Condensing boilers with a 20C temp differential require smaller pipes, making them more economic to fit.
At 11C temp diff:
15mm - 6.0 kW 22mm - 13.4 kW 28mm - 22.5 kWAt 20C temp diff:
15mm - 9 kW 22mm - 24 kW (81,888 BTU/hr) 28mm - 70 kW
It still forms a restriction that and many of them in a system all add up. The inserts also acts a points for sludge to gather too.
Your figures are inconsistent.
Condensing boilers usually aim to achieve the 20C differential by slowing the flow rate, which will also reduce the energy transfer proportionally, so it's not obvious that you can use that higher temperature differential to any substantial benefit in this respect. If you did, then you would restrict the boiler's capability to pump at full speed when heating up the water, which might slow down the system heating up, and restrict max system output in non-condensing mode. And obviously it would only apply in any case for a condensing boiler with appropriately [over]sized rads.
Those could be correct -- they are at least self consistent.
Those are clearly wrong even if you ignore my first point. No more than one of them could possibly be correct as they don't agree with each other.
It's somewhat misleading to use numbers in this way because it depends on pipe length. These figures would be approximately right for 3-4m.
Yes - the old worked very well. It is a fast recovery cylinder and you could shower continuously and never run out of water.
But I've done it as they say - and will be interested to see if there's any difference.
Interestingly, the diagrams in their installation book show the flow at the top - only the text changes that.
Agreed. And also to ultimately use a bigger fraction of the cylinder.
Having read through most of these posts. I'd say that in general the advice is good but somewhat over the top. IME the whole matter is much less critical than many posters would seem to suggest. There are two aspect to heating pipe sizing: One, make sure the pipes are big enough that water won't have to be unduly 'forced' through. having to increase the pump setting could increase the electric bill by £10s.
Two, try to arrange the pipes, in layout and size so the system is naturally balanced as far as practical, this will mitigate the effects of many problems.
If you have a "backbone" type layout with the boiler at one end it suggests you keep the backbone larger for longer so that at the far end you feed just one or two rads per drop and maybe 2 or 3 at the nearer end.
The pressure drop in the 22 is quite small relative to the drop in 15.
hth
They don't. They use a larger heat exchanger. One which extract more heat from the gas.
It gives slightly more hot water from the cylinder, so in effect giving a slightly larger cylinder. Having the flow at the top is more efficient using a condensing boiler as it ensures a cooler return.
I would have used a direct cylinder, a plate heat exchanger and a bronze pump. It heats the cylinder top down using thermal layers (stratification). If the cylinder ran out of hot water it will supply water at the rate the boiler can give, acting like a combi. It also means you can use a smaller cheaper cylinder, about half the size in most cases. It also means the condensing boiler works on high efficiency. Heating a coil from the bottom connection is the worst for condensing efficiency.
He wouldn't listen!
As for most of run time a system is on part load, which mean the rads are too big for heat loss of the house. Then the temp deferential can be widened. Only when -3C outside will the system be flat out and running to its design. Everything is sized for the worst case scenario, -3C outside.
You tend to be the reason why most hate plumbers - spending other's money like it grows on trees. Care to cost out that little exercise? If and when the cylinder needs replacement it will be time to consider such a thing.
FWIW the existing system already acts like a combi in that it will give an adequate temperature shower continuously. As I discovered once having come back from holiday with the system off and wanting a shower near instantly.
There would be no advantage in having a smaller cylinder anyway - it's in a purpose made cupboard which isn't much use for anything else. The airing cupboard is part of the boiler cupboard. And not much use now. ;-)
I was just curious as to why my original boiler had 28mm fittings - and that my expert specified that for the 'primary loop' while the near enough same size modern boiler uses 22mm. And thanks for the various explanations.
Of course my original system was open vented so you couldn't mess about with pump speeds regardless. It was in fact happy on minimum.
Yup - I've done that.
Uh! I'm no plumber.
Cheaper than a large cylinder with quick recovery coil. It also promotes highly efficient condensing, heating the cylinder top down and maintaining a very cool section at the bottom of the cylinder. The boilers heat is dumped right at the top of the cylinder, ready for use immediately at the taps. Within a minute or two from cold, a sink full of hot water is available
Here is one off the shelf:
Not if the cylinder is discharged of heat it will not. It is also inefficient all around. Cheaper to have a small, cheap direct cylinder, a plate heat X and bronze pump. A flow regulator may be required to maintain the pump flow to make the boiler act as a combi when no heat in the cylinder. An adjusted gate valve usually does it though.
The message from "Dave Plowman (News)" contains these words:
I am more than a bit busy atm but I am sure I posted some relevant information when a similar subject was aired recently. A quick google might find it. Now going out for lunch so can't check myself.
The message from Roger contains these words:
What I said in January:
The larger the bore the more water can be pumped though and further but in a normal pumped system you may not need 28 mm even if you have one circuit serving the whole house.
My information is grossly out of date but the figures I have (taken from Ravensbourne Beginners Guide to Central Heating) are:
Length limits at maximum flow rates (3 feet/sec)
Tube size (mm) Max flow (gallons/hr Max length (feet)
15 105 80 22 240 120 28 390 175And at those values the heat capacity and head loss are:
BTUs/hr inches water gauge/foot run
15 21,000 0.90 22 48,000 0.60 28 78,000 0.41
Whatever.
Cheaper than the one I already have?
I have near unlimited hot water already. Be more interesting if you gave a figure for gas saving.
As I said size doesn't matter.
As I said I came back from holiday and switched the system on - then had a coffee. By the time I'd finished that the shower was fine - and continued for as long as I needed. It was however in the summer.
You keep using that word cheaper. I'd need to know the payback time before replacing an existing one.
The idea of my boiler acting as a combi makes me shudder...
On a fully pumped system I would not expect to /need/ 28mm connections until the primary rating is above 30kW (+/-). The old boiler would have had 28mm connections to allow for the possibility of gravity circulation.
Please read what I wrote. What you don't understand, ask. Trying to be a natural plabntpot...oh I forgot! You are a plantpot!
24kW in condensing. Read my post.
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