Vaillant ecoTEC plus 937 - hot water goes cold when running a bath

14 Dec 2004
Reaction score
United Kingdom
Hi all. We have a Vaillant ecoTEC plus 937 combi boiler. It's about seven years old and was installed before we moved in. Been serviced every year and the only problem we've previously had was last summer when the hot water flow rate started to reduce - this turned out to be collapse of the plastic internal outlet pipe from the store (diagnosed with assistance from searching this forum!) - replaced and all good with showers, etc. again.

However, since we've started taking baths again this winter we've noticed that, with the bath mixer tap on full hot water flow, it only runs hot for a couple of minutes before going lukewarm / cool. This issue does not occur when running any of the two showers, or the bathroom or kitchen taps.

I've looked at the information codes on the boiler whilst running the bath: it starts with S.27, then goes through various combinations of S.24, S.20 and S.26. I've been into the diagnosis and checked the faults: one F.28, five F.22s and one F.29 ... although goodness knows how old these are, so they may have nothing to do with this.

We don't recall having this problem in previous winters so it occurs to me that, since having the plastic internal outlet pipe from the store replaced, the hot water flow is now generally greater and the boiler cannot cannot meet the demand from the bath hot water tap (perhaps the two minutes sizzling hot water is coming from the internal hot water store)? A work colleague suggested that the tap cartridge might be awry (replaced, no difference) or even a shower mixer in the house might be faulty and be causing cold water to mix with cold (no action taken on this as it seemed far fetched).

Any thoughts or possible simple things to check before we either accept it or call an engineer please? Thanks.
Sponsored Links
you are wrong to dismiss the advice regarding a thermostatic shower mixer, if you have one that could very well be the cause, close the cold water inlet to your boiler and put the shower all the way around to hot and turn it on and if water comes out that is your problem
Thank you very much for that ianmcd - I shall do that test with both showers.

I wonder ... is it also possible for a (bath) mixer tap to create the problem in the same way as you describe a shower mixer can? I have suspicions about the bath tap, despite changing the cartridge ...
Sponsored Links
Thank you again for your help ianmcd.

I thought it would be useful to update here in case others have similar issues in future ...

I followed your guidance and checked the taps and showers - no problems with any of these, so I concluded it is a boiler issue. Fortunately (perhaps), I recalled that we had "free for the first year" home emergency cover with our home insurance, so called them and they sent a heating engineer to investigate. To be honest, I wasn't hugely impressed with him as he was permanently on the phone to his office for advice. After a while the conclusion was that there was a fault with some combination of diverter value, hot water heat exchange or various sensors (pretty much what I thought based on some Googling). He arrived at this conclusion because he identified hot water going down the central heating pipe when the bath tap was running. He noted that the system water was corroded (black) and that this may have caused the problem, and may not pay for the fix because of this. Irritatingly, he left the systen pressure so low that the central heating didn't come on that evening until I returned home and refilled it via the loop, but that's by the by ...

Predicably, after a few days, the insurance company called to let me know that they wouldn't pay for the fix because of the water quality - this was a clear exlcusion in the polkicy and I was expecting it. The engineering company then called me to arrange a powerflusgh at £650 + VAT and to replace all the components mentioned at a similar cost. Not surpisngly I declined both offers!

We still have heating and hot water (albeit a slightly punctuated bath running experience) so this is not an urgent case - my plan is therefore something like this (after the colkd snap has passed!):
- Clean the Magnaclean every few days (first clean last week yielded lots of black sludge)
- Drain the heating system (with the filling loop going until the water drains clear)
- Refill, including system cleaner
- Let the system clear do its thing for the appropriate number of hours or days
- Drain the heating system again (with the filling loop going until the water drains clear)
- Refill, including inhibitor
- Book a Vaillant fixed price repair for £275

I'd welcome any comments on this approach or further advice. Thanks everyone.
Draining and refilling the system won’t shift much muck.

Chuck a dirty shirt in the sink with detergent and it won’t do much.

That’s why God invented the powerflush machine.

That being said, all Vaillant ecoTECs lunch their diverter valves regularly. One of my customers is on his 3rd in 6 years and the system water is clear.

Then theres the expansion vessels, flue seals, water sensors, pcbs, filling handles/valves, condensate pipes, knobs, the list goes on. All the 937s suffer from the pipe fault eventually, on top of the aforementioned.
Thanks simond. Based on that list of faults, if I was in any doubts of the merit of a Vaillant fixed price repair I'm not now ...

However, we did have the condensate dip pipe changed last summer (it had collapsed and disinegrate, reducing the flow of hot water), so at least it won't be that. Mind you, I suppose all the little bits of plastic will have gone somewhere ... maybe stuck in the diverter valve ...

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local