Ariston Eurocombi SX20 DHW cycling between hot and cold

[Logidex]

I know this is an old boiler. But I'm hoping that the issue I am experiencing can be resolved without resorting to replacement of boiler.

CH works fine. The problem is with DHW. Open HW tap and boiler fires up. Water at tap gets hot. Temp gauge rises to about 70C and burner goes out (as expected) but the temp drops to 50C before boiler fires up again by which time water at tap has gone cold. Then the temp rises back up to 70C before burner goes out and so on.

Had CH system power flushed. Replaced DHW thermostat which is set to max. Divertor valve is working fine (pin lifts and operates micro switches, no leakage to CH circuit). Descaled secondary heat exchanger (by pumping Furnox DS3 round DHW circuit).

So the 20 degree spread from 50C to 70C seems rather high which looks like the cause of the wild temp fluctuation at the HW tap.

Does anyone know whether this is normal or what likely cause is and fix please?

 

[Agile]

Descaled secondary heat exchanger (by pumping Furnox DS3 round DHW circuit).

So the 20 degree spread from 50C to 70C seems rather high which looks like the cause of the wild temp fluctuation at the HW tap.

If any part is blocked its likely to be the primary side of the secondary heat exchanger.

Tony

 

[DP]

Logidex get a heating engineer to check min and max burner pressure. Also check you are getting stipulated delivery of water flow at the hot tap. Get him to check burner not going out when mod coil wires are removed. Checks need a competent person in front of the boiler to ascertain why burner shutting down when heat demand still in place. You need a heating engineer, not a plumber, to get to bottom of this problem

Your description would suggest HW operation is correct as it should run forever at 70 to give you 9.5l of hot water at the tap. Especially with temperature of mains water (in Glasgow around 5 now), boiler would not even modulate.

Blocked secondary would indicate higher temperature. OP, when boiler has not been used for several hours, does the temperature gauge indicate around 20 degrees?

 

[Logidex]

Thanks for prompt replies.

Interesting point about boiler should be on constantly if HW tap is on full. Certainly would imply heat from primary circuit is not being tranferred efficiently to DHW circuit.

Due to age of boiler, it has the mini tank type heat exchanger. I thought the power flush using Sentinal X800 would've taken care of any blockages or scale in primary circuit in the secondary heat exchanger
secondary.

Could a failing pump also be a possible cause? Although CH side as I said seems to be working OK. This is a large 2 bed flat on one level BTW.

I don't really want to be spending anything substantial on a boiler of this age so if the pump is on its way out, I'd rather bite the bullet and replace the boiler. What's the most easy way to check that the pump is still up to it?

 

[Agile]

Due to age of boiler, it has the mini tank type heat exchanger. I thought the power flush using Sentinal X800 would've taken care of any blockages or scale in primary circuit in the secondary heat exchanger
secondary.

Who did a power flush and what did he say about the problem?

Few I hear of ever include the boiler in a power flush!

Tony

 

[Logidex]

I'm surprised to learn that it's normal to exclude the boiler in a power flush. I would've thought it was essential to include it unless there's a good reason for not doing so perhaps due to chemical incompatibility.

Power flush was done by gas safe plumber. He's says he did the boiler as well. But I have to admit, having spoken with him, I'm not 100% confident.

 

[DP]

If the pump was suspect, boiler would not fire for HW..

It has to be said, one must be gas safe registered, else one cannot work on gas appliances. But that does not mean Gas Safe guy or gal can fix boilers. Might be able to instal them, but repairing these is another ball game

suggest you stick with the boiler and install some controls to reduce fuel usage. Quite possibly present boiler might outlast some for sale now

 

[Logidex]

Thanks. I forgot about the flow switch.

DHW thermostat senses temp of heating circuit (rather than DHW). When DHW stat is on max, temp of heating circuit rises to 70C before burner is switched. With DHW stat on min, temp reaches about 50C before burner is switched off. Unless I'm mistaken, this to me indicated burner is operating OK and the (new) DHW thermostat is what's calling for heat (or not).

If as DP points out the pump must be OK, the only remaining cause has to be scale or partial blockage in the primary side of DHW circuit which in turn means secondary heat exchanger and/or diverter valve. Would people agree?

I have a spare diverter valve lying around. So I'll replace that and see what happens - I have nothing to lose.

 

[DP]

Logidex, The fact you can alter the primary temperature by positioning the HW stat, suggests the stat is working, BUT is the flame responding to the HW stat?

If there was scale or partial blockage, one would expect higher temperature than you see but would be wise to check input and output temperatures at the HWHE.

At this moment in time, I do not think the fault lies in the diverter, but go ahead, I could be wrong

Incidentally, if I am on the road and someone gives me way, I acknowledge the kind tester with flash of headlights or something similar. Someone holds the door open for me, I say thank you, I even thank the person at supermarket checkout for serving me (and I am paying her for the shopping). Perhaps I am old fashioned.

 

[Logidex]

Thanks for all your advice.

I did some further testing and was able to establish that when the burners shut down, all the sensors (flow switch, thermostat etc) are still calling for heat. From this I'm concluding that the problem isn't mechanical (blockage or scale) or to do with the water pressure and temp sensors. I think this leaves the PCB or something on the gas side perhaps. Either way I think it will cost easily over £100 to get fixed. I don't want to spend any more time and money on this boiler given it's age and so have decided to replace it.

 

[Agile]

If you wanted to know what is going on in your boiler you would need to have the min and max gas burner pressure checked and then to measure the temperatures at all four relevant points within the boiler!

Tony