Central heating question...

[Adrian]

Hi All

I'd welcome your comments on the two (simplified!) heating system diagrams shown in this link.

original system is 'as fitted' to our new-build out here in the South-west of Ireland. Domestic hot water tank and radiators on the same flow & return circuit from the original oil-boiler. The only 'regulation' on the hot water temperature was by means of restricting the flow through the hot water tank coil with a simple lever-valve.

Individual rads have TRVs fitted.

When the new heatpump was fitted to replace the oil boiler - we wanted to isolate the water heating from the central heating circuit.. The controller on the heat pump can run the central heating pump OR the domestic hot water pump - the two pumps are connected in a 'tee' configuration from the 'hot' end of the heatpump..

For some odd reason, the plumber ran one new 'flow' pipe to the airing cupboard - and reckoned that the common 'return' pipe would be sufficient...

What we're now seeing is that some of the radiators are becoming warm

- when the 'hot water only' option is selected. The 'central heating' pump doesn't appear to be running, and the controller thinks that it's only 'doing' the hot water....

The plumber believes that the pumps incorporate a non-return valve - and that his revised layout should effectively isolate the radiators from the hot water cylinder - so that DHW should be available without the rads heating up....

....I'm not convinced - something in the back of my mind thinks that there ought to be a separate 'return' pipe from the hot water tank coil to the heatpump....

So - any comments ??

Thanks in advance

Adrian

[Dave Baker]

I doubt if the pumps incorporate a non-return valve although one could be fitted separately.

There's no need, or indeed any point, for that. Any 'separate' return pipe would still have to tee into the other one somewhere. You need either a zone valve to select the two lines or a non return valve on each line.

[Doctor Drivel]

Adrian

Your plumber should stick to drains and leave things alone he doesn't understand. There should be a non-return valve just after each pump. The return pipe from the cylinder should be the last tee into the main return before the boiler. Then it will work OK.

[Dave Baker]

Where the return from the cylinder tees into the main line isn't going to make a scrap of difference. The flow could still go back up the main return the other way quite happily. All the OP needs is the non-return valves fitted. The rest is fine.

[Doctor Drivel]

Should be "before the heat pump".

[Doctor Drivel]

Best read my post of this. :-)

[Doctor Drivel]

Nope. Look at the drawing. With only the CH pump on the flow will through the cylinder coil, out to the return pipe, into the main return pipe up the return to the first rad (flowing in the opposite direction), out of the first rad and along the CH flow in the right direction and around the rads and back via the main return. The cylinder must always be the last in the return to the boiler/heat pump.

[Dave Baker]

I don't follow you. With only the CH pump on the flow can't go through the cylinder at all because there's now a non-return valve by the HW pump.

>

[Doctor Drivel]

Sorry the DHW pump.

[Dave Baker]

OK, I see what you mean. Yes as it's drawn the HW pump could feed some of the rads even with a non-return valve fitted by the CH pump so the return from the cylinder does need moving to somewhere between the heat pump and the return from rad 1.

[Doctor Drivel]

As rule of thunb the cylinder should return should always be last before the boiler/heat pump. Then no probs.

[Adrian]

Clarification..... !

My two sketches are of the nature of a rough diagram - not an actual layout - sorry if that was misleading....

As the central heating plumbing is all behind plasterboard or embedded in the walls, there's no real way to tell exactly what the order of connections is - so the hot-water cylinder 'could' be at the end of the circuit, or the beginning, or even somewhere in the middle - who knows ??

The one thing that's certain is that there is a (new) direct piped connection between the heatpump flow and the hot water cylinder - the old 'flow' connection has been capped off in the airing cupboard - and the original 'return' from the hot water tank has been left as it was...

There's a new photo of the pump arrangement on the website

big copper manifold is the flow from the heatpump, then there's the domestic hot water pump, then what looks to me like an isolation valve (rather than non-return valve) then a tee with (out of shot) and air bleed valve.

Going down from the manifold (also out of shot) the arrnagement is the same for the central heating pump - with the exception of the air bleed valve...

Can't spot a non-return valve there....? - unless I'm missing something ??

Thanks again Adrian

[Doctor Drivel]

I would have a non-return on each pumps and have the cylinder return back to the heat pump. They all should be fine.

[Dave Baker]

Then we can't really help you much because the exact details of where things connect into the system in relation to other things is what we actually need to know. You could change one thing in a system like the location of a T or the direction of flow of a pump and the whole thing could cease to work.

However, clearly the addition of a separate HW pump is creating enough flow in the CH part of the system to heat the rads but what you need to do to fix that depends on the exact layout of the system.

[Doctor Drivel]

If the pipework is difficult to change, a non-return valve can be fitted on the return of rad No. 1. This will work. The problem is when it fails and passes. Best get the pipework right and rely on valves.

[Dave Baker]

What I might add to that is you at least need a non-return valve immediately downstream of each pump but you may then need either

1) Additional non-return valves in some of the returns in the CH part of the system depending on how everything is linked up to the HW return or 2) Relocate the return from the HW cylinder.

Which option is easier and cheaper to do obviously depends on the location of the items and how much new pipework might be involved and what you need to pull apart to fit it.

Frankly the plumber should have worked all this out to start with so if you've had this done based on a fixed quote to his design it's up to him to put it right. I'm also slightly curious why he didn't do it with a single pump and a zone valve rather than two pumps.

[John Stumbles]

I think the flow through the DHW circuit and will create a pressure drop between the point where DHW return meets the common return pipe and where rad 1's return meets it. That will give (reverse) circulation through rad 1's return, through rad 1 itself, via the flow pipe from rad 1 to rad 2, through rad 2 and back down rad 2's return to meet the common return.

Non return valves on the pumps won't help but one on rad 1's flow or return should do the trick. However don't use a spring return valve: the force required to operate this will be too much for the CH circulator and will cause all sorts of horrible noises. If you can, fit a gravity swing-check valve, or alternatively a normal check valve with the spring removed (on some types it's easy to pry the spring out).

[Doctor Drivel]

Two pumps is better as there is more flow and more reliable too.

[Adrian]

As I said in the 'addendum' - the diagram wasn't necessarily an exact representation of how it's actually plumbed....

Seems that non-return valves after both of the pumps are a necessity (not sure they're actually fitted at the moment). There doesn't seem to be a diagram of the actual route the plumbing takes.... which makes it rather more complicated.

Even with the non-return valves, I could imagine a situation where 'returning' water from the dhw circuit could find its way back through some rads on its way to the common return - which might account for the problems we're having with gratuitious rad heating...

My thoughts were to insist on a separate return from the airing cupboard back to the boiler..... but whether this would cure the problem I don;t know ?

Thanks Adrian

[Doctor Drivel]

Are you kidding?