Expansion Vessel on heating circuit

[Ginga]

Hi,

Hopefully someone can help? I have a system boiler ( Ideal mini S24 ) which forms part of the unvented system I have in our house.The boiler sits in the first floor of a three storey townhouse, right next to the unvented cyclinder. There are two valves, one for the unvenetd hot water cyclinder and one for the heating circuit. There are about 8 rads on the system. I have had the system power cleaned about a year ago and also the boiler heat exhcnager replaced about 2 yeras ago.

In order for the CH flow switch to make when the heating only is called for, the cold pressure needs to be at about 1.3 bar, as shown on the boiler pressure gauge. The issue is that as the house heats up, the pressure gauge on the boiler shows about 3 bar and water starts dripping into the tundish ( as designed to do ). So when the heating shuts off, the cold pressure is down to about 0.75 bar which is not enough to bring the CH flow switch in when CH is called for. I am relucatant to keep topping up the pressure from the filling loop as this will sludge up the system eventually ( I think this is what may have clogged the heatexchanger a couple of years ago.)

My thoughts are that the water in the CH circuit is heating up, expanding and that the boilers internal expansion vessel ( 6 Litres ) is not man enough to cater for the increase in pressure due to expansion ( hence dumping out the tundish ) of water in the CH circuit

We do have a 12 litre external expansion vessel which is appear to be connected to the hot water cylinder cold inlet, so I think that this caters for increased pressure in the hot water tank only.

We don't have an external expasnion vessel on the CH Hot Water circuit. Is it normal to have an additional expansion vessel on the CH cirucit to cater for increased pressure when on CH only?

This problem seems to have been teased out due to the addition of an additional rad.I have bled all air from the system completely. So I am thinking that we now have increased volume in the system and now need an external expansion vessel on CH circuit.

If anyone can offer advice, I would be most grateful!

Best Regards

 

[HDRW]

Have you checked that the expansion vessel (in the boiler) is properly pressurised? Hopefully the boiler instructions tell you how to do this.

If it is, then you need more expansion volume and an external expansion vessel would do that, but perhaps that's not a DIY job, most people would say.

Cheers,
Howard

 

[Ginga]

Hi HDRW,
Thanks very much for the prompt reply. I am looking at the boiler expansion vessel afternoon. I'll let you know what I find. The boiler manual doesn't say anything about how you recharge the expansion vessel - only how to replace it. I'm going to take alook to see if thats possible. I've got a feeling it is factory charged and only replaceable. I'll let you know what I find.
Regards
Chris

 

[denso13]

//www.diynot.com/wiki/plumbing:faq:faq7

 

[ianmcd]

All expansion vessels are re-chargeable follow the link you have already been sent

 

[ianmcd]

All expansion vessels are re-chargeable follow the link you have already been sent

 

[Ginga]

Gentlemen,
An update
so I followed the steps in the link above, located the expansion vessel and the Schrader valve on top. I pressed the Schrader valve pin and heard air escaping, so I presumed diaphragm in vessel ok.

I then went to halfords and bought a mini digital type gauge from the car dept for eight quid.

Back at home, I isolated the CH flow and return, opened the boiler drain cock ( white fitting just behind wilo pump) and drained the boiler. I then re measured the vessel air pressure and it was almost nothing. So at this point I presumed that the expansion vessel had lost its 10psi per charge over time which would explain the large increase in pressure from cold startup pressure to operational pressure. With drain cock open, I pumped air into the vessel Schroeder valve until I had 10 psi in there, as measured with the gauge. I then de isolated the boiler, charged the system to 1 bar and fired up the boiler.

So with a cold starting pressure of 1 bar and the boiler temperature stat to max, I cranked up the room stat to 25 and the hot water tank to 60 to see what the pressure differential when system was maxed out. It floated at around 2 bar so happy days so far. At the same time I cleaned out the maga clean and in the week will top up with inhibitor once I'm convinced it's all good.

Interestingly the cold starting pressure used to have to be 1.3 bar at least to get the Flow switch in And I also had to stagger the hot water and Ch startup times otherwise the flow switch would not come in. Now , both Dhw and ch can come on at same time with only 1 bar cold starting pressure....weird but most welcome.

Thanks for you steer gents, much appreciated. I even managed to get the kids to rugby!
Thanks again!

 

[HDRW]

Thanks for the update - glad it worked out!

...
Interestingly the cold starting pressure used to have to be 1.3 bar at least to get the Flow switch in And I also had to stagger the hot water and Ch startup times otherwise the flow switch would not come in. Now , both Dhw and ch can come on at same time with only 1 bar cold starting pressure....weird but most welcome.

Expansion vessel has probably been low on pressure for a while, causing over-sensitivity to changing pressures.

Thanks for you steer gents, much appreciated.

Glad to help - I've had lots of useful advice from this place so it's good to be able to give some back.

I even managed to get the kids to rugby!

Yes, but I bet they'll find their way back again. You need to move house before they return - that's what my parents did!

Cheers, Howard