I think (as a non-plumber) you and John are right – the boiler can’t transfer the heat well enough via an indirect coil so its cycling on and off as the return is too hot – direct connection would be best – flow into the top of the tank, return from the bottom, mixed to suit the boiler. The secondary flow tapping could be used like you say. Ideally, you could do with more holes in your cylinder – direct would be best for both the boiler and the ufh – and as you say, stratification isn’t a problem as there’s no hot water section to worry about – you just need a whole tank of warmish water for your ufh, so mixing it all up probably won’t make much difference, provided the odd draft of water doesn’t trip a single cylinder stat on or off.
Using two or even all three indirect coils in series would be a second option, but you might still find the boiler cycles if it’s only dropping a few degrees through one coil.
Your existing connections being upside down with the flow at the bottom of the coil seem odd – no idea why that’s been done, but hey, I’m not a plumber.
Oil boilers need a well controlled return temp I think? – as John says - so a thermostatic mixing valve or loading valve would be good (£££), and would make the boiler more efficient. Laddomat/esbe (sp?) do specific loading valves for gravity-fed stoves, no difference to a tmv here though.
You say the tank is not heating beyond 50 degrees – if it’s just for ufh, chances are it shouldn’t get much hotter anyway, to protect any hard flooring, unless as above there’s a separate mixing valve on the ufh manifold. Most ufh co's recommend a separate stat that cuts the ufh pump if the flow goes overtemp anyway.
Hotter the cylinder gets, the more wasteful of energy it is. If you have rads running off the cylinder as well as ufh, then yes, you might need it warmer unless the rads are seriously over-sized.
You’re right - any ‘standard’ (if there is such a thing) buffer/thermal store only taps off for the ufh halfway down on the cylinder as the top half is kept much hotter to provide hot water supply – you’ve got that hot water covered with a second cylinder, so I’d have thought you can run the tank at 50-60 and take the ufh right out of the top. Set temp might depend on whether it’s open vented or not – legionaires – plumber should know about that. I would hope. He’ll have a book on it somewhere.
Your ufh shouldn’t need an additional pump if you already have one on each manifold. Plumber should have told you that, unless he's on commission from Grundfos. If there’s two pumps with a manifold each, you might need to split the flow with zone valves?
If you want heated towel rails, again you’d have to split the flow from the cylinder (Y plan or S plan, I can’t remember which is which buzzword, tee’d/split anyway) – from the cylinder top tapping it would be tee’d with one side with a 2 port valve to the ufh, the valve controlled by the ufh manifold wiring centre using the switch in the valve to power the manifold pump, and the other side with a 2 port valve to your towel rad circuit, with it’s own pump, set on a timer. So if you’ve got two manifolds plus a set of towel rails, you’d have a Z plan. W plan? Double Y plan? Not sure if they have a name for three zone valves. Plumber may get a migraine at this point and give you a sick note written by his mum. Or at least start sweating, mentioning part P and mumbling about an electrician.
As said above, more for safety than anything else, you need a decent plumber/heating engineer who's done this sort of thing before. Your design is complex, and needs a guy with real experience.
Best of luck.
Using two or even all three indirect coils in series would be a second option, but you might still find the boiler cycles if it’s only dropping a few degrees through one coil.
Your existing connections being upside down with the flow at the bottom of the coil seem odd – no idea why that’s been done, but hey, I’m not a plumber.
Oil boilers need a well controlled return temp I think? – as John says - so a thermostatic mixing valve or loading valve would be good (£££), and would make the boiler more efficient. Laddomat/esbe (sp?) do specific loading valves for gravity-fed stoves, no difference to a tmv here though.
You say the tank is not heating beyond 50 degrees – if it’s just for ufh, chances are it shouldn’t get much hotter anyway, to protect any hard flooring, unless as above there’s a separate mixing valve on the ufh manifold. Most ufh co's recommend a separate stat that cuts the ufh pump if the flow goes overtemp anyway.
Hotter the cylinder gets, the more wasteful of energy it is. If you have rads running off the cylinder as well as ufh, then yes, you might need it warmer unless the rads are seriously over-sized.
You’re right - any ‘standard’ (if there is such a thing) buffer/thermal store only taps off for the ufh halfway down on the cylinder as the top half is kept much hotter to provide hot water supply – you’ve got that hot water covered with a second cylinder, so I’d have thought you can run the tank at 50-60 and take the ufh right out of the top. Set temp might depend on whether it’s open vented or not – legionaires – plumber should know about that. I would hope. He’ll have a book on it somewhere.
Your ufh shouldn’t need an additional pump if you already have one on each manifold. Plumber should have told you that, unless he's on commission from Grundfos. If there’s two pumps with a manifold each, you might need to split the flow with zone valves?
If you want heated towel rails, again you’d have to split the flow from the cylinder (Y plan or S plan, I can’t remember which is which buzzword, tee’d/split anyway) – from the cylinder top tapping it would be tee’d with one side with a 2 port valve to the ufh, the valve controlled by the ufh manifold wiring centre using the switch in the valve to power the manifold pump, and the other side with a 2 port valve to your towel rad circuit, with it’s own pump, set on a timer. So if you’ve got two manifolds plus a set of towel rails, you’d have a Z plan. W plan? Double Y plan? Not sure if they have a name for three zone valves. Plumber may get a migraine at this point and give you a sick note written by his mum. Or at least start sweating, mentioning part P and mumbling about an electrician.
As said above, more for safety than anything else, you need a decent plumber/heating engineer who's done this sort of thing before. Your design is complex, and needs a guy with real experience.
THAT is the hardest bit.
Best of luck.
