Hi - first post here. sorry its so long!
Firstly my system - installed 10 years ago when we bought a run-down cottage and renovated and extended - effectively a 'new-build'
4 bed semi - UFH upstairs and down (no rads at all) all running off of a DPS Heat-Bank direct thermal store. Hot water also runs off the thermal store through a plate heat exchanger.
There is a plan this summer to make the control system a whole lot more elegant, but for the time being the UFH pump is controlled from room stats, the boiler's "heat-demand" signal comes from an immersion-stat in the store - about 1/3 of the way up. currently set to mid / high 60's (its set lower in the summer when the incoming mains water is warmer and needs less delta-T added to it). The system worked pretty well for 10 years with a Baxi-Barcelona boiler, except that the boiler was a piece of **** and kept going wrong again and again and again. The boiler finally kept going into lockout and only resetting after multiple attempts a few weeks back and I've just had a new boiler installed.
The new boiler (Vaillant 428) has been somewhat problematic and I think the problem is down to flow rate, but I can't understand why. The boiler pump is attached to the store, and was a grundfos 15/50 on speed 3.
The baxi seemed to do what a boiler on a store is supposed to do and run at full tilt, until the store was almost up to temp, and then start to modulate down (I guess as the flow from the boiler reached the boilers max temp and the return started to catch up with it) and then get shut off by the store stat. I've got some very primative pipe stats near the store and delta-T was around 20 degrees, maybe a touch less with the baxi. There was some cycling, but that was due to the hysterisis on the stat being insufficient I think. (part of the reason for the new control system being fitted later this year).
The new boiler however (set to 85 degree flow on the dial) reaches 85 very very quickly and often shuts down in its anti cycle then starts up again. When the store is just topping off the boiler seems unable to modulate down quick enough and the temp shoots through 85 degrees and it shuts off. Using the D-codes (D-40 and D-41) on the boiler to get the temps I can see almost 30 degrees Delta -T across the boiler when its running full chat (30kw). I did a bit of research and reckon that means insufficient flow rate. But I can't understand that. Same power boiler, Same pipework, Same pump. So the only thing is the flow restiance of the boiler itself. Can the vaillant really be so much more restrictive than the baxi?
I reckon I'm getting around 850-900 litres/ hour flow (30kw at 30 degrees) and i need around 1200-1250 for a 20 deg delta-T at 30kw, so today I fitted a 15/60 pump, which seems signifiantly more powerful in the specs. Its a bit better, but not much. I've got about 25 degree delta-T. The boiler behaves better, but still not 100% ideal. with 20 degree delta-T it would be about perfect as the hotest return the boiler is going to see is going to be around 65-70 and the boiler should have time to modulate down as the temp won't shoot up to 85 too fast for the boiler to deal with. And 20 degree is what the boiler is designed for so its logic is probably having a bit of a paddy with high delta-T's
Anway - any ideas? Is there a more powerful direct fit replacement pump for the 15-60? The boiler feed / return is in 22mm with 28mm near the boiler. As part of the summer strip down and refurb I could replumb in 28mm with less sharp bends, which could improve the flow rate with the 15/60. I'm surprised that I can't get the flow through, even with 15/60. Does anyone know what the system version of this boiler has pumpwise?
Is it possible to calculate what 'power' pump I need given the above flow rates and the required flow rate?
Firstly my system - installed 10 years ago when we bought a run-down cottage and renovated and extended - effectively a 'new-build'
4 bed semi - UFH upstairs and down (no rads at all) all running off of a DPS Heat-Bank direct thermal store. Hot water also runs off the thermal store through a plate heat exchanger.
There is a plan this summer to make the control system a whole lot more elegant, but for the time being the UFH pump is controlled from room stats, the boiler's "heat-demand" signal comes from an immersion-stat in the store - about 1/3 of the way up. currently set to mid / high 60's (its set lower in the summer when the incoming mains water is warmer and needs less delta-T added to it). The system worked pretty well for 10 years with a Baxi-Barcelona boiler, except that the boiler was a piece of **** and kept going wrong again and again and again. The boiler finally kept going into lockout and only resetting after multiple attempts a few weeks back and I've just had a new boiler installed.
The new boiler (Vaillant 428) has been somewhat problematic and I think the problem is down to flow rate, but I can't understand why. The boiler pump is attached to the store, and was a grundfos 15/50 on speed 3.
The baxi seemed to do what a boiler on a store is supposed to do and run at full tilt, until the store was almost up to temp, and then start to modulate down (I guess as the flow from the boiler reached the boilers max temp and the return started to catch up with it) and then get shut off by the store stat. I've got some very primative pipe stats near the store and delta-T was around 20 degrees, maybe a touch less with the baxi. There was some cycling, but that was due to the hysterisis on the stat being insufficient I think. (part of the reason for the new control system being fitted later this year).
The new boiler however (set to 85 degree flow on the dial) reaches 85 very very quickly and often shuts down in its anti cycle then starts up again. When the store is just topping off the boiler seems unable to modulate down quick enough and the temp shoots through 85 degrees and it shuts off. Using the D-codes (D-40 and D-41) on the boiler to get the temps I can see almost 30 degrees Delta -T across the boiler when its running full chat (30kw). I did a bit of research and reckon that means insufficient flow rate. But I can't understand that. Same power boiler, Same pipework, Same pump. So the only thing is the flow restiance of the boiler itself. Can the vaillant really be so much more restrictive than the baxi?
I reckon I'm getting around 850-900 litres/ hour flow (30kw at 30 degrees) and i need around 1200-1250 for a 20 deg delta-T at 30kw, so today I fitted a 15/60 pump, which seems signifiantly more powerful in the specs. Its a bit better, but not much. I've got about 25 degree delta-T. The boiler behaves better, but still not 100% ideal. with 20 degree delta-T it would be about perfect as the hotest return the boiler is going to see is going to be around 65-70 and the boiler should have time to modulate down as the temp won't shoot up to 85 too fast for the boiler to deal with. And 20 degree is what the boiler is designed for so its logic is probably having a bit of a paddy with high delta-T's
Anway - any ideas? Is there a more powerful direct fit replacement pump for the 15-60? The boiler feed / return is in 22mm with 28mm near the boiler. As part of the summer strip down and refurb I could replumb in 28mm with less sharp bends, which could improve the flow rate with the 15/60. I'm surprised that I can't get the flow through, even with 15/60. Does anyone know what the system version of this boiler has pumpwise?
Is it possible to calculate what 'power' pump I need given the above flow rates and the required flow rate?