Central heating pump sizing

[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