Circulation pump

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Gravity hot water. Gas condensing boiler. 4 bed detached.

Radiators balanced. System bled. House slow to heat. Low return temp to boiler. Pump intermittent cavitation.

I would appreciate some advice on trouble shooting my central heating. It's slow to heat up and the pump is intermittently very noisy once the system heats up.

My central heating circulation pump is making what sounds like a cavitating noise. The noise is intermittent and starts after the system has been on for a while. See attached video. The pump is quite old (British Gas Multihead G, part number 59506180). Maybe twenty years old. The speed is on the highest of its three settings.

I have 15 radiators and a mix of 15mm and 10mm tails. The main flow and return lines at the boiler are 28mm. I've recently balanced the radiators as some weren't heating up at all. All the radiators now heat up at the same rate. There are no cold spots on the radiators. However, the house heats up very slowly. It increases in temperature by about 1°C per hour. So it takes around 5 hours to go from 14°C to 19°C. It struggles to get above 20°C (4 bed detached).

The boiler is a Worcester Greenstar 24 Ri. I don't know its age, but I would guess around ten years old. It's rated at 24kW. I've monitored the gas consumption at the incoming gas meter and it is only consuming the equivalent of around 12 kW when the system should be running at full load to achieve the thermostat set point temperature (20°C).

The flow temp is set to max on the boiler. I haven't measured the temperature on the flow line, but from touch it seems around 70°C. The return line is around 15°C. That indicates a flow rate of around 3 litres/min (0.05 litre/sec), assuming 12 kW and a dT of 55°C.

I appreciate I could have a few problems but I was hoping for some direction on where to start. The boiler is consuming around 1/2 of its rated capacity. The boiler is serviced annually so fouling shouldn't be that bad. There is a filter on the return line and the water out of the bleed points looks clean.?

If the pump isn't delivering the mass flow then the boiler can't run up to capacity otherwise it would overheat the water.

The pump may not be delivering the flow rate because it is faulty or the system resistance could be too high? The last radiator in the circuit is full open, but I had to close off the lock shields on the other radiators quite a bit to balance the system. So the system resistance could be significant.

Do I take a chance and change out the pump. E.g. for a more modern Grundfos UPS3 15-50/65. The UPS3 has a greater flowrate at a higher head so could help if the system resistance is the issue.

I'm not sure a new pump would solve my cavitation issue though? The system is vented so I can’t increase the pressure other than to fill up the header tank in the loft, which hasn’t solved the cavitation.

Any help appreciated.
 

Attachments

  • IMG_2730_small.zip
    1.6 MB · Views: 26
Last edited:
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Is the boiler running constantly while heating the house, or does it run for a bit, stop for a bit then start running again? If so it points to a circulation problem. The boiler can't get rid of the heat fast enough, so overheats, turns itself off and waits until the water cools and then starts again. The big difference between flow and return temperatures reinforces this possibility.
 
The boiler seems to be firing constantly. 12kW is 50% turndown. Perhaps the boiler can run continuously at 50% load. The manual doesn’t give a max turndown.
 
I’ve turned down the flow temp by around a 1/4 on the boiler temp control knob tonight. There is no cavitation tonight. Of course the house is heating up even slower but at least it’s quieter.
 
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Suggest removing the pump head (after isolating) and inspect pump impeller and ports for obstruction, a tie wrap can check/ clear the impeller vanes. I would not, under most circumstances, install any A rated pump that doesn't at least display the power in watts, W. Most now, also display the flowrate in m3/hr, the UPS3 , while a powerful pump, displays neither, suggest a 6 M DAB Evostata3, but not model 2, or, like my own, a 6M Wilo Yonos Pico, both of which display the power and flowrate.
I'm surprised the boiler keeps firing with dT of 55C.
 
Thank you for the advice Jon. The pump is installed in an awkward position. I’ll have a look at access to the pump head bolts. I may need to take it out either way for a look.

The pump is in a noise sensitive area. I can't find noise ratings for either the evosta 3 or the wilo yonos pico. The ups3 is rated at "lower than 32 dB(A)" according to its datasheet, so I shouldn’t hear it running in a closed airing cupboard from the outside. Your point on flow rate and power feedback is a good one. The performance feedback would be helpful to troubleshoot the system. How noisy do you find your pico?
 
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The Wilo is very quiet and can be incrementally set in 0.1M steps to give almost infinite settings in both CP & PP modes, also has the traditional fixed speed settings at 2M, 3.8M & 6M.

