Ariston Eurocombi SX20 -

[woodburyboy]

The central heating works fine. The domestic hat water does not. The only way to get dhw is to fire up the ch and then turn on a hot tap. I've just replaced the diverter valve according to advise given to me on another boiler forum site but...no joy

Supplementry info:
1 - Extractor fan starts up when a hot tap is turned on
2 - pressure is 1.5 bar
3 - rising pin on left hand diapraghm rises on ch but doesn't on dhw. If I close the microswitch with the tip of a screwdriver, then the boiler will fire up
4 - the two small dia pipes leading to the diapraghm mentioned in point 3 are clear.

Any advice would stop me going over the edge !!!

Thanks

Mark

 

[weargas]

it's a nasty habit that sx 20's have. you can't tell whether it's a
1. blocked/partially restricted main heat exchanger
2. blocked/partially restricted dhw heat ex
3. pump not quite turning at the correct speed.

the symptoms you describe could be any, i've had them all and there's no way to tell which one it is. The best i can say is it's most likely to be the pump. Before all of those though you need to be sure the pipe coming from the right hand side of the main heat exchanger and the manifold that connects it to the diverter haven't got any blockages.

 

[woodburyboy]

Thanks for the advice. I'll go with the main heat exchanger first since that is going to be easier to remove than the secondary exchanger. Is it just a matter of filling it with the correct Fernox, plugging the ends and leaving it to stand?

Why the pump? That works fine on the central heating

 

[weargas]

Why the pump? That works fine on the central heating

when the diverter valve is in the ch position you have the weight (for want of a better word) of the ch system (unless you live in a bungalow in which case ignore my advice) that will lift the primary flow switch. When the diverter valve is in dhw mode it cuts off the weight of the rad system and relies purely on the flow of the pump to lift the switch.

like i said my money's on the pump.

 

[woodburyboy]

No...not in a bungalow...in a house...and what you say about head of water on ch makes sense. Hmmmm sounds £££££££ !!!

 

[ChrisR]

I had precisely this situation on the same boiler recently and was talked though it (thanks you know who you are!) It appears to be the back pressure from the ch system which keeps things going when hw stops. (This was a single storey!)

I don't follow how a blocked HW heat exchanger would cause the problem - if I remember correctly the main(primary) flow switch senses the circuit pressure before the dhwhe so a blocked one would put the pressure UP not down.
Whether it 's original or not I'm not sure, but there was a washer on one of the dhwhe connections with a reduced centre, to increase flow resistance that way. It was helping, but not enough.

Nothing was furred up but the system had been passing on the prv so the water was darkening. Local boiler breaker tells me the pump -Wilo Gold (funny sort of gold colour at apinch I suppose) - wears out v rapidly in dirty water.

You see the back pressure difference if you watch the pressure gauge - it'll go up a bit more for CH than for HW. Removing said reducing-washer made indicated pressure rise even less on HW.

The bearing in the old pump was very worn, and a new pump solved the problem. Was also shown a slow pump (same) with no sign of wear though...

Pump comes with air vent, and a pump head off a normal wilo 50 will not fit.

 

[woodburyboy]

Thanks for talking the time to give all the info Chris.

Do you remember the approx cost of the Wilo Gold-V pump with air valve? Is there a good on-line supplier? Would you beleive that HRPC ( a national outlet I believe) wanted £110 for a diverter valve whearas ezypart.co.uk only wanted £35 !! The genuine Italian article. Trouble is ezypart don't do the Wilo pumps.

 

[ChrisR]

Yes I know about the prices - HRPC ones are "retail" and everybody else shows a discount. Some supply chain short-circuiting goes on too. To build a boiler from spares would cost thousands! Like in any trade, after a while we get to know where to buy things, with varying prices and hassle...
Try all the suppliers Google comes up with for Boiler Spares - I expect you'll pay about £80, check the exact boiler GC number.

 

[weargas]

ariston part number for the pump : 570590

 

[darkskiez]

What did you do to fix this issue in the end?

 

[woodburyboy]

I replaced the pump (as I was told ) and that cured the problem. Been running fine ever since.....

 

[woodburyboy]

5.5 yrs on and I think it may be a pump problem again, not sure, so i thought I would check on here.

Central heating is fine. DHW starts off by gaining heat, shortly followed by going cold. Replaced pump 5.5 yrs ago so I guess it's had a good innings? Any thoughts anyone? Boiler must be about 14 yrs old now. Where is the cheapest place to buy the pump - Ariston part number for the pump : 570590