (From what I can see, the factory installed valve did not use any paste / tape)

Cheers,

Gerard.

 

[kevplumb]

ptfe wont do any harm

 

[gwiseman]

OK. PTFE vanished. Found some nice pipe jointing compound instead.

Kevplumb / others, I feel like a wallaby, but can you confirm precisely how I figure out what the EV air pressure should be, and when in the refilling process I should be achieving this. I ask because the boiler manual states:

"Set the system pressure by filling the system until a pressure of 1.5 bar shows on the gauge and check for water soundness. Release water from the system through the PRV until the system design pressure is obtained.

System pressure in bar = Static head +0.3

The minimum system pressure should be 1 bar"

OK, so as my house is two storey plus loft conversion, I will be OK running the system at the minimum of 1 bar.

However, the manual continues:

"The charge pressure of the expansion vessel must not be less than the static head at the vessel. The intial charge pressure of the expansion vessel is 0.5 bar. A schraider type valve is fitted to the vessel for the charge pressure to be increased if necessary."

This last paragraph clearly implies air pressure of 0.5 bar as being an 'initial norm.'

Q1: Should I repressurise the air valve on the EV to make it approximately equal to the water pressure (ie. 1 bar, possibly 1.2 - 1.3 bar) ?

Q2: Should I do this only after I have achieved system water pressure of 1 bar (as per instructions above) ie. cold, no pumps operating. ?

Thanks.

Gerard (getting colder)

 

[doitall]

push the Scharder valve and tell us if you air or water.

 

[gwiseman]

Hi, Doitall,

The EV is sound: schraider valve does not drip water. Only air comes out when we offer up the pressure gauge.

I am just trying to understand if at my 1 bar minimum system operating pressure (as per WB manual), my EV should just be pressurised to 0.5 bar (factory setting), or it should be set to something else (and why) - people in the forums often seem to ball park 1 - 1.5 bar for the EV)...

Thanks, Gerard (gas fire's on)

 

[doitall]

Hi, Doitall,

The EV is sound: schraider valve does not drip water. Only air comes out when we offer up the pressure gauge.

I am just trying to understand if at my 1 bar minimum system operating pressure (as per WB manual), my EV should just be pressurised to 0.5 bar (factory setting), or it should be set to something else (and why) - people in the forums often seem to ball park 1 - 1.5 bar for the EV)...

Thanks, Gerard (gas fire's on)

No take it up to 1bar with the system pressure at zero

 

[kevplumb]

initial charge system pressure plus the head above the boiler

if not sure go up a tad

will somebody get rid of that bl**dy xmas smiley

 

[croydoncorgi]

You need to understand how the EV works and the relationship between the EV 'preload' pressure and the Static Head of the system.

The EV consists of a steel can with a thick rubber diaphragm across the middle. Air on one side, water on the other. If water ever comes out of the Schrader valve, then the diaphragm is holed and the whole EV must be replaced.

The purpose of the EV is to provide EXTRA volume in the system to allow for expansion of the water when it heats up. Obviously, as the prssure oin the water increases, the disphragm gets pushed out of the way and compresses the air.

The pre-load pressure is set with zero pressure applied to its water-side. This can be achieved by draining (a lot of) water out of the system or (easier) by isolating and draining just the boiler. The required preload is calculated from the appled Static Head: the distance to the top of the system from wherever the boiler is fitted. Most EVs come pre-loaded to 0.5 Bar. Allow an EXTRA 0.5 Bar for each 15 feet of head. So in a typical building / boiler setup, the preload should be about 1 Bar. The trick is to allow enough EXTRA space with the system filled and pressurised to allow for some movement of the diaphragm.

If you over-pressurise the system (ie. the pressure on the gauge on the boiler is set too high) there's no capacity left in the EV and when the system heats up, the pressure will immediately go much higher.

 

[gwiseman]

Cheers to all. Brain expanding pleasantly.

How come, then, if EV pressure needs to be optimally set at approx 0.5 + static head (in my case say 5m or 0.5 bar), and low EV pressure can result in excessive system pressure when all rads on, pumps circulating, does the industry not fit air pressure gauges to the EVs, knowing the schraider valves don't hold 100% air in year in, year out...

 

[bripl]

thats why the makers recommend a service every year not just wipe the casing or blow on the burners

 

[kevplumb]

if not sure go up a tad


howay cc

 

[gwiseman]

Thanks all, especially croydoncorgi for my lesson in the black art of EV pressurisation. I am now toasty.javascript:emoticon('')

For any people following this thread trying to learn how to check / pressurise their EV, I did indeed need to dump *a lot* of water out the system to drop the pressure to zero *and to keep it at zero even whilst increasing the pressure in the EV* (the water pressure climbs as you pump otherwise !). I used the PRV to do this as it was brand new so I felt the spring / seat should be OK and it gave me a good chance to test it for soundness. I preloaded the EV to 1.2 bar and it took a fair amount of air even with one of those big boy pro bike pumps.

My system pressure now rises from 1 bar to a maximum of 1.5 bar when the rads have been running for several hours / overnight.

Do you pros think this 1.5 bar max system pressure when hot is a little too high for this boiler or is it OK ? (WB 35 CDi Mk1, 1 bar water pressure when cold / off).

If you do think this is too high, and I need to 'add a tad more' air to the EV (to quote the wise Kevplumb), is it OK to just add a couple more strokes to the EV with the system pressurised (and then just dump a bit of water to bring me down to 1 bar) ?

Or must I follow the procedure again: system pressure to zero, this time taking the EV up to, say, 1.5 bar air instead of my initial 1.2 bar ?