Pump on return is there a problem?

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.

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.

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

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?

mgw said:

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?

Click to expand...

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?

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.

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?

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.

mgw said:

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

Click to expand...

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.

Is this still going

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.

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.

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.

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?

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.

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.

Chris-1984 said:

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.

Click to expand...

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.