DIY Doctor

Main navigation

Lukewarm Hot Water - Hole in HWC Coil or Faulty 3 Way Valve?

Postby Diy3r » Wed Dec 10, 2014 9:27 am

Hi guys and gals,

After a bit of advice please. We have a very odd ch system which was in the house when we moved in. Basically you cannot control the HW and CH separately, if you want one you have to have the other. In summer months we just turn off rads from the valves.

This morning the HW was only lukewarm (its usually very hot) but the rads were hot as normal, hence I don't think the issue is the boiler. My thinking is either a split in the coil in the HWC or perhaps an issue with the 3 way valve which is next to the HWC? We have a HWC in the airing cupboard in small bedroom, the CWS tank in the loft with the F&E tank next to it. The CWS tank and the F&E tank sit on the same platform, the bottom of both tanks are at the same height, however the CWS is a much bigger tank so the height of the water level is a lot higher than the water level in F&E tank which is small (hope that makes sense).

Is there a surefire way to test whether there is a split in the coil?

Appreciate any advice / knowledge - many thanks (I know the ideal scenario is to upgrade to either a combi or upgrade the current system but I hope to move house early next year so doesn't make sense financially).

Thank you
Rank: Apprentice
Progress to next rank:
Posts: 14
Joined: Fri Oct 22, 2010 12:23 pm


Simply Build It

Postby proptech » Wed Dec 10, 2014 7:09 pm

Hi Diy3r

As the two tanks ate the same height, the coil is OK. otherwise the F+E would be forever overflowing.
It's not easy to diagnose problems of this type over the internet, but a proper diagnosis is required.
Perhaps it's time to call for some professional help, and get back to us with the results.
Rank: Foreman
Progress to next rank:
Posts: 415
Joined: Wed Dec 05, 2012 11:22 pm

Postby Trev37 » Thu Dec 11, 2014 12:03 pm


I had a similar problem, if that's any help.

I see you've checked the water level, in the F&E. So that's OK (sometimes they run dry is the ballcock sticks).

If this is the cause, the simple way I solved a similar problem was racking up the thermostat for a short while.

Basically some heat is getting to the HW tank but is could be blocked half way the HWC coil (maybe by an airlock):
1) So turn off all rads using the valves.
2) Crank up the thermostat (is there just a thermostat on the boiler like some very old single pump vented systems?)
3) Listen out, after 5-10 minutes the system will make a bit of a racket, especially the expansion pipe going back to the F&E tank.
4) Turn down the thermostat to its normally setting.

let me know how it goes.
Rank: Apprentice
Progress to next rank:
Posts: 7
Joined: Tue Jan 20, 2009 4:21 pm

Postby proptech » Thu Dec 11, 2014 6:34 pm

Trev37 It sounds like your boiler thermostat is well out of calibration, allowing the water to boil on the highest setting. In some cases this can be a serious danger. Not a good idea to use it in an attempt to rectify some other fault.
Rank: Foreman
Progress to next rank:
Posts: 415
Joined: Wed Dec 05, 2012 11:22 pm

Postby htg engineer » Mon Dec 22, 2014 9:54 am

Quite common on gravity systems. A reduction in heat to the hot water if the heating is working as it should, is normally a blockage in the primaries. Once you're happy the system is filling (try draining a radiator and then refilling, to check cold feed isn't blocked) drain the system and remove the primaries from then tank/cylinder, if there's elbows on the tank connections check there for a build-up of debri/limescale etc.

As long as you don't have a primatic cylinder, you could try a descaler or sludge remover additive as there is still flow through the coil you might be lucky.
htg engineer
Rank: Project Manager
Posts: 3256
Joined: Tue May 22, 2007 5:22 pm

Display posts from previous
Sort by
Order by

  • DIY How to Project Guides
  • DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!

  • Related Topics