Central heating/hot water All aspects of plumbing questions and answers, help, tips and information

[Carnegie]

I have recently had a new & extended central heating system installed utilising new boiler, extra radiators & pipework, new programmer & unvented (Santon) hot water cylinder. The installer is at his wits end and cannot find the answer to the following problems (1) When the hot water only is on, the hot water in the cylinder return flow reverse heats some radiators by going down the radiator return flow which is connected to the cylinder return flow in the same cupboard about 18" away. The plumber did fit a one way check valve to stop the reverse flow down the pipe but this appeared to have effect of restricting radiator flow/return when heating on! (2) When the hot water and heating operate together the water in the radiator circuit, as at (1) does not circulate although the initial part of the 22mm flow pipework gets hot. When the hot water is turned off and the heating operates on its own the flow to these radiators starts immediately. Also, and seperate to the above - a number of rads in the house warm up via their 'return pipe' rather than the flow pipe! The pump in the boiler (Vaillant) has been adjusted to give heating preference over the water but to no avail! Any ideas would be gratefully received by me & particularly the plumber!!!

[stoneyboy]

Carnegie,
Difficult to provide a solution without seeing the installation but I have had a similar problem which turned out to be the pump being too powerful. If the pump can be turned down try this first.
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[Carnegie]

Thanks for your interest stoneyboy. Boiler engineer & Plumber have looked at pump & it is operating correctly. Someone suggested that its a 'return flow' problem in that all radiator circuit return flows should be fed back and linked together near the boiler and then teed into the primary return as close to boiler as possible. This would prevent my problem where the return of one radiator circuit is teed into the cylinder primary return to close to where the pipe comes from the cylinder. This means that as soon as the hot water leaves the cylinder, instead of heading off back to the boiler, it is immediately taking a 'right turn' down my radiator return pipe - the static water in the radiator circuit (when heating not on) is either being 'pushed back' or the static water is being heated up and thereby back heating the rads, etc. When the hot water and heating are operating together, the hot water from the cylinder still flows down the radiator return pipe but is confronted by the radiator water wanting to go the other way, almost a stalemate as the flows confront each other. When the hot water is turned off or the heating operates on its own, the radiator circuit operates perfectly because it doesn't have to contend with the reverse flow from the cylinder (if that makes sense?) By having all return flows linked near to the boiler and then linking them to the primary return flow, also near the boiler, means the flow pressure is definitely more towards the boiler with little chance of the flow being sidetracked. Hope this makes sense - what do you think?

[stoneyboy]

Carnegie,
The only ways I can see the problem you describe is if there is a vertical return pipe to the radiators connected to the return from the HWC - this may lead to gravity flow. Or. Somewhere there should be diverter valve(s) to separate the circuit flows - are these working correctly?
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[Carnegie]

Hi Stoneyboy

Yes you are right - the 22mm radiator return flow comes vertically thro' the bottom of the cylinder cupboard and the cylinder return flow is connected higher in the same pipe (about 18"/24" above). As the hot water from the cylinder flows it takes a 'left turn' downwards into the radiator return flow causing the problem. I don't know about the diverter valve(s) but I think they are working ok. I might add that also connected to this return flow pipe (above where the cylinder return is connected), is a 15mm return flow for a bathroom radiator which operates when water and water+heating are on. So, to repeat myself and just to make sure you can understand my description - the 22mm radiator return flow enters vertically thro' base of cupboard, takes a couple of bends and then rises towards top of cupboard. The cylinder primary return joins this pipe about 18"/24" higher and the 15mm bathroom rad pipe is about 12" above that. The return then goes into ceiling heading off to the boiler!
Thanks for your interest

[Carnegie]

Hi Stoneboy - just to let u know that had 2 second opinions on the pipework by the Santon cylinder and both independently agreed that the 22mm heating return for most of the downstairs radiators should NOT have been connected so closely to the cylinder primary return but rather it should have been teed into the 'upstairs' 22mm heating return which feeds back to the boiler - the installer acknowledged his mistake and has now done this and the problem of 'reverse' radiator heating to the downstairs affected rads appears to have been cured - so far so good! Also, the installer, again on advice of 2nd opinions, altered pipework by boiler to ensure a second heating return for 3 rads at the other end of the house, was joined into the main heating return, by the boiler, so that only one heating return pipe was teed into the main primary return - apparently, this is and should always be best practice.

The only problem I now have is that circulation to a group of 10 rads on the ground floor (there are 25 rads in the complete system including towel rails) is taking too long and it might be the pump is struggling to push the water round and then upwards to join the upstairs heating return- the pump is in the boiler which has an output of up to 35-40KW (up to 126000BTU?) - I am hoping to solve this by rad balancing to get a stronger flow/return to the affected 10 rads.

Hope you understand my layman's description and many thanks for your interest.