Central heating pump sizing

[Last Plumber]

Thanks

I suspect that adding the by-pass to your system would (if set weak enough) prevent the delay/waiting code from being activated because you would likely increase the return temperature to something the boiler wants to see. However, you would still have a circulation problem and I personally think the anti-cycling timer would come into play more.
These are just my thoughts on what I have read regarding your heating system.

 

[John.g]

What about taking the TRV rads out of the picture altogether by noting the TRV settings on each and then just shutting them and just run the boiler on the other 4 rads, the heating demand from them may still be > 5 kw to allow the boiler to run continuously if the boiler flow temp is 70/75C, this will also give a reasonable feel for the flow rate and deltaT?.

 

[Reddwarf4ever]

What about taking the TRV rads out of the picture altogether by noting the TRV settings on each and then just shutting them and just run the boiler on the other 4 rads, the heating demand from them may still be > 5 kw to allow the boiler to run continuously if the boiler flow temp is 70/75C, this will also give a reasonable feel for the flow rate and deltaT?.

Hi
if I shut off some TRV rads, won’t that further restrict the flow through the boiler ?

 

[Last Plumber]

Hi
if I shut off some TRV rads, won’t that further restrict the flow through the boiler ?

Here is a few lines from post 32: -

Are all your lock shields open?
Is the pipework off the top of the boiler vented properly? (no possibility of air lock in flow or return).
Are the pump valves fully open and clear?
Once the obvious has been ruled out, look for partial blockages.

Is there any reason that doesn't make sense? Am I missing something or have I missed something you said earlier?

 

[John.g]

4 rads with wide open valves should have no problem in circulating adequate water through the boiler, if the deltaT is again too high then IMO there is restriction somewhere or possibly a pump problem if set to speed 2 or speed 3.

 

[Reddwarf4ever]

4 rads with wide open valves should have no problem in circulating adequate water through the boiler, if the deltaT is again too high then IMO there is restriction somewhere or possibly a pump problem if set to speed 2 or speed 3.

happy to change the pump.....but if I am doing that anyway, maybe a higher throughout would be best......want comfort more than a low wattage pump.....any thoughts ?

 

[SJB060685]

If you double your flow rate through fixed pipework you will double the velocity of the water. This can not only cause noise problems throughout the system, it can also cause valves etc to fail earlier through erosion. More importantly though if you double the flow rates this increases the frictional resistance against the flow by around 4 times, which means you would need an increase in pump power to overcome the system resistance.
Just a thought but have you considered placing a flow monitoring valve on the flow or return by the boiler? This would at least give you an indication of the flow through the boiler.

 

[John.g]

Its a vented hot water cylinder and if also a vented primary system then with (and if) that circ pump is on speed 3 there should be some signs of vent pump over in the F&E tank if there is a blockage in the system. I think running on those 4 rads only would tell a lot especially if their rated outputs or dimensions are known.

 

[Reddwarf4ever]

Its a vented hot water cylinder and if also a vented primary system then with (and if) that circ pump is on speed 3 there should be some signs of vent pump over in the F&E tank if there is a blockage in the system. I think running on those 4 rads only would tell a lot especially if their rated outputs or dimensions are known.

All 4 radiators 100cmx50cm, the CH vent to the FE tank was joined to the outlet of the FE tank several years ago, as I kept getting excessive air into the system, the plumber told me this was the usual thing to do with that problem......never had the problem before the CH system was expanded during the extension.....that’s the trouble with modifying stuff, especially plumbing....

I tried running the boiler at 65oC ( set at 7 KW ) and all went really sweet for a while, DT 24oC across the boiler.....but then for an unknown reason it tripped, then just kept overshooting the target temperature and tripping......wish it controlled better, just doesn’t seem to cut back enough when close to the target temperature ......set at say 70oC often goes to 71oC, which is acceptable, but then at 72oC it trips......wish I could adjust this to make it more ‘ lenient‘

 

[John.g]

All 4 radiators 100cmx50cm, the CH vent to the FE tank was joined to the outlet of the FE tank several years ago, as I kept getting excessive air into the system, the plumber told me this was the usual thing to do with that problem...never had the problem before the CH system was expanded during the extension..that’s the trouble with modifying stuff, especially plumbing.***

I tried running the boiler at 65oC ( set at 7 KW ) and all went really sweet for a while, DT 24oC across the boiler..but then for an unknown reason it tripped, then just kept overshooting the target temperature and tripping...wish it controlled better, just doesn’t seem to cut back enough when close to the target temperature ...set at say 70oC often goes to 71oC, which is acceptable, but then at 72oC it trips...wish I could adjust this to make it more ‘ lenient‘

You have a combined vent and cold feed, mine was installed like this 48 years ago and works very well to prevent air ingress etc so even if you have restrictions it won't pump over. The 4 rads, if doubles, should emit ~ 5.5kw total at 75C.
The normal reason for a boiler cutting out is because the heating load is less than the boiler minimum output, and it is quite normal then to exceed the target temperature, normally by 5C, before it trips as you can't have the target temperature and the cut out temperature the same except it is a non modulating boiler like a oil fired boiler, some gas fired boilers then seem to have a problem in modulating down fast enough to avoid again cutting out after ignition especially with very low heat demand, which of course was the reason for tripping in the first case.
You have your boiler set to 7kw, this shouldn't prevent it modulating to 5 kw if this is its minimum output. You could consider opening the TRVs fully on another few rads to enable the boiler to run continuously to see how it performs.
The boiler shouldn't have the trip temperature set only 2C above the target temperature as there is bound to be a bit of overshoot after coming up to its target temperature.