New cylinder

[AJSmif]

I've had some useful insights & advice from this usergroup... (Read more: //www.diynot.com/forums/plumbing/a-good-hot-bath.336315).

I was just looking for some last advice before getting the plumber back for the annual service

System was updated a year ago from a gravity system to vaillant 428 boiler + 3 port valve, cylinder stat, room stat & grundfoss pump, plus Danfoss FP715 programmer.
The HW cylinder (see photo) though is still the original red jacket (not replace when system updated)… with a lot of the HW circuit 28mm piping.


Now I understand with these boilers they cycle off if the return water is not much colder than the feed (it assumes there is no requirements?)

On HW only I have problems getting the HW to really hot ... the boiler starts cycling after 15-20 mins as the hw is then returning to the boiler at almost the same temp as it is flowing.

In the winter with CH+HW ... the HW heats up better as the boiler stays firing more (less cycling off as the return water is much lower than the feed from the boiler .... but the radiators furthest from the boiler do not heat up for hours (even with lock shield fully open)… but heat up straight away if I turn HW off.

When I think about this, both of these could be symptom of the HW circuit being too easy…

To my mind if water flows round the HW circuit much easier than the CH it explains why the further radiators do not get hot until HW is switched off, and also explains why the boiler spends so much time cycled off when on HW only.

I’ve seen recommendations elsewhere that the cylinder should be changed… I believe this is pretty costly – would it definitely fix the problems….

Are there any other options to a) stop the boiler cycling off so much when HW only & b) to balance flow between HW & CH?

Thanks for your thoughts.
Andy
View media item 63683

 

[gigz]

Is there a lsv on the bottom connection of the coil ? If so it may need balancing (throttle it down) so it goes round the coil a bit slower. Have you checked the cylinder stat is turned up far enough ? And is the boiler stat turned up far enough ?

 

[AJSmif]

Is there a lsv on the bottom connection of the coil ? If so it may need balancing (throttle it down) so it goes round the coil a bit slower. Have you checked the cylinder stat is turned up far enough ? And is the boiler stat turned up far enough ?

no - there isn't a lsv on the bottom of the heating coil - should there be?
yes if water went round slower I could see that there might be more chance for the heat to be exchanged and hence the cylinder to get hotter (and the return might be cooler hence the boiler would fire a little longer?). I guess this might also help with the CH to the far radiators?

yes the boiler & cyl stat are set high - but the clinder doesn't reach the temp.

thanks

 

[mickyg]

should have an auto bypass not manual
pump should be minimum 6m head on max
3 port valve would need to be 28mm with 28kw boiler but probably boiler is grossly oversized so downrate it to 15-18kw.
as the cylinder has 28mm primaries it was probably specified for a gravity system and will have a very small coil, so you might end up needing to replace it anyway