Additional pump with system boiler?

[lukewb01]

Hi all, just wanted to get some peoples opinions.

A new customer of mine has asked me to look into what he can do about his heating system not heating up downstairs radiators sufficiently. He has recently had a extension and a new 32kw oil system boiler with 15-60 130 pump fitted outside. The system is quite large as a long house with ground floor and first floor and around 16 radiators average sizes with a unvented cylinder in loft.

The pipework from boiler goes up to the loft where it splits for the cylinder and radiators. I have been advised that the pipework feeding the radiators goes around like a loop (instead of dropping down multiple times to feed a number of radiators). This pipework is in 15mm and cannot be changed now. This means the radiators at end of the 'loop' downstairs struggle to get hot enough. The radiators have been balanced the best they can, while this has helped a little it hasn't solved the problem enough.

Due to the the pump being outside on ground level and having to be pumped up to loft before dropping down the rest of the house the customer is worried it is not big enough for the job and has asked me to install a bigger pump up the in the loft to boost the pressure. I understand the correct way of doing this would be to pipe correctly or fit a low loss header however the customer would like to try the cheaper option first of just installing the pump on its own.

I wanted to get your opinions on how this may work if at all and where the pump should be fitted. Do I install on the flow before the zone valves so it boosts flow to cylinder and CH or just on the flow after the CH zone valve? My concern if I install on just CH pipework is this will cause a issue with the cylinder being bypassed when both are calling for heat?

Many thanks Luke.

 

[Johntheo5]

What is the make and exact model number of the existing pump?. Is the pipework 1mm copper?

 

[lukewb01]

Thank you for your response, please see image of current pump in boiler.

I assume you mean 15mm copper? I would have thought so but cannot confirm what is in walls or under floors.

 

[Johntheo5]

Yes, meant 15mm copper which has a ID of 13.6mm as against 12.0mm for " plastic," so more than likely copper otherwise your 6M existing pump would only circulate ~ 10LPM.
A LLH (low loss header) can be very beneficial with a gas fired boiler as its heat exchanger loss alone can be up to 3.5/4M at 15 to 20LPM, leaving only ~ 2 to 2.5M for system circulation but a oil fired boiler's heat exchanger loss is tiny so a LLH is of little or no great benefit from that point of view, IMO. Not sure about a booster pump or its location but if you were to replace the UPM2 6M pump, ~ 5.5M @ 16LPM with a UPS2 8M pump then, because it will pump up to 54LPM at 8M, the flow rate should increase by ~ 21%, this may be sufficient?. A 6M booster pump, on the other hand should increase the flow rate by ~ 40%, theoretically.

 

[lukewb01]

Hi I think the idea behind the LLH was as a buffer between the two pumps so they are not working against each other as opposed to the heat loss side of things. Upgrading the pump isn't possible so a additional one will need to be added further up the line in the loft.

I was thinking of adding a UPS3 15-50/65 unless there is a better pump to install.

My main worry is the position of the pump whether to put before the cylinder or after. I still want the cylinder to heat up just as quickly when both are calling for heat as well as the radiators heating up better when just heating is on.

 

[Johntheo5]

I've read that all right re pumps fighting each other but not sure why they should as there must/should be some equilibrium final head point, the UPS3 isn't a bad choice IMO but do seem to suffer from more than their fair share of premature failures.
I would boost the lot, cylinder included.

What make/output oil fired boiler is installed?. the only other thing to watch is, assuming a sealed system, then some manufacturers like Grant have their PRV (pressure relief valve) set to 2.5bar (3.0bar on most gas/oil boilers), if you have the expansion vessel with a precharge pressure of 1.0bar installed (properly) on the pump suction, then if the booster pump head is set to say 6M , you have a cold boiler pressure of 2.1bar which might rise to 2.5bar (Grant PRV setting) when the system heats up and may require a bigger expansion vessel.

 

[lukewb01]

It is a Worcester Greenstar Danesmoor External System boiler. Max CH output 32kw. Yes sealed but not sure on size of expansion vessel or PRV setting think it is 3Bar.

Think customer wants to try putting a additional pump in on its own (maybe bigger expansion vessel too if needed) and see how it goes with the idea that if a LLH is required then the pump and expansion vessel will still be needed anyway so worth a shot without the price of the LLH header first.

 

[Johntheo5]

Yes, but if a 6M pump is selected and for whatever reason doesnt work in series with the other one and a LLH is installed, then, from a system head requirement, your back to square one as the primary pump in a LLH does not provide any boost to the secondary pump. (Except you install a 8
M pump as a precaution)

 

[lukewb01]

Yes but pump now being installed in the loft and not needing to push the water from ground floor to loft before going to radiators not give a better overall performance to the system? Or if so fitting next size pump up will overcome this such as 8m instead.

Thanks.

 

[Johntheo5]

Yes but pump now being installed in the loft and not needing to push the water from ground floor to loft before going to radiators not give a better overall performance to the system? Or if so fitting next size pump up will overcome this such as 8m instead.

Thanks.

If booster pump is installed in the loft then the boiler circ pump must overcome the pipe line losses up to the pump so the inlet pressure into the booster pump will be reduced by the same amount ( + the elevation but that doesn't matter as its gained on the way back down) so less of a boost pump head, overall, I can't see any advantage in installing it up high as against after the boiler circ pump assuming the same pipeline losses and system losses, the circulation is through a loop, in effect.

 

[muggles]

The physical height of the system isn't relevant here. You need to know the resistance in the pipework. Running two circulators in series is likely to cause cavitation

 

[Johntheo5]

The physical height of the system isn't relevant here. You need to know the resistance in the pipework. Running two circulators in series is likely to cause cavitation

I can't really see running two pumps in series causing cavitation as long as the second pump is installed just after the boiler flow pipe as it exits the boiler, the boiler pump, a UPM2, is internally mounted? , not sure if its before the boiler HEX or after it but shouldn't matter as the oil boiler HEX has minimal resistance, if the EV is installed where it should be, before the boiler pump inlet/suction then the boiler pump head will be the EV pressure + the pump head, there is only a very slight chance of second pump cavitation if its mounted high up due to pipework resistance.

The small bore pipework will also control the flowrate.
I had a look at the pump curves, the UPM3 (on its own) might be circulating ~ 16.7LPM at 5.6M, if a UPS3 is installed as the second pump then I reckon that the flowrate might increase to ~ 22LPM with a UPM2 head of 4.62M & a UPS3 head of 5.62M, total, 10.22M, even if a UPS2 8M pump is installed as the second pump then a flowrate of ~ 25LPM with a UPM2 head of 4.39M & a UPS2 head of 8.0M, total 12.39M, apparently not a lot to be gained by installing a 8M pump but if a LLH has to be installed for whatever reason then the UPS3 is useless.