CH comes on with DHW

[plumbamateur]

Guys, just moved to a new place. Boiler is a Vaillant combi about 8 years old. I've not been using the CH yet, other than to test it, and all seems ok- all the rads heat up. This issue is that 2 of the rads upstairs heat up when I'm drawing hot water. The effect is particularly noticable when I run the DHW for a decent length of time e.g. Fill the bath. The rads concerned heat up quite a lot, almost to the extent you'd expect with the heating on.
I'd be grateful for any recommendations on how to approach troubleshooting this issue
cheers

 

[kevplumb]

run the h/w with the heating cold

do the heating pipes warm up

which vaillant combi btw

 

[plumbamateur]

Hi Kev

It's a Turbomax Plus 828e

Just to clarify what you want me to test - just run the h/w with the heating off (cold)? If so that's what I've been doing - and the rads upstairs heat up (so yes, the pipes warm up). Would this suggest some sort of problem with a valve or something?

Cheers

 

[kevplumb]

diverter valve would be a good place to start

 

[plumbamateur]

OK thanks. Can you give me an idea where to look for this? How would I check it for faults? Sorry I've not attempted fixing a boiler before but i'll give it a go!
Thanks again

 

[kevplumb]

if you stick an e mail in your profile i will send the mi's

 

[plumbamateur]

Kev, it's happyeater(at)gmail.com
thx

 

[ChrisR]

This is the beast:
//www.diynot.com/forums/viewtopic.php?p=1451932#1451932

 

[kevplumb]

sent

 

[plumbamateur]

Kev thanks, that's great. Loads of info in there - there wasn't a manual like this with the boiler when I moved in so appreciate you sending it over. It looks quite simple to replace - is it just a case of buying a new one and seeing if it works; or are there any specific tests I can do to prove the fault to the diverter?

Would I need Vaillant parts or are these generic?

Thanks again

 

[plumbamateur]

Thanks ChrisR
Crikey, she's not cheap is she?!! 85 quid-odd. I'm actually going to be embarking on some quite major building work in the new year, and will probably replace the boiler then as it's got to move location in the house. What would your opinion be of just leaving it? Whilst rads heating up when the h/w is run is a bit of an annoyance, is there any other reason why this should (i.e. needs to) be replaced?

 

[plumbamateur]

OK another issue with this wretched boiler. The pressure in the system drops to zero over a week or so. I originally thought this would indicate a leak somewhere, and have been simply topping it up as required. However, I've just noticed that when running the CH, the pressure goes all the way up into the 'red' zone (3+ bar) once the system gets hot. Does anyone have any suggestions as to what might be causing this? Could it be related to the problem described above with rads warming up when using the DHW? thanks.

 

[kevplumb]

expansion vessel needs recharging see the heating FAQ up the top of the forum

 

[plumbamateur]

Thanks Kev, appreciate it. I'll have a go at it tomorrow

 

[plumbamateur]

Kev, I read the section on re-pressurising. I think the PV membrane must be ruptured as water escapes from the valve when I press the central pin. I'm not going to replace the PV in it's existing position as it means taking the boiler off the wall. I read the relevant section on replacing the PV in the manual you kindly sent previously, and an alternative exists:

"A suitable replacement expansion vessel
can be fitted externally to the boiler
as previously described (if possible
on the central heating return in an
accessible position). In these circumstances,
the replacement vessel must
be correctly sized ignoring the original
expansion vessel, which can be
left in position on the boiler."

Given I'm going to be replacing the entire boiler in the next 6-9 months, would a suitable solution be to leave the rad at the top of the house half full to allow for expansion? I'm thinking of the bathroom rad in the loft, which we don't use much.

Cheers