S plan heating system pump location

[Snapford]

I'm replacing an old back boiler with a combi and am advised the pump needs to be relocated from under the floor on ground floor next to the boiler to the loft next to the hot water tank otherwise the system won't work. Does it matter where the pump is ? Thanks

 

[cross thread]

If it’s a combi the pump is built in !

 

[JohnD]

however if it is a non-combi (conventional) Condensing Boiler (sometimes people get the name wrong) you can put the pump where you want. It is usually next to either the boiler, or the Hot water cylinder.

Are you going to have a HW cylinder? Combi's don't have cylinders (except sometimes!)

 

[ianmcd]

are you sure you are getting a combi ?

 

[Snapford]

Sorry - not sure why i said combi - it's a regular boiler feeding 2 hot water tanks (for a power shower) and central heating.

 

[Snapford]

And does it matter whether it's on the supply or return ? Thanks

 

[FiremanT]

however if it is a non-combi (conventional) Condensing Boiler (sometimes people get the name wrong) you can put the pump where you want. I

Not really

And does it matter whether it's on the supply or return ? Thanks

Flow

I'm replacing an old back boiler with a combi and am advised the pump needs to be relocated from under the floor on ground floor next to the boiler to the loft next to the hot water tank otherwise the system won't work. Does it matter where the pump is ? Thanks

Is the boiler going into the loft? The pump would normally be on the flow pipe between the boiler and the motorised valve(s), so th acyaul location is dictated by practicalities. Having the pump under the ground floor is just generally daft, tbh, and I would look at every available option before siting a boiler in a loft.

 

[fixitflav]

Sorry - not sure why i said combi - it's a regular boiler feeding 2 hot water tanks (for a power shower) and central heating.

Is the system open vented (with F/E tank) or pressurised (with expansion vessel)? Preferred arrangement is boiler - open vent - cold feed (or expansion vessel connection) - pump, with some stipulations about distances and levels we can comment on if you want. How is it piped currently? Are you thinking of changing the boiler location?
Is it a 2-storey house?

 

[ericmark]

And does it matter whether it's on the supply or return ? Thanks

In a way, the old system no did not matter, but new idea is the by-pass valve opens if all the TRV's have closed so water returns to boiler hot and turns down boiler output. So the pump needs to be placed so it can open by-pass valve, unlikely to be able to do that if on the return.

Also don't want pump to suck in air, so being mounted low down on the system less likely to suck in air.

However there are exceptions to every rule, so there may be a case where pump can be high.

 

[fixitflav]

So the pump needs to be placed so it can open by-pass valve, unlikely to be able to do that if on the return.

I don't see why on the return is a problem. He could run a pipe from boiler flow, through the ABV to the pump suction pipe, all local to the boiler. That's if he wants to bother with an ABV, if at least 1 rad is always open, not essential.

If he goes for pumped return, anybody know the current thinking? Is open vent - cold feed - pump - boiler the way to go, or is pump - boiler - open vent - cold feed OK? If the latter, and depending on the existing setup and the house layout, that could be a solution, as the vent and feed connections could be in the airing cupboard, below the F/E tank.

 

[FiremanT]

Why is he bothered?

 

[ericmark]

I am an electrical engineer not a heating engineer or plumber. So I am willing to be educated. I was told that a by-pass valve has a pressure range 0.1 to 0.6 bar so 1.5 psi to 8.5 psi, and with a vented to atmosphere system we have a maximum of 14.7 psi i.e. one atmosphere.

I was told a by-pass valve was unlikely to ever open when on the return due to this, when my system was being re-plumbed the plumber has got a by-pass valve ready to be fitted, but the heating and ventilating engineer said no point in fitting as it would never open.

I did wonder at the time, as comments were made about the towel rail being always open, however can't see what that has to do with it as towel rail on the DHW circuit so would not allow any water to by-pass the closed TRV's.

In real terms the shower room TRV is non programmed so likely never fully closes, and the thermo syphon will likely allow enough flow to turn off boiler, in theory the pump could cavitate. What I am not sure of is how much damage is in real terms caused by cavitation?

I look forward to being educated. I must admit I am only level 5 educated, and as an electrical engineer cavitation was not really gone into, however did work for SLD pumps and so have some back ground as to problems with pumps running when no liquid is being pumped.

I know my father-in-laws pump failed when the micro switch failed to open and the pump was pumping against a closed motorised valve, but this was on the supply side not the return side.

 

[fixitflav]

the pump was pumping against a closed motorised valve

Loss of flow due to all valves being closed is more of a problem for the boiler than the pump. The water in the pump might get hotter than usual, but CH pumps are designed for hot water

 

[ericmark]

Loss of flow due to all valves being closed is more of a problem for the boiler than the pump. The water in the pump might get hotter than usual, but CH pumps are designed for hot water

I am unsure of the way a modulating boiler works, but the old non modulating gas boiler did not seem to suffer when the water did not flow, it switched its self off in the normal way. I know with oil boilers having a C or Y plan is seen by some as better than S plan due to being able to cool through heating the DHW.

I am unsure about all TRV being closed, with mechanical type the difference between fully open and fully closed is quite high, so set to heat a room to 20°C likely still passing coolant at 21.5°C, the electronic type however seem to be less of a range between full on and fully off, so a higher chance of all being off and the wall thermostat still on.

However it seems they also don't simply turn on and off, I hear the motor running in a series of short bursts, so likely the wall thermostat will turn off before all TRV's are fully closed.

However again with an oil boiler the boiler looks at the out going water temperature, the return temperature is not monitored, only with boilers designed to modulate and extract the latent heat is the return water temperature important. i.e. modern gas.

Running my boiler to give just DHW the motorised valves are closed, and pumps don't run, so can't see in my case how any boiler damage can be the result of closed motorised valves, that is what should happen with a C Plan for DHW only. The only worry is the pumps.

 

[fixitflav]

but the old non modulating gas boiler did not seem to suffer when the water did not flow, it switched its self off in the normal way.

Yes, I should have said my comment relates to modern-ish boilers which must be fully pumped. That's what the replacement is likely to be.
The old back boiler could be gravity HW, pumped CH, and as you say if given a live feed it would fire until the control-stat reached with no problem, because plenty of water and metal in the boiler. The early systems (sometimes called "traditional" in the literature) had no valves, boiler powered for HW, boiler + pump powered for CH + HW. CH only not possible. Inefficient because if HW left calling, the boiler goes On/Off on control-stat indefinitely, and in the ideal place to lose heat up the flue when off. There aren't many of those left.

Honeywell C-plan was an improvement, attached. If the zone valve is closed (cylinder stat satisfied) and CH called with all the rad valves closed, whether the boiler would have a problem I don't know, probably depends on the particular boiler. But I doubt there are many C-plans about now.