Pump on return is there a problem?

[mgw]

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.

 

[Rob Foster]

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]

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

 

[Rob Foster]

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

agree but its much more serious at the industrial scale
centralheatking

 

[rpm]

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. 😀

 

[mgw]

Pipework not sure but think it is something like this

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

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.

 

[Rob Foster]

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. 😀

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

 

[mgw]

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]

agree but its much more serious at the industrial scale
centralheatking

Captain obvious or what! 😀

 

[rpm]

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

 

[mgw]

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

Thank you, and I hope some one can tell me if I should worry or not?

 

[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

 

[mgw]

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?

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.

 

[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.

 

[Rob Foster]

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

 

[Harvest Fields]

The open radiator is usually a bathroom radiator. centralheatking

No it isn’t Rob. On most domestic installs it is usually the hallway radiator.

 

[Rob Foster]

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

 

[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.

 

[mgw]

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.

 

[doitmyself]

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?

 

[mgw]

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.

 

[Chrisjones.7976]

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

 

[Alex Edge]

Just skim reading install a bypass valve?

 

[Chrisjones.7976]

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

 

[mgw]

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.

 

[mgw]

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.

 

[mgw]

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.

 

[Chrisjones.7976]

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

 

[mgw]

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?

 

[doitmyself]

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?

 

[mgw]

I suppose it does not really matter in my case if pump before or after valve, as the boiler can send its output to the cistern hot coil which can't be turned off, so the by-pass is only protecting the pump, however all valves must be same side of pump, both the motorised valve and the TRV can both stop the flow, well the TRV is a motorised valve in my case.

This is where I can't get the plumber to agree, he says the by-pass valve always goes on pump output.

With by-pass on the output it must be set below the pump maximum pressure, this could be 15 psi, however on return 14.696 psi is the maximum and clearly that can't be attained, so likely looking at 1 or 2 psi? So it would seem the by-pass on return is not the same device as used on feed.

This is what I think would be correct order, to stop pump cavitating but the pressure would be rather low.

My plumber is telling me I am wrong, maybe I am? But I can't understand his logic.

 

[doitmyself]

Provided the static head, i.e vertical distance from the feed and expansion tank to the pump input, meets the pump manufacturer's requirements, cavitation should not occur.

How are the pumps controlled? It can be directly, via the auxiliary switch in the MV (which is controlled by room stat/programmer), or it is controlled by the pump overrun in the boiler. If directly then, when the boiler goes off, there will be no circulation in that circuit. It's only if the pump is controlled by boiler overrun can there be any circulation through the circuit.

Where do your numbers come from - 15 and 14.696?

 

[mgw]

The 14.696 psi is one atmosphere so max pressure on suction side, the pumps were originally controlled one from a wall thermostat in the granny flat, the other on a 13A plug. This is why I am altering things, I think walking down a floor level outside the house is not a good way to switch on the central heating, the two pumps had problems as thermosyphon and back flow resulted in radiators working when only wanted DHW, and due to mauled hand I realise I have not the ability to fit motorised valves or by-pass valves, so found a local plumber.

It seemed best way to run pump is using the micro switch in the motorised valve so pump can't run until valve opens, however I have known micro switches to stick, so not really can't it more should not. But clearly that only works on the master motorised valve, all the TRV heads are electric so they are a type of motorised valve, and in the case of the TRV there is no micro switch so to my logic the motorised valve and the TRV's should be same side of the pump, and the by-pass valve should by-pass both the motorised valves so if either is shut there will be no damage to pump as water will still circulate.

Of course don't really want boiler to run either, but since DHW is thermosyphon it will not really do any harm, the boiler will simply start to cycle.

So the by-pass is just to protect the pump. And this is where the plumber and me can't see eye to eye, I say motorised valve and TRV together and plumber want pump between the TRV and motorised valve.

 

[doitmyself]

The 14.696 psi is one atmosphere so max pressure on suction side

The static pressure at the inlet is determined by the system's static head, i.e the vertical distance betwen F&E tank and the pump inlet. It has nothing to do with atmospheric pressure.

