Hot Water temperature with Greenstar Ri boiler

[Tommohawk2]

We have conventional Greenstar Ri central heating and hot water system with tank. When heating water only, it seems to take forever for the water to reach the target temperature of 60 deg C set on the tank stat. The pump works fine, but the boiler is modulating to the point that it only heats for maybe 20 seconds before reducing heat - it sounds as though it almost cuts right out, though the pump continues to run.

Once the tank water temp reaches say 55 degrees, the return isnt going to be less than this value. So if condensing boilers are supposed to maintain delta T of 20 deg C, how is the system ever going to reach target of 60 degrees unless it runs at 80 deg C?

From memory this is less of a problem in winter ironically, presumably because with the rads on, delta T of 20 is easier to maintain.

Any thoughts?

 

[Johntheo5]

We have conventional Greenstar Ri central heating and hot water system with tank. When heating water only, it seems to take forever for the water to reach the target temperature of 60 deg C set on the tank stat. The pump works fine, but the boiler is modulating to the point that it only heats for maybe 20 seconds before reducing heat - it sounds as though it almost cuts right out, though the pump continues to run.

Once the tank water temp reaches say 55 degrees, the return isnt going to be less than this value. So if condensing boilers are supposed to maintain delta T of 20 deg C, how is the system ever going to reach target of 60 degrees unless it runs at 80 deg C?

From memory this is less of a problem in winter ironically, presumably because with the rads on, delta T of 20 is easier to maintain.

Any thoughts?

What is the boiler target flow temperature and post a photo of the circ pump (running) showing its (any) LEDs and make/model/mode/setting, if known. What model/output is the Greenstar RI?.

 

[Tommohawk2]

Thanks for that. It's 18 Ri, but no idea what target flow temp is.. how do I tell? Temp dial is set to about 2 o'clock.
Pump is Grundfoss Ups3, and currently it's set to CP2. I did try it set to CP3, and PP2 or PP3 and it seems the same.

 

[Johntheo5]

Set the boiler stat to around 2/3 of its range which should give a flowtemp of ~ 65C, the boiler has a minimum output of 6KW which requires a flowrate of 5.7LPM at a deltaT of 15C to keep the boiler from cycling which it might still do as the cylinder temperature approaches say 50/55C, set the pump to CC1, 4.5M. Feel the cylinder coil flow/return, there should be a fairly noticeable temp difference, (15C), check for a gate valve in the cylinder coil return and ensure its not less than 1 full turn open from fully closed. Check for any pump over through the feed and expansion vent with pump running and also watch for pump over while you get someone to start the pump, and then stop it a few minutes later.

 

[Tommohawk2]

Thanks for that - a few things for me check when I get a mo.
Where is gate valve in cylinder return likely to be - right by cylinder? Access anywhere else would be a problem.
To add to the data, theres a bathroom rad plumbed before 3 port valve so always on. Strikes me that if the return from this is hot that will also cause modulation - maybe I should reduce flow on that circuit?
One other issue is that in the winter, I had issues with the boiler seemingly modulating due to overheating on lower pump settings - on CC1 the flow got really hot and return quite cool. It would only function and heat properly with higher pump settings.
BTW the boiler was serviced couple of months back, but no system flush or other work in 6 yrs. Far as I can tell pump is functioning as it should.