Baxi 80e pressure issues

[AndyMcG]

1st post so thanks for reading.

I have a 9 year old Baxi 80e driving a DHW and 6 radiator CH system. Recently I turned on the CH and noticed by chance the CH pressure gauge was at max 4Bar!!!! immediately I turned off the system and waited for the system to cool, pressure dropped. My observations were that the discharge pipe was not draining. On reading the manual i noticed that the PRV on this system should activate at 3 Bar, so surely this should have been working to reduce this over pressure in the CH system.

I also observed the pressure rose abnormally in the CH system, something to do with the Expansion vessel at the back of the boiler.

I also noticed that the CH pressure gauge fluctuated (increase slightly) when the DHW was operating. I thought the two circuits were separate so how could pressure be getting from one system to the other? Something to do with Plate Heat exchange from my research?

My thoughts on the issues are

1. The PRV has failed, it should open at 3 bar, so this needs replacing.

2. The expansion Vessel is failing, why would the CH pressure rise above normal operating pressure when at temperature.

3. The Plate Heat exchanger is somehow failing.

I have a professional coming on Friday to advise but wondered if i was on the right track ( I do like to have some knowledge of what is going on).

Many thanks for any advice

Andy

 

[MANDATE]

I have just answered '32tybeams' on a similar issue.
Reduced/lost air charge in expansion vessel results high pressure and the opening of the PRV.
The HW circuit and CH are separate up to a point.
In the CH circuit heated water can be made to go to the radiators or by use of a divirter valve it can be made to around a much shorter circuit which includes a heat exchanger which is used to transfer heat to HW.
So cold water comes into boiler and passes through the heat exchanger and comes out hot.
It's the opening of the hot tap that controls the diverter valve and so you can not have both HW and CH at the same time.
The amount of water used when the heating circuit is used for HW is limited to the smaller circuit so the water pressure does not rise as much as when CH is on

 

[AndyMcG]

thanks for the reply.

The diverter valve works when the i open a Hot water tap, the micro switch is activated by the small rod that pops out the front (return when tap is switched off and flow stops).

Can i test the pressure of the expansion vessel with a small digital tire gauge? what pressure should be in there? Thats if i can get to it, boiler is in a cupboard and limited access to the top

Do you think i have an issue with the PRV since it didn't allow any pressure to escape via the discharge outlet when the system CH gauge went past 3bar?

 

[MANDATE]

Yes you can test the expansion vessel pressure but you'll not get one of those extending pencil gauges on the valve. I use a push on type dial gauge or if using a foot pump, the foot pump has built in gauge.
The main problem is not knowing the position of the diaphragm.It is essential there is no water pressure and the water side open to atmosphere.
If air has been lost, the diaphragm gets moved towards the air side and volume of air reduced. So its not just about getting the pressure correct, it's also about making sure the diaphragm is as far as possible to the water side to give maximum air volume.
Air charge pressure about 0.8bar (10psi).
When you later pressurise the water side to 1bar it will hardly have moved the diaphragm so you will have ample air and with heating on it should only rise about another 1bar and never get near the 3bar to open the PRV
Yes I think there is a problem if the PRV has not opened at 4bar

 

[AndyMcG]

It is essential there is no water pressure and the water side open to atmosphere.

There is a drain on my hall radiator, if I attach a length of hose from this nipple to the outside drain and leave this open it should drain my CH system and release all pressure?

While i am doing this i should be able to replace the prv with http://www.akgas.co.uk/Baxi-Pressur...MBI-COMBI-HE-DUO-TEC-PLATINUM-PRV-248056.html

I believe my friend has a http://www.reginproducts.co.uk/flowandpressure/REGK30 so i should be able to recharge the EV at the same time.

Would i then need to close off the CH system and then recharge and bleed the radiators before i perform a test at full working temp and pressure?

Also as a test of the PRV should i be able to open the CH filling loop (at cold temp) and push the pressure of the CH system past 3bar, thus firing the PRV and water would then escape down the drainage pipe?

Cheers for all the responses, much appreciated.

 

[MANDATE]

Don't see any point in using radiator drain.
Options are (a) open existing PRV or (b) remove existing PRV, rectify expansion vessel and then fit new PRV.
You may want to open and close the new PRV before fitting, but I would not advise opening it once fitted by using the water pressure just in case it any sediment gets trapped on seating face. this would mean you may have corrected one problem but created another.
When filling system again, any air will be pushed out through the AAV (automatic air valve), except that trapped in radiators.
Note also bleeding radiators will allow the water pressure to drop so will need adjusting.
The key to success is noting the pressure rise and not going too near the 3bar mark.

 

[AndyMcG]

Just got home with spare PRV. I drained the existing pressure out of the system by opening the existing PRV. I then attached a hose to the drain on the hall radiator, leaving the PRV open i drained the system.

PRV replacement was a straight forward job. Detached the discharge pipe, loosened the grub screw under PRV and twist pulled the PRV out. The inside seemed rather grubby and furry. Anyway reverse installed for new PRV.

