Central heating pump sizing

[Reddwarf4ever]

you should be getting around 15dc split / delta across the flow and return at the boiler

eg

flow 65
Return 50

Hi
get 20-25oC DT when running normally

 

[ShaunCorbs]

Hi
get 20-25oC DT when running normally

what about when it’s playing up ?

 

[Reddwarf4ever]

what about when it’s playing up ?

Difficult to tell, as the temperature falls quickly when the boiler trips, but must be greater than D30oC according to valiant....when it moves from 5.4 to 5.3 hold state, it’s typically around 15-20oC DT but the outlet temperature is only around 60oC on low fire, while it’s on ‘hold’ in its 5.3 state.....would like to know if the 30C DT is for boiler protection or efficiency, can’t seem to find this out, or if the value can be changed..... altered D02 to 10 minutes from 20 minutes to stop it staying in 5.3 so long.....

 

[SJB060685]

I actually thought this posted ages ago but obviously not. Your thinking is correct yes. By increasing the flow rates you will shrink the DT, which is your problem by the sounds of it. But installing a larger pump will increase running costs, cause other problems and may not even help. If there's a restriction somewhere in the system then that is affectively governing the flow through the system and boiler.
They are centrifugal pumps yes. I think John's suggestion above was to determine the pump head and flow by cross checking the wattage.
I will leave you in Shauns capable hands as I'm an oil man by trade and im not familiar with the ins and outs of specific gas boilers.

 

[John.g]

Its quite simple really!, a 18 kw boiler will require a flow rate of 12.9 LPM to maintain a deltaT of 20C at max output and assuming a min output of 5 kw, 3.6 LPM to maintain the same deltaT of 20C.
But what must be remembered is that where TRVs are installed its they that determine the deltaT across the rads and boiler and not you. If you had all rads fitted with TRVs and assume that the rads emit say 15 kw at a delta t of 15C, and start throttling in when the heating demand falls to 11 kw, the flow/return then will be 75/45C with a deltaT of 30C, boiler trips. I know some of the above rads are full open but just maybe the TRV rads are throttling enough that when mixed with the higher return temps still (sometimes, in mild weather) give a deltaT of 30c across the boiler. Maybe a ABV should be installed.

 

[Reddwarf4ever]

Its quite simple really!, a 18 kw boiler will require a flow rate of 12.9 LPM to maintain a deltaT of 20C at max output and assuming a min output of 5 kw, 3.6 LPM to maintain the same deltaT of 20C.
But what must be remembered is that where TRVs are installed its they that determine the deltaT across the rads and boiler and not you. If you had all rads fitted with TRVs and assume that the rads emit say 15 kw at a delta t of 15C, and start throttling in when the heating demand falls to 11 kw, the flow/return then will be 75/45C with a deltaT of 30C, boiler trips. I know some of the above rads are full open but just maybe the TRV rads are throttling enough that when mixed with the higher return temps still (sometimes, in mild weather) give a deltaT of 30c across the boiler. Maybe a ABV should be installed.

However, as the radiators are Not connected in series, if TRVs close then the water would simply bypass those radiators and return to the boiler hotter.....if all rads were closed, the Delta T across the boiler would simply be due to pipework heat losses....it’s been agreed that a higher flow pump would assist with the Delta T across the boiler, by returning the water quicker ( hotter ) to the boiler, but what are the negative sides of this ? Apart from pump energy consumption ?thanks

 

[Last Plumber]

The radiators should not be connected in series anywhere. If this is a 2 pipe system, once the TRV's close, water will cease to flow through that particular rad (or be reduced obviously). Water will only bypass rads in order to supply others further on in that circuit but it won't return to the boiler unless there is an open part of the circuit or bypass for it to flow through. i.e if they all close as John G is saying, there is no flow.
If this is a one pipe system, then there will always be a flow through the pipe work and that would work as you are describing.

Having skim read through this, it sounds to me like a restriction in flow on a 2 pipe. I would check the pump over to ensure it is working correctly to rule it out but as this has always worked the same way from new, I think you should be looking more at a restriction.

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.

Hope this helps.
Might have missed it but is this sealed or open vented?

 

[John.g]

More water will pass through the non throttled radiators thus reducing their deltaTs. (if this is what you mean?) and by installing a more powerful pump will reduce it still further??, you would have to double the pump head to increase the flow by 40% through the non throttled rads, this will decrease the detaT by ~ 4/5C. Maybe prudent to invest in a thermal gun and take a few temperature before proceeding. you might consider reducing the boiler temperature temporarily to say 60/65C, the TRVs will then open further with a reduced deltaT but probably a higher return temperature.
Is there a automatic or manual by pass installed anywhere if the boiler has pump overrun?

 

[Reddwarf4ever]

Will try reducing the boiler outlet temperature and see what happens, maybe if the boiler runs stable more often, will compensate for the reduced radiator temperature there is no bypass, but has a boiler over run

 

[Sparkgap]

Modern self regulating pumps should modulate down as TRVs close but may not keep up the minimum flow required for the boiler. Two ways: (1) fit an auto bypass valve to the heating to keep minimum flow through the boiler or (2) fit a separate pump for the heating and hydraulically separate the boiler with a low loss header (more expensive and usually done on larger systems).

 

[Reddwarf4ever]

Thanks, will try the lower outlet temperature first, but if not, where would this auto bypass be fitted exactly ? Thanks

 

[Sparkgap]

Thanks, will try the lower outlet temperature first, but if not, where would this auto bypass be fitted exactly ? Thanks

Basically between the boiler flow and return. I usually put them at least a couple metres away if possible but before any zone valves or controls. An AVDO or similar valve will start opening as the TRVs close and maintain the boiler flow rate. Also since there will not be more water forced through the open rads any noise will be reduced.

 

[Reddwarf4ever]

Basically between the boiler flow and return. I usually put them at least a couple metres away if possible but before any zone valves or controls. An AVDO or similar valve will start opening as the TRVs close and maintain the boiler flow rate. Also since there will not be more water forced through the open rads any noise will be reduced.

If this valve is between the boiler flow and return, is it effectively a boiler bypass ? Can’t see how that increases the flow through the boiler ?

 

[Last Plumber]

If this valve is between the boiler flow and return, is it effectively a boiler bypass ? Can’t see how that increases the flow through the boiler ?

It creates a short circuit which opens more when the circuit closes down to help the boiler maintain a minimum flow rate. I personally do not think this will make any difference to the issue you have, although I agree it needs to be there.

 

[Reddwarf4ever]

It creates a short circuit which opens more when the circuit closes down to help the boiler maintain a minimum flow rate. I personally do not think this will make any difference to the issue you have, although I agree it needs to be there.

Thanks

 

[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.