Lukewarm 'hot' water - secondary heat exchanger already changed

[Neil Sutherland]

Hi, I'm looking for help with a persistent problem. I have a Vaillant TurboMax Plus 837E combination boiler which appears to be in fine working order. However, it provides only lukewarm water to the taps and shower. To date three engineers have looked at it but failed to resolve the problem. The diverter valve appears to be working normally, closing the supply to the radiators when a tap is turned on. The boiler temperature remains stable at 60 degrees or higher. I have had the secondary heat exchanger replaced and there seemed to be an improvement immediately afterwards; but it proved short-lived. As things stand, we are able to get hot water to the en suite shower but only by opening a hot tap in the main bathroom. The tap then runs cold, but the shower is warm! It works, but is very wasteful of water. You'll understand that I'm reluctant to call yet another engineer without having some idea what is going on, so if you have any ideas for things to check I'll be most grateful. Thank you.

 

[Gasmk1]

Sis the hot water flow pipe from the boiler hot to touch, if it is your problem could be a faulty mixer tap allow cold into the hot.

 

[Neil Sutherland]

Sis the hot water flow pipe from the boiler hot to touch, if it is your problem could be a faulty mixer tap allow cold into the hot.

Thanks for the suggestion. But no, the hot side pipe on the shower mixer is cold to the touch (unless the hot tap in the other bathroom is open).

 

[Rightjob1]

Turn off all your colds to fixtures in the house and run the hot on its own..if it works fine then you have a tap that mixing tthr hot and cold

 

[Darren Jackson]

We are trying to determine whether this is a boiler problem or a backflow problem with a TMV. The hot water pipe "directly" underneath your boiler as it comes out, does this pipe get hot to the touch when a hot tap is turned on?

 

[jtsplumbing]

Possible problem boiler not going onto Max burn when hot water demand calling

 

[Neil Sutherland]

Turn off all your colds to fixtures in the house and run the hot on its own..if it works fine then you have a tap that mixing tthr hot and cold

Any hot tap on its own runs lukewarm, but I can get hot water in the en suite by running a hot tap in the main bathroom ...

 

[Neil Sutherland]

We are trying to determine whether this is a boiler problem or a backflow problem with a TMV. The hot water pipe "directly" underneath your boiler as it comes out, does this pipe get hot to the touch when a hot tap is turned on?

Yes it does; but the water isn't hot when it gets to the tap (unless a main bathroom hot tap is on at the same time. Does my head in!

 

[Neil Sutherland]

Thank you all for chipping in your ideas - we'll get there eventually. I can't rule out some oddity in the pipework; when we moved in I found the toilet in the en suite had a hot water flush! We live in an old pub that was refurbished in 1995 by a small developer and I think he did some of the plumbing work himself ...

 

[Rightjob1]

Did you turn the valves off to the tap as in turn the colds off and run the hot only..this will tell if there is a problem in a fixture of cold crossing into hot

 

[townfanjon]

Forget individual taps for now .

Just turn off the cold supply to the boiler , NO water should come out of any hot tap , if it does then you can start to try pin point which one

 

[Neil Sutherland]

Forget individual taps for now .

Just turn off the cold supply to the boiler , NO water should come out of any hot tap , if it does then you can start to try pin point which one

That sounds like a good idea. Would I be right in thinking that what I need to shut off is the valve shown in the attached picture? Seems a bit stiff; is it just a matter of turning it 90 degrees?

 

[Chalked]

That’s it.
An Allen key fits inside.

 

[Neil Sutherland]

Aha! When I turn off the cold supply to the DHW heat exchanger and open a hot tap, water flows (pretty much as it would with the water turned on). Just to check the supply was truly off, I tried using the en suite shower with the bathroom hot tap open - that's what we usually do to get a hot shower (!) - and the shower ran cold (as it should). I think we're making progress - what's next? (Can't say how much I appreciate the help.)

 

[Chalked]

You shower needs non return valves fitted to th hot and cold feeds to it. Mains cold is overcoming the hot, so you have a cold shower.

 

[Gazzzz101]

It is the flow sensor, the boiler is going into preheat and not max burn.

 

[Gazzzz101]

Sorry, just read the thread again.

 

[Darren Jackson]

As #townfanjon says, you should not get any water flowing at all from the hot taps or shower ect.. if you have isolated the cold feed to the boiler! If you do there is a crossover somewhere in the pipework. Most likley a mixer valve, mixer tap or a mixer shower that is letting cold water into the hot supply cooling the hot down at the outlets. If you have iso's fitted to your pipes under the taps, then turn off all the cold ones leaving the hot ones on. Then turn the supply back on to the boiler and see if you now get hot at the outlets. Also check that you have double check valves fitted to all the mixer taps and showers if not get some fitted. If you do get hot coming through then one at a time turn back on a cold iso to a tap then try the hot again if it stays hot you can rule that tap or shower out and it is ok. If it runs Luke warm again then leave that particular one isolated and move on to the next tap and repeat untill you have tried them all. Hopefully this will identify the crossover. If that still does not work then there is a pipework problem and someone has inadvertently piped a cold supply to your hot pipe! That will need to be found and rectified but that may involve pulling up floor boards and taking off the covers to any boxed in pipes to trace the pipework and hopefully find the problem.

 

[Neil Sutherland]

Thank you. I shall persevere and let members know what I find. I'm gradually resigning myself to having to pull up the bathroom floor to trace the pipework, but will try to pin it down a bit further before I do that!

 

[Neil Sutherland]

Progress report. Turns out isolating valves are fitted to the bath mixer taps in both bathrooms, but not to either of the showers. So I've ruled out the bath mixers, but still have suspicions about the shower mixers. These are Grohe Avensys exposed thermostatic valves (34 029) and I note from the installation manual that they have internal non-return valves. I'm thinking of replacing those (though at £18 a pair it would be cheaper just to fit check valves in the supply pipes). Worth a try before lifting floors? Also, I'm having to replace the valve mechanism in one of the showers (over £100) because it drips constantly. I wonder if that is exacerbating the problem?

 

[Neil Sutherland]

And I'm wondering now whether I should fit flow restrictors to the shower inlets - but that seems to require measuring the hold and cold pressures which I'm not equipped to do. I always assumed the pressures were similar as the hot water is essentially under mains pressure when a combination boiler is used?

 

[diptin dogshi..]

An isolating valve is not a double check valve? Did you do as suggested and turn off the cold isolator then see if the tap ran hot or did you mistake isolating valves for one way valves and eliminate them on that basis? Confused reading your progress update sorry

 

[Neil Sutherland]

An isolating valve is not a double check valve? Did you do as suggested and turn off the cold isolator then see if the tap ran hot or did you mistake isolating valves for one way valves and eliminate them on that basis? Confused reading your progress update sorry

Sorry if I wasn't clear! There are no isolating valves on the shower supply pipes (hot or cold) so I wasn't able to isolate them. More news at the weekend, I hope!

 

[diptin dogshi..]

Very confusing thread this...
Must be my eyes

 

[Vee]

Sounds like the thermistors need replacing. Had exactly this on that type of boiler.