Possible central heating pump replacement

22 Dec 2005
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United Kingdom
I currently have a noisy central heating system.

The noise appears to be flow noise and there is no air in the system and the pipes and pump are properly secured etc. The noise is at its worst in the night when the heating is cutting in and out under the roomstat control in the hall and you can hear the noise throughout the upstairs of the house. It is particularly bad in the room adjoining the airing cupboard where the pump is located which happens to be our 1 year old son's room :(

You can distinctly hear when the motorised valve closes and the bypass comes into use. The flow noise is around the same volume but goes up in pitch and it just sounds like too much water is being pushed though the pipework.

The system was fitted around 3-5 years ago (before we bought the house) and consists of a gloworm ultracom 24 hxi which heats 9 radiators downstairs and 7 radiators upstairs in a 2 floor house. The system is sealed and all but 3 of the radiators are fitted with TRV's. The system is on S plan and has a bypass valve.

The pump is a Grundfoss 15-60 set to the intermediate speed setting.

My current thinking is that the pump is the cause of the noise in one way or other. There is either excessive flow around the system or the closing of the TRV's is causing too much flow through to few radiators and hence making the noise or the pump itself is on the way out. I had a similar problem at our previous house that was fixed by replacing the pump.

Firstly, is this pump the right pump for this size of system?

Secondly, can i experiment by turning the pump speed down without risking damaging the boiler. I would expect the boiler to modulate the burner in line with the water temperature in the heat exchanger so i'd have thought i can do this but thought i'd check. I'd expect this to reduce the noise but it will obviously also increase the heat up time for the radiators so it would only be a temporary solution to prove that it is the pump or excessive flow that is causing the noise.

Thirdly, would fitting a smart pump help with the flow noise? If so, would the Wilo A rated Smart pump be a suitable replacement. It can achieve the same flow rate as the current Grundfoss pump but gives a 4M head rather than the 6M head the the current pump can produce.
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Oh and the system is piped in standard 28,22 and 15mm copper piper. No microbore or small bore pipe.
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So can anyone tell me if a lower head pump like the wilo smartpump will be suitable?
Whole house website says boiler should be 25kw.

Is balancing likely to be involved when you consider that the volume of the noise remains the same when the motorised valves are closed and the pump is only circulating water through the bypass?
The 15 50 would probably be fine with a condensing boiler.

A smart pump would probably give considerable improvement.

What exactly is mostly causing the noise?

Tony Glazier
I can't pinpoint where the noise is coming from, its seems to reverberate through the upstairs radiators. But it sounds like flow noise.
Whole house website says boiler should be 25kw.
So it is a large poorly insulated property?

Did you work out your rad output? It could be relevant.

Is balancing likely to be involved when you consider that the volume of the noise remains the same when the motorised valves are closed and the pump is only circulating water through the bypass?
Yes, the noise is cause by a high flow rate, i.e high pump speed. The speed of the pump can often be reduced when a system is correctly balanced.

What are the actual flow and return temperatures (Parameters d.40 and d.41)?

Which auto bypass valve do you have and what is it set to?
I don't have the kit to measure flow and return temperatures.

I turned the pump speed down over the weekend and initially all seemed to be fine. The flow noise was substantially down but the radiators all seemed to heat up quickly and evenly across the house.

However, i think i either have a faulty bypass or it needs adjusting.

A few times over the weekend, when the motorised valves closed and the system went into bypass, the bypass valve made a noise not dissimilar from a trumpet.

The first few times i didn't understand what the noise was, but the last time i got to the airing cupboard and turned the pump speed back up. The 'trumpeting' stopped straight away.

I've not idea what make of bypass valve i have, but it looks like this:

I don't have the kit to measure flow and return temperatures.
You don't need any special kit. Dig out the Installation Manual and go to page 35 - Diagnostic Menu. This explains how to obtain various parameters. Parameter d.40 is the Flow temperature and d.41 is the Return temperature.

I've not idea what make of bypass valve i have
Don't recognise it from the pic. Are there any markings to indicate a part no etc?

Are there any numbers or markings etc to indicate the current setting? If so what is it set to?

The 15-60 is probably the correct pump for your installation as the system resistance will be fairly high, thanks mainly to the heat exchanger used in the 24hxi boiler.
Lower>> How did you solve your problem?

I think I have something very similar.

My system is 5 years old with Grundfos 15-50 + Glowworm 15hx, water pressure~1.2bar.

For the last 10 days or so I have had a vibrating noise coming from the pump and/or one (or more) of the copper pipes in the airing cupboards when the pump is running.
The pump has always been set on speed 3. If I reduce to speed 2 the noise marginally reduces. I can further reduce the noise slightly by holding some of the copper pipes (especially the top one on the image with the black box on top) together with the pump. But still very noisy.

There is no air in the system and at this stage I think the pump might be starting to fail causing the pipes in the vicinity to vibrate.
I would appreciate any advice.

If it is the pump then I would be looking for a high end ultra silent pump as replacement. Any suggestion?

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