How do I size up the pump required for 56kw oil fired fully pumped ch. 20 rads, large 5 bed, 2 storey house? The pipes and rads are about 20 years old and the rads furthest from the boiler take a long time to get up to temperature. The boiler is three years old. The system is reasonably well balanced after recent powerflush and pipes are 1inch tapering down as the load decreases. When we moved in, 2 years ago, the previous owner had recently replaced the pump with what looked like the smallest domestic Wilo he could find, I replaced it with a larger Wilo 50 now I am thinking I may need something bigger, a Wilo 60 or to go for a light commercial type maybe. The boiler is too large for the house I know and it starts to cycle even before the furthest rads are even warm, hence thinking it would take a larger pump if thats the way to go, or do I spend more time balancing the nearer radiators? Any help appreciated.
Replacing CH pump
- Thread starter hottotrot
- Start date
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Which boiler do you have (make and model)?
Do you know what size boiler you should have? If not, use the Sedbuk Recommended Boiler Size Calculator to find out.
Do you know the total heat output of your radiators? If not use the Stelrad Elite Catalogue to estimate what it is.
My first comment is that the boiler is likely to be seriously oversized and probably quite inefficient being used that way.
Its difficult to guess at the heat loss of your house as so much depends on insulation etc. but a detatched five bed house is usually about 16 kw with cavity walls and about 24 kW without.
Then there is the question of the heat output of the rads! Without seeing them I would guess at 1.5 kW each which would give 30 kW but that would be far higher output than I would have expected.
In any case, I would expect that a 50 pump would be almost adequate but in view of the difficulties you have mentioned changing it for a "60" is cheap and easy. Dont expect a massive difference though.
A better appreciation of how its performing would be obtained by a thermal survey.
Its also possible the water heating is bypassing the heating circuit. I would almost be inclined to fit the recovered "50" pump for the hot water circuit as your boiler can easily handle that. Then the "60" pump would be able to give all its output to the heating.
Otherwise fit a gate valve in the hot water circuit to better balance the flow to the CH and HW. You can get an idea of what effect this is having by comparing the heating when the HW is on and off. That assumes you have proper controls and can turn the HW off independently.
Tony
Its difficult to guess at the heat loss of your house as so much depends on insulation etc. but a detatched five bed house is usually about 16 kw with cavity walls and about 24 kW without.
Then there is the question of the heat output of the rads! Without seeing them I would guess at 1.5 kW each which would give 30 kW but that would be far higher output than I would have expected.
In any case, I would expect that a 50 pump would be almost adequate but in view of the difficulties you have mentioned changing it for a "60" is cheap and easy. Dont expect a massive difference though.
A better appreciation of how its performing would be obtained by a thermal survey.
Its also possible the water heating is bypassing the heating circuit. I would almost be inclined to fit the recovered "50" pump for the hot water circuit as your boiler can easily handle that. Then the "60" pump would be able to give all its output to the heating.
Otherwise fit a gate valve in the hot water circuit to better balance the flow to the CH and HW. You can get an idea of what effect this is having by comparing the heating when the HW is on and off. That assumes you have proper controls and can turn the HW off independently.
Tony
Hi guys,
Thanks for your quick replies. Rough calculation is about 40kw. 19 rads all high water content double panel and one towel rail. It is a large country house 5 double bedrooms, 4 bathrooms etc. The domestic hot water is currently timed to come on only after the house is up to temperature as we have a large megaflow to store it all. It does has a proper by-pass and balancing gate valve though. The heating circuit is in basically three loops. The problem rads (largest is 3metres long 600mm high double) are the furthest from the boiler, some 25 metres distance, so the poor old pump has to cope with a round trip of 50 metres+ The last two radiators alone are 8.5kw approx - it is a big system.
Thanks for your quick replies. Rough calculation is about 40kw. 19 rads all high water content double panel and one towel rail. It is a large country house 5 double bedrooms, 4 bathrooms etc. The domestic hot water is currently timed to come on only after the house is up to temperature as we have a large megaflow to store it all. It does has a proper by-pass and balancing gate valve though. The heating circuit is in basically three loops. The problem rads (largest is 3metres long 600mm high double) are the furthest from the boiler, some 25 metres distance, so the poor old pump has to cope with a round trip of 50 metres+ The last two radiators alone are 8.5kw approx - it is a big system.
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As said earlier, the critical piece of info is missing, what boiler?
If the boiler is oversized, it is easily downrated using a different nozzle and recalibrating the oil burner settings.
If you knew the boiler resistance you could calculate the ideal pump for the unit.
However it is likely that the house was extended (they normally are when these type of problems occur) the pipework to the remote ends is probably undersized.
You do not say whether you have a sealed or open vented system. If you have an open vented system then the system may have been pumping over causing the need for a second powerflush (one must assume that the first was done when your boiler was changed out 3 yrs ago).
If you have a sealed system the only disadvantage to a larger pump eg: Grundfos 25-55 would be noise. If your system is open vented you could precipitate pumping over in the tank, which would be a disaster in terms of sludge generation.
If you find the pipework circulation problem appears insurmountable and it is not easy to renew, Honeywell Hometronic might be the solution, but you would need a local specialist to advise you.
My first stages would be to check the boiler sizing (this will not cure the cold rads).
Then I would see if the system could be converted to sealed, and I would check to see if there is a gravity HW circuit open causing a bypass.
Finally, it is a larger flow rate pump, a repipe or Hometronic, in that order.
