Combi Boiler Experts?

[bert]

I had to descale the Hot water heat exchanger in my worcester 350RSL last night as the hot water pressure had dropped below 3psi and would not trip the flow switch which meant no hot water.

I drained the system removed the heat exchanger and desclaed it, refitted everything and refilled to 1.5bar pressure.

Hot water pressure was back but after approx 20-30 seconds the boiler pressure went up very quickly to just below 3bar at which point I turned off the hot water and the boiler pressure dropped back to 1.5bar.

I tried the central heating and that worked perfectly with no change in pressure and all the rads worked fine.

I found that the expansion tank didnt have any pressure so i reduced the boiler pressure to zero via the Pressure release valve and pumped the expansion tank upto 1bar and then refilled the water back to 1.5 bar but I still have the issue with the pressure rising when the hot water is turned on

I thought that maybe the heat exchanger was holed so turned the boiler off and turned on the hot water tap, the water pressure in the boiler stayed the same so i dont think its at fault.

Overnight there was no change in boiler pressure.

Any ideas as to what the cause could be?

Thanks

 

[the snooper]

is the pump kicking in when the hot water is turned on if it has one for the hot water, if you run a boiler with no water flowing it only heats up what is in the heat exchanger but rapidly and the safety valve usually blows off

either way i would guess at lack of flow or a blockage

 

[octo0072000]

sounds like a air lock let the hot & cold water run for 30mins useing all the taps so if that helps

 

[bert]

I believe the DHW pump is working, I can feel it vibrate a little when running and a continuity test shows its ok.

Any other way of testing it?

Im not sure the running the hot water tap will help with an airlock in the boiler as the systems are separate.

 

[dai]

press the valve center on the expansion vessel
does water come out??
my pressure gauge was giong up n down coz the bladder was goosed

 

[upinsmoke]

re combi boiler

first thing to do is to bleed all the radiators and make shure that the system is fully pressured to about 1-1.5 bar. then try setting the boiler to hot water only. If the boiler is cold it should start up ready to produce hot water,or you could run the hot tap as this should also make the boiler fire. next see if it can do the radiators when whatever control system you have tells it to heat the the rads. if the boiler starts for a short time and stops after after a few mins, say 5 mins and is not producing heat then the pump could need replacing,but if it does hot water but not heating,or if heating but not hot water.Its the 3 port valve. Both should be easy to replace

 

[wrinx]

press the valve center on the expansion vessel
does water come out??
my pressure gauge was giong up n down coz the bladder was goosed

This...although ours didn't leak water immediately, which confused me for a few weeks!

Ended up fitting an external EV.

wrinx

 

[bert]

The expansion tank is fine no water in there.

Its got 2 pumps inside one for the CH which is working perfectly and one for the DHW which isnt. Today im going to take out the DHW one as I think the insides are goosed and not pumping enough water when on allowing the water in the gas/water exchanger get too hot producing steam hence raising the boiler internal pressure when the DHW is on and once cool to return to normal.

well at least i think it is, to say its not a normal kind of failure would be an understatement

 

[the snooper]

is there power to the pump when you run a hot tap it could be the switch not working

 

[bert]

pumps running fine, just swapped the CH and DHW pumps around and its made no difference.

 

[the snooper]

have you checked there is a flow through the heat exchanger from the pump to make sure of no blockage

 

[bert]

im getting some flow through it as i get hot water to the tap, but im beginning to think im not getting enough flow so i have ordered a new heat exhanger to give it a try.

I also ordered a new automatic air release valve incase thats also part of the problem.

if there is still not enough flow then im guessing the back plate is blocked just on the bit where the DHW goes as the CH works perfectly.

 

[the snooper]

you should have just removed the heat exchanger and tested the flow with a hose pipe

 

[bert]

well a new heat exchanger sorted it, i dont really know why as they both flowed the same!

Thanks everyone for the advice