As an example, Grundfos recommend these minimum static heads for the Alpha 2:

Water temperature (deg C) - 75 -- 90 -- 110
Head (m) ------------------ 0.5 - 2.8 - 10.8

On the earlier Selectric circulator, Grundfos suggested 1.4m at 82C.

 

[Riley]

Is this still going😀

 

[mgw]

Yes still going, although I worked for SLD pumps for a time, so understand the general principle, I am an electrician not a plumber, so not really knowledgeable with domestic pumps, I know my father-in-laws pump failed after the micro switch stuck on the motorised valve and caused it to run with no flow, however the pump could have been on last legs anyway.

But in that case the motorised valve was on pressure side of pump, not sure what happens if on suction side? Also just fitted the motorised valve to wiring, although not as yet plumbed in, and I note mounted one way it could be forced open by pressure the other way pressure would hold it closed, so if mounted one way around maybe it would simply be forced open and do no harm?

I have got it wrong before with domestic central heating, so not going to tell the plumber his job, but still would like to get it straight in my own mind.

The plumber said fitting a motorised valve would not stop thermal syphon through the radiators, so I asked him to do simple experiment, turn off the maintenance valve at the pump, if the radiators then did not warm up, clearly a motorised valve would help, if they still warmed up, then clearly a motorised valve would not help, and they stopped warming up, so fitting a motorised valve seemed to be way forward.

That is not to say some sort of pressure release valve like a by-pass valve would not do the same job, however I would then need to fit relays or pressure switches to replace the micro switch in the motorised valve, so for me as electrician the motorised valve was easy sure way to cure the problem.

I am sure this heating was fitted by a professional in the first place, so some thing about this house has clearly caught him out, so I am being careful, after all the ark was built by an amateur and Titanic by professionals so the professionals can get it wrong. As an electrician I have made mistakes, so why should I think other will not.

I think the problem all hinges around the conversion of the garage into a flat, they were clearly well out of their comfort zone, with loads of over sights during the conversion. This includes a fuse box between the original garage ceiling and new false ceiling with a hole just big enough to reach the main isolator. So nothing surprises me in this house.

 

[doitmyself]

Th.e bypass flow will be in the direction of the red arros in this edited pic. Whether the pressure at the input of the ABV would vary correctly to cause the ABV to open as required is difficult estimate.


Where do the feed and expansion pipes connect?
What is at the other end of the "unknown" pipe?

PS It would be less complicated if you ditched one pump, then moved the other pump and both valves to the flow. At the same time the HW should be pumped and a MV fitted to the HW circuit. It's called an S Plus system. The feed and vent pipes would need to be connected before the pump. You would then need one autobypass after the pump, connected to the boiler return.

 

[mgw]

I wish I knew how it was plumbed, the plumber is telling me the T's in the airing cupboard are going to the radiator feeds, personally I think it goes to just the bathroom towel rail, I think the feed and expansion is on the thermo syphon connection which is to right of boiler and there is no connection to left hand side of boiler which does central heating other than through the boiler. I can see two pipes as shown going into the loft, would assume to a header tank, the problem is when the garage was made into a flat, the walls were clad with plaster board hiding all pipes except in the kitchen cupboards either side of the boiler, with the flat being under the house it is in essence three story, so thermo syphon works well, plumber suggested just using the TRV to turn off radiators, however not sure they are accurate enough, where the wall thermostat will turn off the central heating completely at temperature set, the TRV tends to modulate over a couple of degrees, so when in summer the DHW is called for, the upper radiators still get warm.

 

[doitmyself]

You, or your plumber, need to stop guessing and do some investigating. For a start, look in the attic and check how the F&E tank are connected. Then find the other end of the "unknown" pipe shown on the heating side of your diagram. As shown it is not doing anything, so why is it there?

 

[mgw]

The house is built into the side of a hill, originally it had a garage under the house which had it seems the central heating boiler, electric meter and fuse box in the garage, when the garage was turned into a flat a consumer unit was added for the flat, and walls and ceiling lined so pipe work and original fuse box hidden, there is a small hole below fuse box just big enough to get your hand in to switch it off, may be able to change a fuse, but never taken one out in case can't replace it.

The central heating for house was controlled by going outside and down to flat and plugging in the pump, which was on a 13A plug, the flats central heating had the pump on a thermostat, but no temperature control for the house.

There was an old programmer in the house, but the three core cable house to flat only had two intact cores, so all programmer did was turn whole system on/off. There was a hole in the hall wall with a cable showing, this is seems was originally the location of the thermostat, there was a wireless thermostat receiver in the flat, but no transmitter.

There were three FCU's supplying central heating, one in house and two in flat. In essence only control was the TRV's fitted to every radiator other than house bathroom.

Short of gutting the flat, which is on the cards, no way to trace where wires change colour or water pipes go, so in mean time I need to have a working system at least until wall and ceiling plaster boards are removed, as latter is a major job.

So much for buyers report where the surveyor completely missed all this, even missed the floor was breaking up under the cistern.

We had a pair come in, plumber and chippy, to repair floor under cistern and fit new shower, however when shower removed beams rotten below, so the job took longer than planned, so it was agreed they would return latter to finish the work, and in the mean time I would wire up ready.

So Nest fitted and two motorised valves wired up, but just sitting in the cupboard not as yet fitted, the plumber returned to say he has put his back out, so there will be a delay, he clearly had messed up his back, and it may mean getting some one else, or DIY, and although I could not fit the valve to flat pipe work, or fit the by-pass valve, fitting the motorised valve for house does seem possible.

So looking at what I can do if required.

 

[Chris-1984]

Your type of system is a typical pumped heating gravity hot water system.

The only difference is there are 2 circuits on the heating side. The open vent cold feed will connect into the cylinder flow and return(gravity side).

The best thing you could do is convert the system to a fully pumped setup. Cylinder thermostat to control hot water temp and set it up as a s plan plus. As you the system you have now cannot have a new boiler simply added to it.

Seperate electrical feeds to pumps etc is just a bodge and could potentially be a hazard as the heating system should be fed by one fuse spur.

As its also a old type of system chances are it could be a 1 pipe system and trvs won't work properly anyway.

Reverse circulation will be caused by poor system design we're the circuit downstairs has been added on poorly.

So my advice if your renovating would be start over and do it properly otherwise you will give yourself a head ache.

 

[mgw]

Your type of system is a typical pumped heating gravity hot water system.

The only difference is there are 2 circuits on the heating side. The open vent cold feed will connect into the cylinder flow and return(gravity side).

The best thing you could do is convert the system to a fully pumped setup. Cylinder thermostat to control hot water temp and set it up as a s plan plus. As you the system you have now cannot have a new boiler simply added to it.

Seperate electrical feeds to pumps etc is just a bodge and could potentially be a hazard as the heating system should be fed by one fuse spur.

As its also a old type of system chances are it could be a 1 pipe system and trvs won't work properly anyway.

Reverse circulation will be caused by poor system design we're the circuit downstairs has been added on poorly.

So my advice if your renovating would be start over and do it properly otherwise you will give yourself a head ache.

If it was only that easy, there are two wires and earth between flat where boiler, pumps, valves and heat link is, and the house where I want to control it all from, those two wires now have the 12 volt supply to the thermostat and data running down them.

Getting a tank thermostat fitted is the ideal, however looked for wireless versions of a tank thermostat and they seem to have gone, I remember fitting one in my dad's old house, they were cheap enough, but can't find them now.

In real terms come winter there is no so much of a problem, worried about under temperature rather than over temperature to avoid legionnaires, and not so sure about hot water anyway, OK having a bath, but to wash hands you have to run off so much water before hot arrives, it is a bit daft.

What I have considered is a combination of immersion heater which has a thermostat, and the central heating, so run central heating domestic hot water say 7 am Mon, Wed, Fri, Sun 1/2 hour each day, then run electric at 7:30 am Mon for 3 hours to ensure hot enough. Getting a timer for immersion is on my "to do" list.

However at some point likely ceiling in flat will need a hole making in it, the fuse box is between original ceiling and new flat ceiling and clearly needs moving, once this is done, I may find more about how the boiler is plumbed, and also I will be able to isolate a single circuit, at the moment I can reach main fuse board isolator, but would not want to draw a fuse, as may not be able to get it back in again.