Your making this very complicated, forget all the bollozx about the bypass look elsewhere, start by looking at the status in the live moniter on the boiler for a start 
For a start turn d0 down to around 20kw an report back i have a feeling u will notice a huge inprovement
Listen to what Bunnyman is telling you and forget the rest, I appreciate that it is your first post but so many wannabees on here , Bunnyman is giving you the correct advice
Ok, thanks @Bunnyman, I appreciate your time and knowhow and your help is much appreciated!
I will set d.00 to 20; leave everything else as is. I will monitor live status when starting up from cold and report back.
For my understanding (don't get me wrong, not second guessing, just want to get to grips with what is going on) what is it you expect happens when d01 is at auto? I've never seen it overheat at starting up, to be honest, there's a handy graph on the front display and on startup that usually shows 90-100% power, slowly going down after some time to the lowest segments, or slightly over when it's working somewhat harder.
Thanks again
Waldo
(edited typo d.01 should have been d.00)
I will set d.00 to 20; leave everything else as is. I will monitor live status when starting up from cold and report back.
For my understanding (don't get me wrong, not second guessing, just want to get to grips with what is going on) what is it you expect happens when d01 is at auto? I've never seen it overheat at starting up, to be honest, there's a handy graph on the front display and on startup that usually shows 90-100% power, slowly going down after some time to the lowest segments, or slightly over when it's working somewhat harder.
Thanks again
Waldo
(edited typo d.01 should have been d.00)
D0 not d1?
As luck will have it last night was a gentle 10 degrees, nothing like the -4 the nights before...
So not sure if there's any difference, as the house didn't cool down much under 19 degrees during the night.
Ok, set d0 to 20, let system cool down and monitored it for the better part of an hour:
Hope you can see the attached image, tried copy/paste but that didn't work.
I've checked status, d40, d41, and return pipe temp every minute. Flow pipe measured is consistent with d40 so I've left that out (shows my thermometer is at least reasonably accurate...).
As far as I can tell this shows it is increasing the temperature 14 degrees with 20kW power.
Radiators feed all get warm at about the same time, but it takes quite some time for them to get hot.
After over an hour they are pretty hot the water going in is in around the 70 mark, water going back out around 50 to 55 on average over all radiators.
Boiler reduces power after around half an hour. I've checked after an hour and the return pipe is around 55 degrees, and boiler is running at minimum power, 12.5 kW (according to manual)
Any thoughts?
thanks
Waldo
So not sure if there's any difference, as the house didn't cool down much under 19 degrees during the night.
Ok, set d0 to 20, let system cool down and monitored it for the better part of an hour:
Hope you can see the attached image, tried copy/paste but that didn't work.
I've checked status, d40, d41, and return pipe temp every minute. Flow pipe measured is consistent with d40 so I've left that out (shows my thermometer is at least reasonably accurate...).
As far as I can tell this shows it is increasing the temperature 14 degrees with 20kW power.
Radiators feed all get warm at about the same time, but it takes quite some time for them to get hot.
After over an hour they are pretty hot the water going in is in around the 70 mark, water going back out around 50 to 55 on average over all radiators.
Boiler reduces power after around half an hour. I've checked after an hour and the return pipe is around 55 degrees, and boiler is running at minimum power, 12.5 kW (according to manual)
Any thoughts?
thanks
Waldo
I think the return temp difference is normal, I have checked the flow/return temp on my 630 boiler and the return is 8-12c lower with a contact thermometer compared to D41, the flow temp is within 1-2c.
If 70c is going into the radiators and 50c out they may not be big enough to heat the rooms quickly, the radiators may have been sized for a flow/return difference of 10c for an older non condensing boiler.
My radiators were replaced with the boiler and oversized for weather compensation, it was -7c a couple of weeks back and the system had no issue raising the room temps 3-4c within an hour.
If 70c is going into the radiators and 50c out they may not be big enough to heat the rooms quickly, the radiators may have been sized for a flow/return difference of 10c for an older non condensing boiler.
My radiators were replaced with the boiler and oversized for weather compensation, it was -7c a couple of weeks back and the system had no issue raising the room temps 3-4c within an hour.
@hossacd thanks for the info!
Radiators big enough I believe, but rated at higher temps, yes. So increasing flow somehow would benefit output - at 75/65/20 they should be enough but at 70/50 probably not.
Which is where I think the problem is: flow through system is insufficient. Results in less output, slow heating times. Boiler pumping out hot water but not enough makes it to the radiator.
Strange thing is: same happens when reheating cylinder. But that's a simple circuit, good cold return, and still it seems to mix hot water in with cool on return feed.
So I am not sure where or why this happens.
But if that is perfectly normal, that would mean your 630 and my 637 (now downgraded to 20 kW) are hardly ever condensing. Except for a handful of minutes on a cold start.
Surely that defeats the whole energy saving thing?