Can you look into the F&E Cistern and see if the system is pumping over through the vent, or watch it while someone starts and stops the boiler. If you have the traditional system where, from the boiler flow, you have the Vent then the Cold feed, (no more than 150mm further on) then the pump, you may find a blockage/restriction where the cold feed is teed in.
 
Thank you, I'll check the F&E at system switch on.

Just curious on your pump recommendation, why did you recommend the DAB Evosta 3 over the Wilo yonos pico that you currently have?
 
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The WILO is probably a better choice for systems with TRVs since you can set the PP head setting to match the head/flow demand with all TRVs opened.
 
I went ahead and ordered an Evosta 3 before I saw your latest reply. It's due to be delivered today.

I did the test on the F&E cistern. I turned the central heating off and waited for the pump to stop. I then watched the cistern and turned the system on. There was no pumping over through the vent.

My next step is to swap out the pump I think. At least the new pump should give me more data on flowrate, head, and power to troubleshoot if it doesn't solve the problem with the system.
 
What sort of head would you expect in a 4 bed detached house with 15 radiators?
 
15 rads at say a flowrate of ~ 15LPM, 0.9M3/hr will probably require 3.0 to 3.5 M with a clean system, have a look the pump curves, maybe PP3 at 4.7M if you have TRVs
or CP2 or CC2 at 3.0M?
 
I replaced the British Gas Multihead G circulation pump with the DAB Evosta 3 at the weekend. The system is up and running. I’m waiting until the air is bled out before testing the different pump running profiles.

I’ve attached photographs from the replacement, including the pipework and the removed pump. I’ve also included a photo of the new install.

The pipework and pump appeared to have a bit of stuck brown mud cakes up inside. The photos maybe make it look worse than it was because the flanges were caked, but the pipe itself wasn’t blocked off too badly. The silt was inside the flowpath of the pipework and pump. I’ve yet to open up the old pump to inspect it. I managed to clear most of the silt from the immediate pipework with a screwdriver and my finger. I’ve included photos of before and after I cleaned it the best I could.

The bottom of the F&E tank had a similar silt. I completely cleaned out the F&E tank before I did the pump change as I didn’t want any of it getting into the new pump when I refilled the system.

After an hour or two of running on the new pump, the return line to the boiler still seems to be luke warm. I had the new pump on proportional speed 2, not 3 though. I’ve yet to try the faster speed setting.

I have a Spirotech MB2 filter on the system just before the boiler on the return line. I purged the filter before I part drained the system to replace the pump. A few black particles and brown murky water came out. But it quickly ran clear. I part drained around 6.5 litres from the system for the pump change. The water was clear (see photo of orange bucket).

What is confusing me is that the water is clear but the pipes and the pump look like they were clogged (or partially clogged). I need to keep testing and find out what the system head and flowrates that I am achieving now are. However, does it look like I may need to flush with something like X400(?) to get confident that I’ve not got any legacy build-up inside the system that could be affecting my system resistance?

Any tips on things I should try next?
 

Attachments

  • pump change.zip
    5.1 MB · Views: 28
I’ve got instantaneous system data for proportional control speed mode 2 and mode 3 from the Evosta 3 faceplate.

Mode 2:
Power: 8.1 W
Head: 1.8 m
Flowrate: 0.3 m³/h

Mode 3:
Power: 22 W
Head: 3.8 m
Flowrate: 0.7 m³/h

Any thoughts on the data? I reckon I need 1 m³/h to deliver the 24 kW rated boiler heat input with a 1 m/s flow velocity. So at 0.7 m³/h I'm a little low. I could try the fixed speed mode 3 but I don't want the additional noise / load. I think the next step is to try and clean the system with an additive like X400?

I’ve also spotted brown silt in my F&E tank. I fully cleaned it before the pump change. I must have gotten carry over from the vent? I need to do more start-up testing to see if it just happened when I first started the pump and perhaps whatever blockage / restriction has now cleared.

Is the next step to run the system with X400? What is the best approach? E.g. how long should I run it in the system for (days? Weeks?)? Is a once per day purge from the magnetic filter enough, followed by the system drain down at the end of the run?

I have quite a few “U” sections where quite a few downstairs radiators are plumbed from above. I’d need to drain those individually which would be quite a bit of effort. Do I have to do a full drain down of the system after running with X400?
 
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