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Author Topic: How do you size a Central Heating Pump?  (Read 7785 times)
mpooley
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« on: May 18, 2013, 03:31:01 PM »

I know my pump is over sized as my ex plumber couldn't get my system to work properly without an over-sized pump.

I fixed it myself by increasing pipe sizes and re-routing pipes after he left. ( I wont be asking him back)

I have realised that this replacement pump is using a lot of electric (145w) on it's lowest setting so want to do replace it.
I'm hoping to get a ASHP system running next year so pump will be on a lot more.

What criteria should I use to size a new pump ? I am hoping I can afford one of those new A rated ones.

Thanks

Mike
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derekmt
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« Reply #1 on: May 18, 2013, 06:12:17 PM »

Get hold of the flow/head charactistic curves for your present pump. it should give separate curves for eachsetting.
You need a pump whose curve  on its highest setting matches the curve on current ones lowest setting.
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mpooley
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« Reply #2 on: May 18, 2013, 06:41:11 PM »

I'm quite well up on curves honestly but I'm not sure if that's the sort you mean Wink

forgive my ignorance but if the curves are the same wont the performance be the same and ergo the power usage ?

wouldn't that be the same as keeping the old pump?

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« Reply #3 on: May 18, 2013, 08:53:48 PM »

It is quite complicated to calculate the actual pump pressure needed to run any particular system, you have to know the resistance of the components for a given flow rate, and then match that to the pump curves. Of course that will only be accurate for a specific flowrate, but with the proliferation of TRVs on most systems that will only be a small percentage of the running time.
So..................... most domestic systems use a 3 speed pump which will encompass the various flowrates of the vast majority of systems.

You must have a peculiar type of pump if its minimum consumption is 145W, most pumps don't use that much on max, and on min would use about 40-60W. Most domestic size systems of 8 or 10 rads will happily run on the minimum pump power once balanced and bled properly, which is not enough to warrant a great deal of expense to reduce further IMHO.


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derekmt
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« Reply #4 on: May 19, 2013, 12:16:23 AM »

...
forgive my ignorance but if the curves are the same wont the performance be the same and ergo the power usage ?

...


in short no ...  A large pump trying to run at small volumes/heads can be quite inefficient and vice versa.
their are lots of factors in pump design that influence efficiency.

read this http://www.grundfos.com/content/dam/Global%20Site/Industries%20%26%20solutions/Industry/pdf/The_Centrifugal_Pump.pdf
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dhaslam
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« Reply #5 on: May 19, 2013, 12:29:31 AM »

There are two separate  requirements  for  a circulation pump.   The first is  the maximum head and that relates to the resistance to flow and that is greatly dependent on the pipe size.   Parallel pipes  don't increase pressure.  The  other  requirement is flow rate  and  mostly depends on  the overall  heat output  needed.    

http://www.johnhearfield.com/Water/Water_in_pipes2.htm
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mpooley
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« Reply #6 on: May 19, 2013, 01:08:23 AM »

Thanks
The pump is unusual in that it's a fair bit larger than any other domestic pump I have ever seen.

With my new ASHP (hopefully) I will have the system running 24/7 and I'm going to remove the TRV's ( i never liked them anyway because they don't seem to work!)
It's a large system with 21 rads although some are never turned on.

I'll have a look at those links Thanks.

Mike

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Iain
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« Reply #7 on: May 19, 2013, 07:01:25 AM »


Hi

Quote
I will have the system running 24/7 and I'm going to remove the TRV's ( i never liked them anyway because they don't seem to work!)
It's a large system with 21 rads although some are never turned on.

It does sound like there are some other underlying issues with the system that might need sorting first, there is not a lot to go wrong with TRV's, some of the older ones only like the flow in one direction but the newer ones will accept flow in either direction. I don't know if the bigger pump can be causing some of the problems or if there are other problems with the system.
Iain
Iain
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DonL
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« Reply #8 on: May 19, 2013, 08:36:26 AM »

I have just purchased a smart pump to go with the ASHP system I'm putting in and the main difference seems to be that rather than selecting a speed you select one of three bands within the pump curve. I presume this means that the pump modulates to give the same head as TRV's open and close rather than generating unnecessary pressure when flow drops.

When deciding to install an ASHP using the existing radiator system I opened Pandora's box. Because of improved insulation my existing radiators were over sized so I thought they would be big enough, but when I looked in detail they weren't. To get a decent COP you need to get the water temperature right down and I've ended up changing most of them. This need not be particularly difficult or expensive, you can often get higher thermal outputs by changing to double panel or going taller and still using the same supply and return pipes. Also, you need to keep the temperature drop through the system small so you need to have high flows so decent pipes and distribution. I've made sure that at maximum heat load the flow is sufficient to give a temperature drop of less than 4C.

I've actually ended up putting on more TRV's. The ASHP is basically controlled from the outside temperature and should adjust the radiator temperature to suit. This can be biased by a room stat. However, we have a log burning stove and many of the rooms benefit from solar gain and certainly will not need heat on cold sunny days whilst north facing rooms will. So if you want to automatically decrease your heating (and bills) you need to address this and the obvious solution is TRV's.

I suggest you think hard and long before going down the ASHP route and don't necessarily believe potential suppliers. I've found the level of expertise to be generally low and this was reflected in the performance levels reported by the Energy Saving Trust produced a while back. However, I'm convinced a properly designed and installed system can give good results.

Having said all that I'm still a theorist and will only get results when the system is commissioned next month!

Don
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« Reply #9 on: May 19, 2013, 08:44:17 AM »

Mike
Just realised I didn't answer your question. Calculate maximum heat load and then work out the water flow to dissipate that heat at a low temperature drop (less than 5C?? - any views from others) and this gives the flow. I'm afraid the only way of establishing the required pressure drop is by calculation and really this need to be done by looking at the flow and pressure drop through each loop and finding the worst case. There are plenty of methods around for doing this in more or less detail.
Don
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mpooley
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« Reply #10 on: May 19, 2013, 11:11:48 AM »


Hi

Quote
I will have the system running 24/7 and I'm going to remove the TRV's ( i never liked them anyway because they don't seem to work!)
It's a large system with 21 rads although some are never turned on.

It does sound like there are some other underlying issues with the system that might need sorting first, there is not a lot to go wrong with TRV's, some of the older ones only like the flow in one direction but the newer ones will accept flow in either direction. I don't know if the bigger pump can be causing some of the problems or if there are other problems with the system.
Iain
Iain
Maybe it's me Iain but I have had TRV's in all of my houses (4)  and they never have seemed to work to me . I don't think it's just this System.
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mpooley
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« Reply #11 on: May 19, 2013, 11:47:10 AM »

I have just purchased a smart pump to go with the ASHP system I'm putting in and the main difference seems to be that rather than selecting a speed you select one of three bands within the pump curve. I presume this means that the pump modulates to give the same head as TRV's open and close rather than generating unnecessary pressure when flow drops.

When deciding to install an ASHP using the existing radiator system I opened Pandora's box. Because of improved insulation my existing radiators were over sized so I thought they would be big enough, but when I looked in detail they weren't. To get a decent COP you need to get the water temperature right down and I've ended up changing most of them. This need not be particularly difficult or expensive, you can often get higher thermal outputs by changing to double panel or going taller and still using the same supply and return pipes. Also, you need to keep the temperature drop through the system small so you need to have high flows so decent pipes and distribution. I've made sure that at maximum heat load the flow is sufficient to give a temperature drop of less than 4C.

I've actually ended up putting on more TRV's. The ASHP is basically controlled from the outside temperature and should adjust the radiator temperature to suit. This can be biased by a room stat. However, we have a log burning stove and many of the rooms benefit from solar gain and certainly will not need heat on cold sunny days whilst north facing rooms will. So if you want to automatically decrease your heating (and bills) you need to address this and the obvious solution is TRV's.

I suggest you think hard and long before going down the ASHP route and don't necessarily believe potential suppliers. I've found the level of expertise to be generally low and this was reflected in the performance levels reported by the Energy Saving Trust produced a while back. However, I'm convinced a properly designed and installed system can give good results.

Having said all that I'm still a theorist and will only get results when the system is commissioned next month!

Don
Don I have been testing how my system works at low water temperatures all winter. I have computer control of my boiler, pump and zones with temp readings in all rooms so it's been quite easy to do.
I have found it to work well at a minimum of 43c flow temp with the system working 24/7 with a couple of degree setback overnight.
As we are retired this suits us very well.

To keep the house warm with these water temps it does need to keep the house from cooling down too much. So I know that it works well. In fact we have never felt so comfortable. I will increase sizes of a few of the rads so maybe I'll be able to cut the water temp down a bit more I'm not sure.

I'm terrible at maths and don't have much idea about the technical side. I have no idea how to check pressures, flow rates etc but obviously I need to get up to speed on that to choose my new pump wisely.
I don't really understand what " select one of three bands within the pump curve" means lol

I cant change my pipework any more than I have done to improve circulation but what I did find was that this system was installed by an idiot!
So I had to increase the main pipes to the distributor as they were in 15mm !! , I also increased pipe size to furthest away rads but this was all that was possible without major concrete floor upheaval! I also increased pipes from Boiler to 28mm. 

I understand that ASHP systems need a good flow rate to shift the low grade heat. So i did think maybe it's best to keep my present pump until I did some overnight testing of my electric consumption because it was so high and found out it was nearly all down to the pump.
Thats one reason I decided to get rid of the TRV's though as I read they can cause problems if they all start shutting down.
I'm not sure of the technical reasoning though ?

I looked at the type of pump you talked about as they are supposed to save about 80% of electric. They look very expensive though!
what did you pay for yours?
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mpooley
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« Reply #12 on: May 19, 2013, 11:49:57 AM »

Mike
Just realised I didn't answer your question. Calculate maximum heat load and then work out the water flow to dissipate that heat at a low temperature drop (less than 5C?? - any views from others) and this gives the flow. I'm afraid the only way of establishing the required pressure drop is by calculation and really this need to be done by looking at the flow and pressure drop through each loop and finding the worst case. There are plenty of methods around for doing this in more or less detail.
Don
Thanks Don
I think I know the maximum heat load so I need to find out how to measure flow and pressure drop then.  Sad sounds like it means some dismantling ? Sad
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It's not easy having a good time. Even smiling makes my face ache.

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DonL
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« Reply #13 on: May 20, 2013, 09:02:25 PM »

Hi Mike

I really like your approach of using the existing boiler to simulate the ASHP performance. You really know what will work; calculations are all very well but things like air infiltration tend to be little better than a guess. I did a test run turning my boiler down as low as it would go to check the calculated version.

With the radiator changes I expect to need a radiator temperature of 43C for an outside temperature of -3.4C so we're in the same ballpark.

You could do a couple of tests that would help I think. If you measured both supply and return temperature using a contact thermometer with the system running at steady state you'd know if the flow was adequate with the existing pump. If you measured the pump exit pressure and subtracted the inlet pressure (height to header tank if you have one) and then look at this on the existing pump curve you could get the flow rate. This also gives you a pump duty to use to specify a pump to replicate the performance of the existing one or a basis to specify a different performance.

The pump I bought claimed to be "smart" but I didn't go the whole hog and pay the large amount of money for an A rated one. I bought a B rated one from S******x for about £60.

Must go - bath going cold.

Don
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mpooley
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« Reply #14 on: May 20, 2013, 11:57:24 PM »

Hi Mike

I really like your approach of using the existing boiler to simulate the ASHP performance. You really know what will work; calculations are all very well but things like air infiltration tend to be little better than a guess. I did a test run turning my boiler down as low as it would go to check the calculated version.

With the radiator changes I expect to need a radiator temperature of 43C for an outside temperature of -3.4C so we're in the same ballpark.

You could do a couple of tests that would help I think. If you measured both supply and return temperature using a contact thermometer with the system running at steady state you'd know if the flow was adequate with the existing pump. If you measured the pump exit pressure and subtracted the inlet pressure (height to header tank if you have one) and then look at this on the existing pump curve you could get the flow rate. This also gives you a pump duty to use to specify a pump to replicate the performance of the existing one or a basis to specify a different performance.

The pump I bought claimed to be "smart" but I didn't go the whole hog and pay the large amount of money for an A rated one. I bought a B rated one from S******x for about £60.

Must go - bath going cold.


Don
Thanks Don I get constant Flow and return temps graphed so I'd find that easy if I knew what you meant  wackoteapot
how would I know if flow was adequate?

surely i know that just by the fact i'm warm ..... don't I?   wackoold

How do I measure the pump inlet and exit pressure?

Mike

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It's not easy having a good time. Even smiling makes my face ache.

“Science is the belief in the ignorance of experts.” Richard Feynman
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