At system cold i charged up the CH system to check new PRV was working. At 3bar on Gauge water flowed out from the discharge pipe. Fingers crossed this is fitted and working correctly.

I then recharged the CH system to 1 bar, turned CH on at lowest temp 30 degrees and proceeded to bleed the radiators. Only bathroom and one upper bedroom needed bleeding.

I am now in the process of testing the CH at working temp. So far so good. the CH gauge isn't increasing as previously.

Struggling to understand why PRV replacement alone would fix this.

 

[mfarrow]

I know it's a bit late now, but if the 80e is anything like my 105e then there should be ball valves on the flow and return. Shut these off and you can then let the boiler drain via the white drain tap on the right hand side. It may save you a new bottle of inhibitor in the future.

 

[AndyMcG]

Think i have come up with an idea (in my own way) why replacing a faulty PRV would fix my issue. The PRV and EV are all working in tandom. If the PRV is broken i.e. stuck then it isn't allowing any expansion at all, so the EV is having to take all the increased pressure.

I presume the PRV moves slowly to take the increase in pressure and is only pushed to discharge once it gets to 3 bar. Until it gets to 3 bar it is still moving, just not to the point of opening to discharge.

 

[MANDATE]

Got that the wrong way round! the PRV has inside it a calibrated spring that will only move at 3bar and when it does open it snaps shut again as pressure is released.
It's the expansion vessel that absorbs the extra water (that's if there is room for it)
With vessel properly charged the whole of the vessel is 'air' at 1bar.
With heating on that 'air' gets compressed and its pressure increases.
Now the pressure it increases to depends on a few things.
Volume of water in system, Size of expansion vessel, increase in temperature of heated water. and of course the volume of 'air'.
It's this volume of 'air' where the problem arises. If the 'air' pressure has been lost or diminished, the volume is reduced when in equilibrium.
For example if volume is halved then pressure is doubled.
So what used to go to 2.5bar now goes to 5bar, well it would but for the PRV.
I take it you have not checked the 'air' charge pressure of the vessel?

 

[AndyMcG]

understood, the PRV is an all or nothing safety device opening at 3 Bar, its the pressurised EV that takes the normal operating expansion pressure. Thanks

I have run the system for a few hours now and noticed the pressure is still rising by 1 bar to 2 bar (from a 1 bar starting point). This is still an abnormal pressure increase?

I will attempt to check the EV pressure today, the valve is going to be an a$#e to get to.

update - Managed to get to the EV valve. Took a reading of 1.35 Bar on a digital meter, but i took this reading when the system was still warm (after a full heat test), therefore there would have been hot water pressure in the system pushing onto the EV. Doh.

I take it i should have take the reading when the system was cold?

update - I have read this post like I should have before //www.diynot.com/forums/viewtopic.php?t=37170.

Opened the drain cock on the hall radiator releasing the system pressure (CH gauge read 0 bar). Took reading at EV valve, 0.15 Bar. Page 7 of Baxi manual states recharge of 0.5bar so i will pump EV and test system once recharged.

 

[MANDATE]

Well you've come a fair way since Tuesday.
Just keep that 'drain off' open when pumping the air in and allow any excess water to escape. It's so important to ensure the vessel contains very little water before heating is put on.

 

[AndyMcG]

Thanks, really enjoyed learning and diagnosing the faults.

After a quick trip to the local bike shop for a bike pump and a few flex hoses (linked together for reach) I have recharged the EV, opening and closing the drain cock and rechecking the EV pressure. Noticed the Boiler gauge pressure was rising so kept releasing and drain cock and double checking EV pressure. Final EV pressure of 0.7 bar.

The whole System is now recharged from 0 bar to 0.7 bar. Been running for an hour at 80 degrees. CH Gauge rose from 0.7 to 1.25 Bar and stayed there. Looking good. Will let the system rest and cool and perform another full heat test.

So overall i have replaced a faulty PRV and recharged the EV and I appear to have a working system.

I suppose the question is how would the EV lose pressure if it wasn't failing (ready to fail)? Its a sealed unit, the valve was clean as a whistle with a cap, no debris and no water leaking from it when i tested pressure.

 

[MANDATE]

I think it's got to be the valve or how the valve secures to the vessel.
Just the same as car/bicycle tyres. they do lose pressure over time.
Checking/ adjusting the vessel 'air' charge is a requirement of the annual service and I can understand why?
Agree it's looking good 0.7 to 1.25bar could not ask for more?
The PRV you removed can be examined internally, but you have to dismantle first. you would most likely have to make a thin 'C' type spanner to get under the head when it's in the open position.

 

[AndyMcG]

Took the PRV apart. As i mentioned earlier the outside looked furred. You can see in the photo there is a crust around the internal lip. The dust to the right of the PRV is what falls off when i scrapped it with a small driver head.

Stripped PRV

So this is 9 years of build up clogged the PRV i take it.

System seems to be working fine now. Hush thou, will keep that news from the wife, try and stave off her from having the house like an oven for at least a few days

Thanks to everyone who helped and all the useful info on the forum. Great to learn something new and save a few bob.