Pump Speed - Does it matter?

[roverblue]

Hi all,

After my experience of doing maintenance on my open vented system learnt quite a lot about balancing rads, pipework, sludge etc.

One question I was wondering is the speed at which the pump pushes the water around my system. Which is better?

a) To have the pump slowly ease the hot water around and back to the boiler, with valves upstairs balanced.

b) To have the pump on high speed pushing water around with valves open.

I currently have the latter while sludge cleaner is in my system and it feels like i get more heat from my rads.

Thanks for any replies in advance!
Rover Blue

 

[D_Hailsham]

The right pump speed is the one which will provide the correct amount of heat to each radiators:

1. At max boiler output;
2. With the correct flow/return temperature differential;
3. When all radiators are correctly balanced.

You have to balance all radiators, not just the upstairs ones.

A correctly balanced system will often need a lower pump speed.

 

[dcawkwell]

Pump speed 2 usually does most 7 radiator houses.
If driving more radiators you need speed 3.

Speed 1 is usually only if driving a small number of radiators.

 

[D_Hailsham]

Pump speed 2 usually does most 7 radiator houses. If driving more radiators you need speed 3. Speed 1 is usually only if driving a small number of radiators.

It all depends on the system and how well it has been balanced.

I have 13 radiators, ranging from ½kW to 2kW (total 13kW) and the pump (15/50) is only set to speed 2.

That's probably because the rads are now considerably oversized as the house insulation has been improved since they were installed 20 years ago; so I can run the heating with a 15-20C differential, which means a lower flow rate and therefore a lower pump speed.

 

[Agile]

In theory the pump speed should be set to provide the chosen flow/return differential.

Installers always had the option of using the pump valves to set the differential but virtually none ever did that.

The pump makers could easily have provided pumps with variable speed outputs but did not follow that option.

However, from the beginning of this year all pumps had to be higher efficiency which has mostly meant permanent magnet DC motors.

They mostly have more sophisticated options but at the cost of a higher purchase price. The power consumption is very much lower and can save a lot on electricity bills! Often 2-3 times their cost!

When this catches on in a year or so I expect there will be a lot of promotion to replace pumps to save energy costs.

Unfortunately as with all new products, there is an unknown reliability issue. Not just breakdown of the electronics/mechanics but the problems of magnetic oxides from the heating system being attracted into the pump and either jamming it or wearing out the bearings.

Tony

 

[D_Hailsham]

However, from the beginning of this year all pumps had to be higher efficiency which has mostly meant permanent magnet DC motors.

Don't you mean permanent magnet AC motors?

After all, the change of speed is obtained by changing the frequency of the voltage applied to the motor.

Unfortunately as with all new products, there is an unknown reliability issue. Not just breakdown of the electronics/mechanics but the problems of magnetic oxides from the heating system being attracted into the pump and either jamming it or wearing out the bearings.

They may be new to the domestic market, but the Grundfos Magna has been around for many years in commercial applications.

 

[gaswizzard]

Installers always had the option of using the pump valves to set the differential but virtually none ever did that.

NEVER use pump isso valves in order to reduce flow rate , i've pointed this out to you before Tony.

 

[Agile]

I dont remember seeing you or anyone else saying that.

Whilst I have never done that yet it seems a perfectly possible method.

Why shoule it not be used? Technical reply please!

Tony

 

[DP]

Technical question for tony.

1) how do you run a DC motor from the mains voltage

2) why would you want to restrict the pump valves when the speed selector will do the job?

 

[Agile]

Pump valves would give a fully variable control ( but only to a fixed load !!! )

Pump speed control ( whilst not an exact constant speed if loading varied though ) creates a more constant head.

Tony

 

[gaswizzard]

Whilst I have never done that yet it seems a perfectly possible method.

Your previous post seemed to suggest you use isso valves to restrict flow rate although you've not seen it being done.

Why shoule it not be used? Technical reply please!

Tony

I haven't the time to reply but it should of been explained when you took your heating course/s (not sure you did though).

Look up NPSHA/NPSHR.

 

[gaswizzard]

Pump valves would give a fully variable control ( but only to a fixed load !!! )

Pump speed control ( whilst not an exact constant speed if loading varied though ) creates a more constant head.

Tony

Tut tut.

 

[Agile]

To run a DC motor from an AC supply you rectify the AC and use switch mode power supply to produce a lower higher current DC voltage.

Since the desire was for greater power efficiency then I have always expected a heating pump would be using a low voltage DC permanent magnet motor because I have always expected them to be more efficient.

But of course manufacturing costs are important too.

I have not had the opportunity to examine the current range yet and see how they are achieved.

The first common permanent magnet heating pump in common use was the Askol and used by Biasi about eight years ago with a run of about 300 boilers sent to the UK.

They were very unreliable and that was never repeated by Biasi ! For a while they ran out of replacements due to the number of failures during the warrantee.

I think they were low voltage DC !

Tony

 

[DP]

Tony you are wasting you time telling me how to convert ac to dc. I cut my back teeth on caps, tx, 7805/7812/723 etc. The reply I expected was an educational one. Alas that is not the case

Hard to imagine a grundfos connected to mains voltage with a dc supply fitted within and water leaking onto the electronics

 

[Agile]

Hard to imagine a grundfos connected to mains voltage with a dc supply fitted within and water leaking onto the electronics

Why should that be significantly different to water inside the standard Grundfos pump which we all see regularly?

Tony