CH Pump change

[MikeJhn]

My CH pump seems to be making more noise than ever before, it's not noisy as such and can only be heard when in the Bathroom, when in the Boiler House the noise is minimal, the thing is it's over thirty years old and possibly coming to the end of its life cycle, I recently changed the HW Zone valve (thanks Shaun Cobbs) as it was being bypassed and this got me thinking about the rest of the system, I have a new CH Zone valve to fit, but thought it may be time to update the pump as when originally installed it had standard valves and I have fitted TRV's on all radiators, do you guys think a more modern perhaps multi speed pump would be of benefit.

Oil fired Boiler open vent BF in its own Boiler House.
Serving fourteen radiators and hot water tank.
Loft mounted feed and expansion tank.
Remote pump.

Any other information needed?

Thanks
Mike

 

[SJB060685]

My understanding is and im sure someone will say otherwise but modulating pumps are not ideal for a fixed rate burner like the burner in your oil boiler. As the pump modulates it will reduce the system flow and decrease the flow through the boiler, potentially leading to a lot of on/off cycling of the boiler, or even sudden overheating tripping the overheat stat.
I would consider changing like for like and if you have TRV'S on all rads then a suitable by pass is needed.

 

[John.g]

IMO its the TRVs or/and zoning that really determines the boiler flowrate, reducing pump speed reduces head and flow but proportional pressure (PP) control is ideal for TRVs as it only reduces the head enough to ensure that the TRV's are open (much) further to give the same required flow but are still in control. I have a 20kw OF boiler with a pump set to PP control, what I find (similar to my old fixed speed pump) is that as the flow decreases the boiler deltaT (obviously) increases and the burner will cut out sooner at a lower return temperature but no problems with o/heat, no ABV fitted. OF boilers have one big advantage in this regard vs a non modulating gas fired boiler in that the Hx contains 20/25 litres of water which acts as a buffer to soak up excessive heat.

I certainly wouldn't change out a pump for any perceived savings but if you have to then you really can't buy a like for like because even though you can run all the A rated pumps on so called fixed speed or constant curve they all run at constant pressure (even though on "CC" setting) for quite a portion of their flow rate making it very difficult to set up a differential pressure ABV properly.

 

[MikeJhn]

Thanks for the replies guys, what pump would you recommend?

 

[John.g]

What pump make and model have you got and what is its present speed setting?.

 

[MikeJhn]

Hi its a Grundfos Superselctric 15/60 running on setting two.

 

[John.g]

Can you also see the three power outputs (watts) on the label, if available.

 

[MikeJhn]

Hope this helps.

 

[MikeJhn]

Going off line for a while as my OH has just opened a bottle of wine and called me for Dinner, will be back on later, so please don't think my ungrateful if I don't answer immediately.

Mike

 

[John.g]

Someone may have the pump curves for that Super Selectric but a
Grundfos UPS3 15-50/65 should do the job IMO, its a 6.5M, 60 watt, A rated pump which will pump at a rate of 33 LPM (2.2 M3/hr) at a 3.3M head running on fixed speed 2 (5.2M) which should service a 34 kw heating load at a deltaT of 15C or a 23 kw heating demand at 10C.

 

[MikeJhn]

Thanks for the info John most useful.