Vaillant Ecotec Pro 28 - Hot water goes cooler on full flow

[Searcher2]

I had a Vaillant Ecotec Pro 28 fitted in 2016 and have had it serviced each year by the heating engineer who fitted it. This year I noticed that when the hot water tap is on full the temperature is nowhere near the 55C the boiler is set at. I rang the heating engineer and he said it was likely that the incoming cold water was so cold the boiler couldn't heat it up enough and to reduce the flow on the tap. Reducing the flow to about half did indeed result in hot water ask expected. However, this has continued and I don't think it did it in previous years in the winter.

I have just tested my mum's older EcoTech Plus boiler and that didn't have the same issue... however, my sister's Ecotec Pro 28 does seem to have the same issue and my sister has had to set the hot water temperature to 65 so that their shower is warm enough.

My heating engineer is due to service the boiler this week so I wanted to check if this sounds like an issue or is it just due to 'cold incoming water'? If it is an issue what is the likely cause? The boiler is under warranty.

 

[Deleted member 267285]

The incoming cold water can have an effect on this, however it could be a few things,including the diversion of the water.

 

[Searcher2]

I am reviving this topic as I have been able to do a few tests since the lifting travel restrictions in England...

The heating engineer serviced the boiler and said there was nothing wrong. It was the temperature of the water coming in cf power of the boiler that was the issue. This seemed reasonable as my sister's Ecotec produced similar results, though I have never noticed this issue in previous years which were not noticeably colder. However, I discovered that my sister's boiler which had a similar problem has an problem with the diverter valve as hot water was escaping down the central heating output pipe when a hot tap was run (with no central heating on). I supposed that my boiler may have the same issue. I have just been up to the property with my boiler in it and there is no such leakage when then hot water is run so it looks like the diverter valve is fine.

Now the hot water is hot enough for baths etc. when run but I am questioning whether it is as hot as it should be. It's no problem at this time of year but if there it an issue it will manifest itself again in winter.

The boiler hot water temp is set at 65C

The temperature of the water coming in is 11C (according to my temperature probe - which may not be wholly accurate)

The temperature of the hot water at 6 litres per minute is 57C

The temperature of the hot water at 10 litres per minute falls to 50C

The display temperature on the boiler when hot water is running varies a bit from 74-78C

I would have thought that at 6 litres per minute it should be able to reach the set 65C.... or do these figures seem okay for the size of boiler?

 

[picasso]

Combis are normally rated in litres per minute at 35c rise, so a 28kw combi giving about 11 lpm would heat incoming cold at 10c to 45c or you can slow the flow to give a higher temp rise, your boiler is giving 10lpm at a 40c rise so is about right.