Heating circulator incorrectly sized?

7 Jan 2009
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United Kingdom
The house i am moving into has a sealed central heating system with a regular condensing boiler which is a British Gas 330 (Glow-Worm HXI?). The main heating ring main pipework is 28mm and the it has 16 radiators all teed into the ring main on 15mm pipework. All the radiators except for the hallway where the roomstat is have TRV's, and there is also an auto bypass valve between flow and return.

Currently, the upstairs radiators at the far end of the house (4 of them) never get fully hot like the rest of the radiators. The system is 5 years old and all the radiators were brand new and fitted at the same time as the boiler.

The pump is a Grundfos 15-60 standard pump and the boiler has been range rated to 24w with a temp differential of 12 degrees between flow and return, according to the BG commissioning paperwork

In the manual for the BG 330 boiler, it says that at the minimum output of 18kw the flow rate is 773.8 litres per hour with a 1.00 meter head, and at the top end of 31kw the flow rate is 1289 litres per hour with a 2.55 meter head to be able to maintain a max of 20 degrees differential between flow and return.

If i enter this data on the Grundfos pump size calculator, it comes up with a recommended pump as the Grundfos Alpha2 25-40. Would that sound right to you?

My thinking is that the 15mm nominal size of the pump inlet and outlets with the 15-60 pump is potentially causing too much of a restriction. would anybody agree with that?
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The heat exchanger on the 330 has a huge resistance. I forget what the actual figure is now but a 15-60 won't have a huge amount of head left.
And yes you are correct in thinking that the 15mm opening in the pump will offer its own restriction, but the waterways through the HEX are considerably smaller than that
I've not used the grundfoss pump calculator before does it ask for boiler info? Or required flow rate etc.
If it doesn't know what boiler(heat exchanger) is there it can't know the resistance and as such I'd ignore the information it provides
the calculator asks for required flow rates which i input based on the flow and head graph in the boiler manual.
According to the graph set at 30kw req flow rate of 1289ltrs/hr you will loose 2.55m head?

I had a figure in my head more around the 4m not sure where that came from though.

Ultimately as its a sealed system fitting a bigger (more powerful) pump can't really cause any issues other than possibly system noise.

Do these non working radiators work if you turn off some others. Have u tried balancing
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The boiler is expecting a differential, nearer to 20C than the 12C which BG has used. The effect of lower differential is to increase the pressure loss across the boiler by a factor of about 3, so the pump has much more work to do.

You are putting the wrong info into the Grundfos calculator: The head required is the total head (pressure loss) of the system, not just the pressure loss through the heat exchanger.

Grundfos do know what they are doing.

The system needs balancing with a higher differential. This will reduce the output from the rads but, knowing BG, they are probably oversized.
How would one work out the overall head of the system? (In the abscense of this info from commissioning reports and design spec)
@bhm 1712,

Firstly, that is a great system you have fitted.

There are things to consider with such full loop systems...

1. An auto bypass is not required, if any are fitted they must be set not to function, because a greatly varianced flow factor at the boiler is undesireable for full flow loops.

2. Ideally, theoretically and practically, the boiler is the biggest flow resistance and should be placed first downstream *after* the pump, this would give a progressively lowered flow resistance for subsequent lesser resistance items eg: rads valves etc' along the line of flow as having the highest resistance lastly would/can greatly influence (reduce) the required flow rate through the boiler even further.

Obviously, after meeting these rquirements, the system must be finely balanced as has been mentioned.

Out of curiosity, where is your pump fitted in the flow resistance line?

1. Because it is a ring main, full flow loops have no need for bypass, re: -> "16 radiators all teed into the ring main" - with all trv's closed or almost closed, a ring main still has full flow returning to the boiler. Full flow loops extend the life of the boiler as there is little flow restriction and consequently reduces kettling and calcification problems etc.

2. As said, greatest flow resistance first, lower resistances second prefered for good design practise where full 'open' loops and also 'primary' or 'control' loops are concerned.

@bhm 1712

Start up boiler, close all rads and check first that the circulation return temp shuts off the boiler on stat, then open the furthest of the 4 rads and see if boiler comes on the rad gets hot, open any lock shield restriction on this rad then report back.

PS: ensure bypass valve is shut

Last edited:
I need to apologise for sending you down the wrong path. My description of the system as a ring main is wrong. What I meant was that the main system pipe work - the flow and return is 28mm. It is a 2 pipe system and the 2 pipes are only joined by the radiators and the auto bypass valve so most definitely not a ring main.
It might be 28mm from the boiler but probably reduces along the route to/from the rads. You just need a "magic Hand"to feel the rads and a spanner to adjust the lockshields and balance the system. Worked for me on a 30 room annexe to a hotel I worked in. That was fed off a 4inch constant temp. loop;)

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