Don't forget that I increased the set point on the boiler stat last night and that I don't know if the cylinder stat stopped further firng because the cylinder was at temperature. I have turned off the water heating on the time clock so that I can run it when I am up and can monitor what is happening.
Greenstar 24ri - large temperature drop before re-fire
- Thread starter Gwyndy
- Start date
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Same plot as the last one. Boiler off at ~78C after 38 minutes. Cylinder stat operated at 40-45 minutes - zone valve closed, system off. Plenty of hot water but maybe a bit on the warm side! Might tweak the boiler stat back a little.
I'll post again when I hear from WB and / or the installer.
I'll post again when I hear from WB and / or the installer.
Reducing that will mean the cylinder stat will take longer to reach the tipping point.
It is not uncommon to have two input demands on some heat only boilers- one for hw and other for ch
Boiler return needs to be cold as possible to maximise condensing- less than 55 degrees to be in condensing mode which often may not be possible for HW heating.
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True, but it is running at a fairly high temp. I'm leaving it as it is for the time being.
On my boiler just one - common flow and return.
Not a chance with my system. Max differential when heating HW only is around 6C so running at mid to high 60s. CH is probably OK but I haven't checked in detail.
Not a chance with my system. Max differential when heating HW only is around 6C so running at mid to high 60s. CH is probably OK but I haven't checked in detail.
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Your best bet IMO is to run the boiler at its max temperature and get the cylinder heating finished as soon as possible, then get back to efficient CH, this is what priority HW heating does, it increases rhe boiler flowtemp to near max (automatically) and then returns it it to "normal" when cyliner stat satisfied. When not in condensing region then the efficiency loss is ~ 0.4/0.5%/10C return temperature, not a a lot if only a ~ 1 hour cylinder heating period required each day.
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Your best bet IMO is to run the boiler at its max temperature and get the cylinder heating finished as soon as possible, then get back to efficient CH, this is what priority HW heating does, it increases rhe boiler flowtemp to near max (automatically) and then returns it it to "normal" when cyliner stat satisfied. When not in condensing region then the efficiency loss is ~ 0.4/0.5%/10C return temperature, not a a lot if only a ~ 1 hour cylinder heating period required each day.
@Johntheo5 Thanks, I didn't know that. I have got to the point of thinking that pursuing this any further is somewhat academic, although it does interest me. I haven't had a reply from WB and will give them a 'prod' this week. The installer tells me that they do sometimes fit a gate valve in the HW circuit if the CH is struggling when both heating and HW are calling simultaneously. Not a problem for me as I heat HW before CH demand in the morning.
Meanwhile - leave well alone!
@Johntheo5 Thanks, I didn't know that. I have got to the point of thinking that pursuing this any further is somewhat academic, although it does interest me. I haven't had a reply from WB and will give them a 'prod' this week. The installer tells me that they do sometimes fit a gate valve in the HW circuit if the CH is struggling when both heating and HW are calling simultaneously. Not a problem for me as I heat HW before CH demand in the morning.
Meanwhile - leave well alone!
You are right, you have got it very well balanced now and throttling in the return would be a mistake because the the cylinder stat will never be satisfied then and you will simply have timed out cylinder heating with a smaller vol of available hot water.
6 degree differential is the root cause.
You will note ( I suspect) the differential for ch will be greater so boiler behave/ runs correctly
I wonder if cylinder replacement is the way forward
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In an ideal world, probably yes, but I don't think that it is worth the cost and upheaval for a relatively small gain. If the installer had made me aware of the issue 3 years ago when the boiler was installed I would have had the cylinder replaced. (I'm guessing that they didn't know about potential issues!)
What you say makes absolute sense, thank you.
The Tests I did on my 0.62M2 coil many years ago gave a initial coil input of 7.12kw, dT of 10.2C with the cylinder at 19C and 2.45kw, dT 3.5C at a cylinder temperature of 58C, coil circ flow was a steady 10.0LPM. Boiler flowtemp average 65C, (70C/60C), from OF boiler.
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What you are experiencing i would expect from the way you are trying to run the system connected to an old gravity coil cylinder.
