Low delta t

[Matt0029]

I have installed a system and am only getting a 6degree difference between the flow and return. I have knuckled the locksheilds down on the rads closest the boiler and also lowered the pump speed. To increase it all that can be done is to knuckle the locksheilds down further? Though this is just about stopping enough heat getting too the rads. Thanks.

 

[Scott_D]

What boiler
Can you range rate it?

 

[ShaunCorbs]

Sounds like the rooms are upto temperature?

 

[SJB060685]

Boiler not range rated.
Boiler output too high.
Valves with poor authority.
Pump too big.

Could be a couple of things

 

[Matt0029]

It can't be range rated no. But will modulate down to 12kw.

 

[Matt0029]

Sounds like the rooms are upto temperature?

Suggesting the delta t maybe higher when the rooms are still trying to get to temperature?

 

[Matt0029]

The pumps set to variable speed 2. I could try it on variable speed 1.

 

[ShaunCorbs]

Suggesting the delta t maybe higher when the rooms are still trying to get to temperature?

correct you will get a lower delta when the rooms are upto temperature

 

[Matt0029]

Have limited the flow to 65 degrees the flow was at this temperature when tested not sure if the rooms where up to temp. Is there anyway of telling what flow rate the pump is giving me in litres per minute?

 

[John.g]

Are you measuring this deltaT at the boiler? if so, maybe by pass open.
If your boiler is still running (and not cycling) at 12kw then the flowrate is 28.7 LPM (1.72M3/hr) @ deltaT of 6C which seems extremely high from a pump on reduced speed, I would think most systems would circulate at ~ 1 M3/hr (max) which gives a deltaT of 10.3C @ 12kw.
If the boiler is cycling then you can get a small deltaT but with a fairly low boiler flow temp of maybe 40/45C. resulting in a low rad/boiler output.

 

[John.g]

Have limited the flow to 65 degrees the flow was at this temperature when tested not sure if the rooms where up to temp. Is there anyway of telling what flow rate the pump is giving me in litres per minute?

What is the pump model and present setting?.

 

[Matt0029]

Grundfos ups2 25-80 is set on proportional pressure 2 at the momment

 

[Matt0029]

I am measuring delta t on the flow and return connections at the boiler. Is this not correct? There is no bypass near the boiler as no zone valves. There is a bypass near then end of the heating run.

 

[Matt0029]

The boiler isn't cycling no.

 

[Matt0029]

That's what is concerning me.

 

[John.g]

I am measuring delta t on the flow and return connections at the boiler. Is this not correct? There is no bypass near the boiler as no zone valves. There is a bypass near then end of the heating run.

Is it a manual or automatic (ABV) bypass? and if a ABV what is the setting index?.
Will look at PP2 pump curve for that massive pump but you have 32 rads?.

 

[ShaunCorbs]

34 rads

 

[John.g]

Without knowing the boiler output at this 6C differential with 34 rads then impossible to calculate the flowrate.
First check (no) bypass.
PP2 is still a very powerful setting (7.5M) on this pump so would suggest c/o to PP1 (6.0M).
you can always keep reducing the boiler flow temp to get the return temp down to say 45C even though the deltaT will still be very low (with rad output decrease) or maintain 65C and If deltaT still too low, slowly throttling one valve on the main return to the boiler (preferably not the pump suction) until deltaT rises to 15C or so.

 

[fezster]

From my experience, a honeywell ABV is very easily damaged (Im sure other brands are similar) if you try to set it incorrectly. So worth checking if it is constantly bypassing (assuming you have a bypass!).

 

[John.g]

Virtually impossible to set a ABV properly IMO with this pump as it really hasn't any constant curves, and obviously won't work with PP control as the head falls with decreasing flow.

 

[king of pipes]

You have crept into commercial heating here you need a double regulating valve in the return to the boiler and flow monitoring stations, and the equipment to measure said flow and return pressures you will not achieve this with guess work I'm afraid . Do you have a low loss header fitted what boiler is it post a few pictures up bud ?? . Kop

 

[ShaunCorbs]

Don’t need a low loss header as you don’t require separation eg different flow rates as there all just rads

as for drv you can balance the system from the rad lockshield valves your looking for approximately 15dc delta you may only get 10dc delta depending on who originally sized the rads

 

[John.g]

Suggesting the delta t maybe higher when the rooms are still trying to get to temperature?

If the deltaT is 6C with 65C flow and 20C room temp then it will be ~ 8C with 65C flow and 10C room temp but the rad output will be ~ 30% more at the same flow rate.

 

[SJB060685]

If the deltaT is 6C with 65C flow and 20C room temp then it will be ~ 8C with 65C flow and 10C room temp but the rad output will be ~ 30% more at the same flow rate.

Using the spreadsheet you supplied me I get 3% more? What have I done wrong?

 

[John.g]

Which one is that SJB?, even my will is in a spreadsheet.
Using T50, 20kw rads as the base then using flow/return 65/59C & 20C roomtemp I get a output of 15.94kw ("42deg rad") at a flowrate of 38.08LPM.
At the same flowrate and 10C roomtemp I get flow/return 65/57.3C & output of 20.58kw ("51.1deg rad") a increase of 29%.

 

[SJB060685]

Which one is that SJB?, even my will is in a spreadsheet.
Using T50, 20kw rads as the base then using flow/return 65/59C & 20C roomtemp I get a output of 15.94kw ("42deg rad") at a flowrate of 38.08LPM.
At the same flowrate and 10C roomtemp I get flow/return 65/57.3C & output of 20.58kw ("51.1deg rad") a increase of 29%.

 

[SJB060685]

I've run the maths and see my error

 

[John.g]

No problem, its amazing really what effect the room temp has but I suppose they would never heat up otherwise.

 

[SJB060685]

Another thing is and I think its been mentioned above? is the old system my have been designed for a lower delta t for non condensing boilers. Increasing the size of radiators will help promote the displacement of heat and drop the return temperature. I haven't really read my commercial book but will take a look later.

 

[John.g]

Old rads were designed as "60deg" rads and something like a deltaT of 11C (20F), to get the rated output at 20C room temp requires flow/return temps of 85.5C/74.5C. and even the 50deg rad requires 77.5C/62.5C (15C deltaT) to get rated output and a impossible 90C/50C (deltaT 40C) to get rated output with a 50C return.