Discuss Pump on return is there a problem? in the Central Heating Forum area at PlumbersForums.net

mgw

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I know years ago before the TRV the pump went on the return, but new house to me, all TRV's on radiators except for bathroom which does not really count as that is thermo syphon, not worried about boiler as thermo syphon domestic hot water with no motorised valve, but am worried about pump, will it cavitate as the TRV's close and damage it, with pump on supply easy a by-pass valve will allow it to continue to circulate, but will a by-pass work on the return.

The system was in a mess, two pumps one for flat switched on with a thermostat, and one for main house, simply on a 13A plug, there was a time switch/programmer which said it allowed heating and DHW once or twice a day and DHW once or twice a day, plus continuous and off. However in real terms all it did was turn boiler on/off there was no connection to any pump.

The area around the boiler in the flat under the house had a flat thermostat, and a wireless receiver for another thermostat, but no thermostat linked to it.

On trying to use the system found if either pump used on it's own, it would reverse circulate water in other system, also radiators get hot on top floor with thermo syphon even when pumps switched off.

Turning off the maintenance valves on the pumps did stop the circulation, so local plumber was to fit two motorised valves, since the motorised valves have micro switches built in, easy enough to wire so unless open pump will not run.

So bought Nest Gen 3 and wired up ready for when motorised valves are fitted, also paired with 4 MiHome Energenie TRV heads in hall, dinning room and living room, at moment only using for DHW so all OK.

However two things came to mind, one is what if all TRV heads close while pump is running, and two since linked why does the TRV not show same target temperature as Nest?

As yet plumber not returned to fit valves, so if some thing needed I will not need a second supply of inhibitor, so now is the time, but most pipes hidden behind plaster board, will not only pipes, whole fuse box hidden just enough room to put hand through ceiling and turn off power.

So no option but have pumps on return, only two supplies, there are three returns, plus I assume fill pipe.

Looked at modulating pumps instead of three speed, but they need a minimum pressure, so seems can't be used on return, thought about some vac/pressure switch but would need latching and reset.

I can't be only one with pumps on return and all TRV's fitted, so how is it done? I am not doing plumbing, however I am doing wiring, all set out ready, there will be a slight problem in that if flat calls for heat without either DHW or main house calling for heat, it will not turn on boiler, just circulate water, but not worth fitting relays as flat rarely used. And also no thermostat on DHW cistern, just limit time to 1/2 hour every other day, that's enough to have hot water to wash hands in summer, shower is electric, oil only for central heating and hand wash, OK maybe odd bath, but easy enough to press boost.

So all comments welcome.
 
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Pump cavitation is for big boys to worry about in power stations submarines etc...your pump will give up after a while anyway. Are you an engineer or a knowledgabke diyer...it helps us help you I suppose and put up a diagram please ...Rob Foster
centralheatking
 

rpm

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C'mon Rob it really doesn't matter what size of pump, you don't want cavitation happening.
 

rpm

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I'm sure that the op doesn't want to live with the noise and replace it every what 2yrs?
Sometimes I wonder if we read the same post. :D
 
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mgw

mgw

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Pipework not sure but think it is something like this Pipework.jpg the boiler and pumps are in a flat below the main house so would not hear them, only two wires between main house and flat so fitted Nest once motorised valves are fitted will be wired like this C_Plan_My_HouseC.jpg which is not perfect but good enough, I was first trained as a mechanic and then slowly moved to be an electrical engineer which included working for SLD pumps so yes aware of damage cavitation can cause, however as to if a real problem with central heating I don't know.

If the Nest switches off at same time or before the TRV's then it should be OK, but how would I know, I can set Nest one degree lower than the thermostat in the hall which should mean that thermostat is always open, but how accurate are TRV heads? Far better if there is some relief valve which will allow water into the pump, maybe all it needs is a lock shield set so there is always a bleed of coolant, but this is all above my knowledge, hence the question, it may be I don't need any thing? But time to fit anything is before new inhibitor is added, plumber did not turn up today, so have a little more time before I need to know.
 
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I'm sure that the op doesn't want to live with the noise and replace it every what 2yrs?
Sometimes I wonder if we read the same post. :D
if all the trvs are on the correct side with a bypass and the pump is set at a suitable pace it should work fine, all the electronic crap hive etc is intermediate tech and way over priced in my opinion..reason why its fitted is to allow installers extra profit with little labour input.
And people with mobile phones can pretend to be in charge In fact a properly balanced heating system by and large might not need trv but thats my opinion but as ever will keep selling the stuff because its high profit....loads of money !
centralheatking
 
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mgw

mgw

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Reason for Nest was lack of wiring between flat and main house, with Nest two wires with 12 volt DC is only connection between main house and flat below. Also selected Nest as volt free contacts and will link to my existing TRV heads.

But the wiring of Nest is not the problem, it is if I need some protection for the pumps? being on return not sure a by-pass valve will work?
 

rpm

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After 92 views is there nobody able to help and keep the thread on track?
 
A

Aire

Pumps on the return are fine in heating systems, pumps are located on the return side of combis and system boilers and as the water is cooler it's probably better for the pump. Only thing you have to watch is pump location on open vented systems. And if all your radiators have trvs on them then you should really have a bypass to prevent damage to the pump if they all shut off
 
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mgw

mgw

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If all your radiators have trvs on them then you should really have a bypass to prevent damage to the pump if they all shut off
Question I suppose with pump on return, is where should that by-pass actually by-pass? Pipework-question.jpg
The point I am making is the by-pass needs to prevent a depression rather than release pressure, so what setting and where? Flat radiators easy enough just open the kitchen TRV fully, no thermo syphon problem in flat, main house however a different story.
 
A

Aire

Question I suppose with pump on return, is where should that by-pass actually by-pass? View attachment 39452
The point I am making is the by-pass needs to prevent a depression rather than release pressure, so what setting and where? Flat radiators easy enough just open the kitchen TRV fully, no thermo syphon problem in flat, main house however a different story.
The bypass can go anywhere on the flow pipe and then tee into the return after the zone valve but before the pump. And as for setting, turn the valve to it highest resistance, then with your heating running slowly open the valve until you feel water lass through it, then give it a quater turn back, and that should be fine.

You could also change one of the trvs to a lockshied so one radiators remains permanently open. But this will only work if your system doesn't have pump overrun.
 
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The bypass can go anywhere on the flow pipe and then tee into the return after the zone valve but before the pump. And as for setting, turn the valve to it highest resistance, then with your heating running slowly open the valve until you feel water lass through it, then give it a quater turn back, and that should be fine.

You could also change one of the trvs to a lockshied so one radiators remains permanently open. But this will only work if your system doesn't have pump overrun.
The open radiator is usually a bathroom radiator. centralheatking
 
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No it isn’t Rob. On most domestic installs it is usually the hallway radiator.
I live and learn, we always put it on the bathroom rad as this room needs most heat and usually dries towels etc, why hall. Rob Foster aka centralheatking
 
A

Aire

I live and learn, we always put it on the bathroom rad as this room needs most heat and usually dries towels etc, why hall. Rob Foster aka centralheatking
The hall was generally considered the preferable room as the bypass radiator because it's usually the room with the thermostat. Now with wireless portable stats though, it's getting less common.
 
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mgw

mgw

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The bathroom radiator has no TRV, it is only radiator in the house without one, and it is thermo syphon so will not help the pump in any way.

The wall thermostat in the hall, is well away from the radiator in the hall, and has an open stairs between the two, so I set the hall, dinning room and both living room TRV heads to follow the hall wall thermostat, thought was whole floor should be at same temperature, and since all have degrees C as temperature setting can't see why they would not work together.

The flat wall thermostat in the kitchen, seems a daft place for a thermostat, is again quite a distance from the radiator, and the TRV heads are just numbers, so not sure what the settings are, but flat is rarely used.

I have every intention of fitting bluetooth TRV heads upstairs, but not fitted yet, as far cheaper than wifi, and really the aim is to stop over heating.

At the moment having a problem with DHW boiler was running this morning, but no scheduled to run, I could hear the relay drop out when I switched off the system, and all software said it was not running, but it was, Nest blames far sight and the hot water schedule, both on their instructions turned off, however got that feeling in a couple of days time I will have no hot water.

Nest 3 seemed great, but now not so sure.
 
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MGW

Your two diagrams are different! The first shows both pumps before the valve; the second shows the house valve before the pump. Which is correct?
 
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mgw

mgw

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As yet valve not fitted, due now to be fitted next Monday, and it seems a by-pass will also be fitted, I would prefer it with valve where the by-pass will still protect, as have seen the micro switches fail, however I will leave that to the plumber.
 
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Why do you need 2 pumps, wouldn't just one do the job where it goes into the return on the boiler & maybe a motorised valve on the flat if it's not used much. I've seen problems before with too many pumps causing reverse circulation
 
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Why do you need 2 pumps, that's really likely to cause back circulation where they are, surely you only need one where the single pipe goes back into the boiler. If the flat isn't used a lot fit a motorised valve where the flat pump is. The flat is really just another collection of rads on one system
 
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mgw

mgw

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Why do you need 2 pumps, that's really likely to cause back circulation where they are, surely you only need one where the single pipe goes back into the boiler. If the flat isn't used a lot fit a motorised valve where the flat pump is. The flat is really just another collection of rads on one system
Good point I will talk to plumber tomorrow.
 
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mgw

mgw

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On looking at how to wire with a single pump, it actually works out better, giving full central heating control for the flat. So will see what he says, as also means just one by-pass valve too.
 
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mgw

mgw

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Plumber called today to apologize he has bad back so can't do the work at moment, he says the motorised valve must go on the output side of the pump, I just can't get my head around why it's OK to have TRV's on all radiators on the supply side of pump, but not a motorised valve on supply side, to my mind they are both taps, only difference is TRV closes slowly where the motorised valve closes fast.
 
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He's talking rubbish, it is a tap, it's like a light switch if it's in the live or the neutral it will still turn the light off, not good practice with electricity though. On a plumbing note I always think it better on the return side as the water is cooler & stuff lasts longer if it's cooler & no-ones going to get a shock from it if they touch it accidentally. Plus I've never seem anything in any regulations saying it's got to be in the send side. I've been doing this stuff for over 40 years. Regards Chris
 
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mgw

mgw

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I see your point, only question is what happens when flow is stopped, on pressure side the by-pass valve will work, on suction side not sure how the by-pass valve will stop cavitation, however clearly all valves need to be either on one side or other of pump, not both, and my plumber seems to want to fit the by-pass valve and motorised valve on pressure side of pump, even when there are TRV's on suction side, this to me seems wrong?

Whole idea of by-pass valve is to allow flow through pump even when the motorised valve or TRV's are closed, have I got it wrong?
 
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Whole idea of by-pass valve is to allow flow through pump even when the motorised valve or TRV's are closed, have I got it wrong?
No. The idea of a bypass valve is to allow adequate flow through the boiler when TRVs and MVs are closed.

On a traditional system, with the pump and MV on the flow, this is achieved by connecting one side of the bypass between pump and valve, and the other side to the return pipe. The question is: where do you connect it when pump and valve are on the return? And does the order of pump and valve make any difference?
 

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