Central heating pump sizing

[ShaunCorbs]

Have you tried balancing the system should be around a 10 dc drop on the rad flow and returns ?

 

[John.g]

None of the radiators struggle to get hot..they all struggle when the boiler is cycling 5.3 to 5.4..once it settles down, everything is really hot....found a pump on eBay where the wattage is 93W on high compared to 65W on my present one, does that imply it’s more powerful or just uses more power to do the same thing ? don’t want to modify the system....I assume the 30oC Delta T setting can’t be changed, also not sure why it’s there, efficiency or boiler protection thanks

You might consider a A rated pump which displays the power in watts from which you can easily deduce the flow rate and pump head and at least you can rule this in or out then, as suggested above a 6M pump should be quite adequate, I have 10 rads and a 20 kw OF boiler and the pump (6M wilo yonos Pico) power gives a flow rate of 12 LPM @ 3.2M head which is quite accurate as the boiler deltaT is ~ 25C., this pump only cost me ~ £100 two years ago.

 

[Reddwarf4ever]

That wattage will be its power consumption, which obviously increases with a more powerful pump and the higher the setting. With 22mm primaries and a larger pump I imagine more problems with boiler delta t.

Hi
to my way of thinking, a more powerful pump = higher flow rate, the heat dissipation from the rads would not increase as It’s down to Delta T Rad Vs Room temperature, therefore the return temperature would be higher, the boiler has no issue heating, so does this make sense ? Higher flow rate = increased inlet temperature therefore boiler Delta T would fall.....

Also is wattage such an accurate way to determine flow ? As these pumps are centrifugal, power would not exactly relate to flow, as with a reciprocating positive displacement pump ?

 

[Reddwarf4ever]

Have you tried balancing the system should be around a 10 dc drop on the rad flow and returns ?

Hello have TRVs fitted to all rads except the top landing Hall and lounge ( 4 rads ) needs these fully on, works well....don’t think balancing is appropriate.....the rad flow return Delta T, is that the same as across the boiler, effectively, when on heating only ?

 

[ShaunCorbs]

Hello have TRVs fitted to all rads except the top landing Hall and lounge ( 4 rads ) needs these fully on, works well....don’t think balancing is appropriate.***.the rad flow return Delta T, is that the same as across the boiler, effectively, when on heating only ?

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

eg

flow 65
Return 50

 

[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.