Unless of course I set the flow temp so low that even with mixing the boiler gets a cool enough return to work at top efficiency?
Radiators big enough I believe, but rated at higher temps, yes. So increasing flow somehow would benefit output - at 75/65/20 they should be enough but at 70/50 probably not.
Which is where I think the problem is: flow through system is insufficient. Results in less output, slow heating times. Boiler pumping out hot water but not enough makes it to the radiator.
Strange thing is: same happens when reheating cylinder. But that's a simple circuit, good cold return, and still it seems to mix hot water in with cool on return feed.
So I am not sure where or why this happens.
But if that is perfectly normal, that would mean your 630 and my 637 (now downgraded to 20 kW) are hardly ever condensing. Except for a handful of minutes on a cold start.
Surely that defeats the whole energy saving thing?
Unless of course I set the flow temp so low that even with mixing the boiler gets a cool enough return to work at top efficiency?
My boiler is condensing most of the time as weather compensation means the flow is usually in the high 40s low 50s, even at -7c outside I don't think it went into the 70s (highest I saw was 68c but the flow temp drops as it reaches the target room temperature so it may have been over 70 when it started up in the morning) so the return should still have been under 56c
My boiler is running at full output in auto as the heat loss of the house is around 26kw.
My boiler is running at full output in auto as the heat loss of the house is around 26kw.
@hossacd yes, weather compensation is something I'm definitely going to look at - as soon as I'm sure the rest of the system is up to snuff.
Currently it's wired up on 240v on/off so can't set the heating temperature freely or the hot water might not get hot enough.
Thank you for confirming that would work as I thought it would. You have a VR66 and and a VRC470f or something like that?
Currently it's wired up on 240v on/off so can't set the heating temperature freely or the hot water might not get hot enough.
Thank you for confirming that would work as I thought it would. You have a VR66 and and a VRC470f or something like that?
@Bunnyman Yes, I think we can agree the boiler is grossly oversized. Nothing I can do except replacing it - it came with the house. Must have been a case of 'bigger is better' from the previous owner.
It's on 22mm pipes and from what I've read you can't even shift 37 kW through that without crazy flow speeds or temperature differentials. On top of that in spring/autumn heat loss turns out to be around 5kW, a lot less than the 12.5 kW minimum the boiler can modulate down to before going into switching on/off.
Limiting max power with d0 to 20 didn't make any difference I'm sorry to report. If anything it takes longer for the flow to reach target temperature, I've done a run from cool with d0 on 37 and I see the same status (S1,S2,S4...); only more power so bigger increase in temperature, and faster to reach 75.
Reason I looked into bypass was that my return temp measurement is so different from d41.
Most rooms are smaller less important regarding heating time, but the biggest issue is the main living room, 67m2 with 3 large vertical radiators rated at a bit over 1700 Watt. I think they are about the right size - I get about 5kW required for the room.
1700W is specced for 75º flow, 65º return though, which I'm not getting (currently more 70/50). Any thoughts where I could look to see where the problem might lie?
Thanks again for any suggestions!
Waldo
It's on 22mm pipes and from what I've read you can't even shift 37 kW through that without crazy flow speeds or temperature differentials. On top of that in spring/autumn heat loss turns out to be around 5kW, a lot less than the 12.5 kW minimum the boiler can modulate down to before going into switching on/off.
Limiting max power with d0 to 20 didn't make any difference I'm sorry to report. If anything it takes longer for the flow to reach target temperature, I've done a run from cool with d0 on 37 and I see the same status (S1,S2,S4...); only more power so bigger increase in temperature, and faster to reach 75.
Reason I looked into bypass was that my return temp measurement is so different from d41.
Most rooms are smaller less important regarding heating time, but the biggest issue is the main living room, 67m2 with 3 large vertical radiators rated at a bit over 1700 Watt. I think they are about the right size - I get about 5kW required for the room.
1700W is specced for 75º flow, 65º return though, which I'm not getting (currently more 70/50). Any thoughts where I could look to see where the problem might lie?
Thanks again for any suggestions!
Waldo
You can work out the heat output of the radiator when changing the Delta T, I think in the past I have seen a figure of approx 20% reduction in output from a differential of 10c to 20c.
http://www.castironradiators.ltd.uk/blog/2014/09/10/delta-t-and-heat-output-explained/
Your boiler does look to be very oversized a 615 would have probably done, if you have the newer 637 it does modulate down to just under 7kw. Although it doesn't appear to be short cycling excessively if it is holding the 75c flow for over an hour when the outside temp is 10c.
http://www.castironradiators.ltd.uk/blog/2014/09/10/delta-t-and-heat-output-explained/
Your boiler does look to be very oversized a 615 would have probably done, if you have the newer 637 it does modulate down to just under 7kw. Although it doesn't appear to be short cycling excessively if it is holding the 75c flow for over an hour when the outside temp is 10c.
Last edited: