Ariston Euro combi: Hot water only when heating is on.

[icyged]

Hi there,
My sx20 mmfi ce has had the above problem for a while. When a hot tap is operated the switches are activated, but the boiler does not respond. Also, when using the shower, i have to run the basin tap to keep hot water flowing!

I have changed the diverter valve diaphragm 2 years ago and the PCB 1 year ago.

In the last few days, my missus went away, so i decided to do a little tinkering to solve the problem(s). I gave it a de-scale, found i needed to replace the expansion vessel, checked the diaphragm, and managed to remove and de-scale the heat exchanger. Lots of expletives were released.
To my dismay, i found nothing too wrong. A bit of scale and black sludge in the HE and loads in the EV, but the diaphragm was in perfect nick.

Other than the PCB, what else could be wrong? Cheers in advance.

 

[Agile]

It needs carefully checking by someone who likes boilers. Few like that make!

Tony

 

[DP]

Icyged, get the boiler serviced as per the manufacturer's instructions and it will be sorted. There is nothing wrong with the make of your boiler

 

[icyged]

Thanks to both of you guys. It's fixed - for now...

Agile you right about people's sentiments about the boiler! Although funnily enough I almost changed one guys opinion, when I told him the boiler was installed in '97 - he could believe it could still be running!

DP I also share your opinion, having just put the boiler back together after all my hard work, yesterday. In addition to above, I changed all the crappy inlet valves to full-bore lever and fitted a MagnaClean. Stuff that will help that new boiler.

AND.... I can say that everything seems to have improved, water gets hotter for longer and heating flow has improved

I DID get the hot water to bring the boiler on. But intermittently at first. I've noticed that although pressure in the system may be enough the run the heating (you can hear the 'click' as you fill past the limit), it wasn't enough to start the burner for HW. I say that because as the diverter valve switches get hit, you can hear all the other noises kick in, but no ignition cycle. So I filled a little more in, say 1/4 bar and, hey presto!

So in answer to both of you I WILL be getting it serviced, but I was wandering if another component could need a look at or replacement? Another thing, is it normal for the system to lose a bit of pressure after first refill?

Many thanks again, the misses will be happy

Update: after 1 day for some reason hot taps do not bring on boiler anymore. Spent all night working out why and where theres insufficient flow on my shower arrangement to make the valve hit the switches, with lots of dismantling. Perhaps one for a new post. Anyway too tired to give good answers to your Qs, will be back 2morrow on that, but thanks.

 

[Agile]

The flow round the boiler is a different route on HW compared with CH.

You should pressurise to about 1.5 Bar.

It sounds as if you were puting in a lot less!

Tony

 

[DP]

OP, when you run hot water, can you hear the fan running?

If not sure about that, run the boiler for central heating and listen and observe the sequence.

 

[icyged]

Ok i've looked into it a little more. From cold, opening a hot tap causes the CH pump to come on but not the fan. This occurs as the 3 switches atop the diverter valve are closed. I say from cold, because the last time i tried after the CH had been on and gone off, ie the boiler was still hot, the hot tap brought on the boiler as it should.
I operated the switches without turning a tap on, and the boiler lights up! Infact, the left-hand switch of the 3 does it alone.
Next thing i'm gonna check are the the switches atop another diaphragm housing nearer the CH pump. A thin pipe runs to it from CH. I think this tells the the boiler that the pump is running, and to allow the ignition cycle to start. The contacts could be corroded or there could be a blockage in the pipe. If nothings wrong there then at least it wont hurt. Got to be careful with mains though.
Oh yeah, there's some other gizmo with wires running from it on a CH pipe coming out of the diverter. Could this be at fault i wonder?

 

[icyged]

Ok problem identified and solved to a satisfactory level.

The 'gizmo' I last mentioned is the pump pressure switch. It had nothing to do with the problem. There was also nothing wrong with the other diaphragm assembly either. I found it clear and virtually spotless, including the thin pipes running to and from it.
BUT this is where I found the problem. On top of the diaphragm is a micro switch, the Main Circuit Flow Switch. This detects flow present in the main heating loop. When DHW is required, the diverter valve hits the switches and turns on the pump. Flow in the loop is detected and the micro switch allows the boiler fan to come on, then ignition. Obviously this is also the case for CH.
In my boiler that switch wouldn't make from cold for DHW - for whatever reason! Perhaps the travel of the diaphragm increases when the system gets hot...? One thing I can understand is that the push on the switch is much firmer when the boiler is operating in CH mode. The flow through the heating system is higher than through the DHW calorifier.
It may well be that my efforts to de-sludge the thing may not have been good enough. However I did get a lot of crap out of it, and by the time it went back water ran free and clean through it.
I doctored the switch by bending the armature just a tad, so that it would make. Phew it worked.

 

[playnotwork]

hi icyged.
recently found this thread and it appears to be the same problem that is developing on my 1998 euro combi.
Hot water is at the moment intermittent once the boiler has warmed up and then will only fire once the chw has lit the flame.
Have raised pressure in the system and opened the cold water supply fully.
Not sure if this problem began when i changed the diaphragm in the diver tor valve,
really posting to see if you found a permanent solution to the problem.

Cheers Martin

 

[Gasguru]

He modified a safety device ie. he bent the microswitch arm a little to get the boiler to fire up in DHW mode. So it's not necessarily safe anymore.

This is a very common problem on this particular boiler.

The differential pressure switch is "checking" there is sufficient water flow around the primary circuit by detecting the diffential pressure across the pump and primary heat exchanger.

If the switch doesn't operate there will be a problem with either (or all) the following....

Weak pump, clogged impeller, partially blockages in the pipes and heat exchangers, partial blcokages at the washered connections, weak or perferated diff pressure diaghragm, scaled up pin and gland etc etc

So it can result in a hefty labour bill and maybe a pump to sort out. Often though at this age the boiler get scrapped.