Your boiler is 24kw and the gravity coils at best around 6 to 8kw.
the reheat time would be around an hour from cold (10 to 60) and use about 6kw of gas.
The flow and return differentials are very small and indicate poor heat transfer. Coil is probably scaled within the cylinder and most likely also not overly clean on the heating side. As it was connected as gravity most have large amounts of iron oxide coating the inside of the coil reducing heat transfer.
The Worcester Ri boilers like to see around 10 to 12 degree differential.
But as the boiler runs for around 35 mins before it cycles means that it is able to release a minimum of 4kw to the cylinder until that point.
You will always have a problem with this setup due to the flow point into the cylinder being so high
To solve it i suggest you have a new cylinder fitted. The ones designed for pumped setup have a coil rating of 12kw min normally and the coil is very low down in the cylinder
Your boiler is 24kw and the gravity coils at best around 6 to 8kw.
the reheat time would be around an hour from cold (10 to 60) and use about 6kw of gas.
The flow and return differentials are very small and indicate poor heat transfer. Coil is probably scaled within the cylinder and most likely also not overly clean on the heating side. As it was connected as gravity most have large amounts of iron oxide coating the inside of the coil reducing heat transfer.
The Worcester Ri boilers like to see around 10 to 12 degree differential.
But as the boiler runs for around 35 mins before it cycles means that it is able to release a minimum of 4kw to the cylinder until that point.
You will always have a problem with this setup due to the flow point into the cylinder being so high
To solve it i suggest you have a new cylinder fitted. The ones designed for pumped setup have a coil rating of 12kw min normally and the coil is very low down in the cylinder
That is what is happening - see earlier posts.
The current system is working but obviously not very efficiently.
Your advice is sound but I really do not want the hassle of changing the clinder at the moment. Maybe in the Spring next year. I can live with what I have for now.
For the sake of completeness in this saga, the following is an extract from a recent e-mail from WB.
"The only time you would normally have to wait for a large temperature drop before the boiler fires back up is if the primary sensor on the heat exchanger gets excessively hot.
For example, you have set the boiler flow temp to 70C. If there was any circulation issue with the pipework to the cylinder or through the cylinder coil and the primary sensor gets hotter and hotter due to poor circulation and goes to say 90C the boiler shuts down and it wouldn’t then re-fire until the flow temperature goes down into the late 30’s or 40 degree temperature range.
If the boiler was working as it should once the temperature drops a couple of degrees below what your asking for it would re-fire."
From this and looking at my plots of the flow temperature it is almost certain that the boiler is going over temperature and shutting off. (Installation and maintenance manual suggests set point + 5C which is about what I am seeing.) Typically 15+ minutes and significant temperature drop before re-ignition.
Options -
1. Have new cylinder - disruptive and costly. May consider next year.
2. Gate valve on return line to throttle flow through the coil - easy to do without draining system. (@Johntheo5 You advised against this in an earlier post. Do you think that it is a viable option?)
3. Do nothing for the time being - I do have sufficient hot water as I have said before. Probably the preferred otion at this time of year.
"The only time you would normally have to wait for a large temperature drop before the boiler fires back up is if the primary sensor on the heat exchanger gets excessively hot.
For example, you have set the boiler flow temp to 70C. If there was any circulation issue with the pipework to the cylinder or through the cylinder coil and the primary sensor gets hotter and hotter due to poor circulation and goes to say 90C the boiler shuts down and it wouldn’t then re-fire until the flow temperature goes down into the late 30’s or 40 degree temperature range.
If the boiler was working as it should once the temperature drops a couple of degrees below what your asking for it would re-fire."
From this and looking at my plots of the flow temperature it is almost certain that the boiler is going over temperature and shutting off. (Installation and maintenance manual suggests set point + 5C which is about what I am seeing.) Typically 15+ minutes and significant temperature drop before re-ignition.
Options -
1. Have new cylinder - disruptive and costly. May consider next year.
2. Gate valve on return line to throttle flow through the coil - easy to do without draining system. (@Johntheo5 You advised against this in an earlier post. Do you think that it is a viable option?)
3. Do nothing for the time being - I do have sufficient hot water as I have said before. Probably the preferred otion at this time of year.
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