If the boiler is oversized, it is easily downrated using a different nozzle and recalibrating the oil burner settings.
If you knew the boiler resistance you could calculate the ideal pump for the unit.
However it is likely that the house was extended (they normally are when these type of problems occur) the pipework to the remote ends is probably undersized.
You do not say whether you have a sealed or open vented system. If you have an open vented system then the system may have been pumping over causing the need for a second powerflush (one must assume that the first was done when your boiler was changed out 3 yrs ago).
If you have a sealed system the only disadvantage to a larger pump eg: Grundfos 25-55 would be noise. If your system is open vented you could precipitate pumping over in the tank, which would be a disaster in terms of sludge generation.
If you find the pipework circulation problem appears insurmountable and it is not easy to renew, Honeywell Hometronic might be the solution, but you would need a local specialist to advise you.
My first stages would be to check the boiler sizing (this will not cure the cold rads).
Then I would see if the system could be converted to sealed, and I would check to see if there is a gravity HW circuit open causing a bypass.
Finally, it is a larger flow rate pump, a repipe or Hometronic, in that order.
Hi Simon,
It is a Mistral 56kw according to the plate. It is a closed system and yes the house has been extended. The house was professionally redecorated throughout since the boiler change and the radiators were removed so I don't believe the system was re-inhibited when they were put back.
For my part, I have replaced the pump, powerflushed and inhibited. Given the boiler is only three years old and maybe not working at optimum efficiency, I'll have to live with that, I thought a larger pump could cure the problem hence my question on sizing.
Regards,
Dennis
It is a Mistral 56kw according to the plate. It is a closed system and yes the house has been extended. The house was professionally redecorated throughout since the boiler change and the radiators were removed so I don't believe the system was re-inhibited when they were put back.
For my part, I have replaced the pump, powerflushed and inhibited. Given the boiler is only three years old and maybe not working at optimum efficiency, I'll have to live with that, I thought a larger pump could cure the problem hence my question on sizing.
Regards,
Dennis
I doubt the lack of inhibitor is the culprit, you need regular water changes to create a significant sludge contamination.
As it is sealed, a larger pump won't do a lot of harm.
I suspect the house extension is the culprit and the cooler rads are in the newer bit.
Hometronic does not require any redecoration but is not cheap, if you google it you will find a few sites (mine included) describing how it works.
As it is sealed, a larger pump won't do a lot of harm.
I suspect the house extension is the culprit and the cooler rads are in the newer bit.
Hometronic does not require any redecoration but is not cheap, if you google it you will find a few sites (mine included) describing how it works.
Wrong Question - deleted.
What was I thinking of?
Start again
More details of the boiler model, please, as there are several Mistrals about that size.
8.5 kW would need a 22 mm pipe to feed that power.
Even 4.25 kW is the limit of what a 15 mm pipe can carry.
Its possible that careful balancing can improve the situation and a larger pump than a "60" would be the correct choice but its surprising just how well a "60" can manage even when its undersized.
Can yuo measure the flow and return temperatures at the boiler end and on each of the problem rads?
Tony
It all depends on what temperature drop is used.
A 22mm pipe can easily carry 24kW if the drop is 20°C. But if the drop is only 11°C , then 12kW is about the max.
Similarly for a 15mm pipe. At 20°C the capacity is about 11kW but only 6kW for an 11°C drop.
If a 15mm pipe is only carrying 4.25kW the water velocity will be about 0.64 metres/sec, which is on the low side as sludge is liable to settle at this sort of velocity.
Hi Guys,
Some more facts. Outside temp now -5deg C. Boiler flow is 55 rising to 65 when it cycles, return is 45 not rising much more. All rads have trv's except two close to the boiler. These are well balanced. The problem rads after a long time reach 55 to 60 flow with return about 35 to 40. I think this demonstrates that the pump is not big enough to feed the demand. I don't think it matters on the specific model of boiler - its too big I know, but I can live with that. My original question remains unanswered, how do you size a pump? I know you can calculate using load, water temps, pipe sizes, number and quanity of fittings, radius of pipe bends etc, surely there's a rule of thumb somewhere? Spill the beans, how do YOU size up a pump for a large house?
Thanks for the replies so far - all good stuff.
Regards,
Dennis
Some more facts. Outside temp now -5deg C. Boiler flow is 55 rising to 65 when it cycles, return is 45 not rising much more. All rads have trv's except two close to the boiler. These are well balanced. The problem rads after a long time reach 55 to 60 flow with return about 35 to 40. I think this demonstrates that the pump is not big enough to feed the demand. I don't think it matters on the specific model of boiler - its too big I know, but I can live with that. My original question remains unanswered, how do you size a pump? I know you can calculate using load, water temps, pipe sizes, number and quanity of fittings, radius of pipe bends etc, surely there's a rule of thumb somewhere? Spill the beans, how do YOU size up a pump for a large house?
Thanks for the replies so far - all good stuff.
Regards,
Dennis
Most installers don't bother sizing the pump! They just fit a Grundfos 15-50 or 15-60, depending on their gut feeling.
To do the job properly you need to calculate the index circuit, this is the one with the greatest head (resistance) and know the designed flow rate through the boiler.
Alternatively you could go to Grundfos Web Site and use their Webcaps Pump Sizer. But it's not easy to use if you are not familiar with the procedure.
If you want to have a go at calculating the pump size yourself, read Copper Tubes in Domestic Heating Systems
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