boiler & feed pipes BTU capacity

mymedi

Hi everyone,
I must be misunderstanding something in the plumbing science of things.
We need to provide CH to about 17 large radiators which in total are about 85,000BTU. The boiler is a Worcester CDi550 HighFlow with the capacity of about 30kW which would be sufficient. However, from what I understand a standard 22mm pipe can carry roughly up to 13.4kW of heat or about 45,700BTU. The boiler has got a single 22mm outlet for CH.
How does that work out - if 22mm can only carry about half of what we need, we would need to split the boiler output to two 22mm pipes, but would that not limit each of them to half of the boiler's outlet limit?
Or is it not just a matter of capacity?
Many thanks!

Plumber90

All boilers I've seen come with 22mm connections on them. Just run your flow and return pipework in 28mm and it will be fine.

Surely whoever is going to be fitting this for you should know all about this.

mymedi

It's not a matter of me not trusting the plumber that this will work.
I just want to understand how and why this works, because on the surface it shouldn't...

jayyar66

Without going too deep (and TBH it's been so long since I actually needed this information that I'd have to dig my books out to swot up on it!!) I'll try to help.
A kilogram of water at whatever temperature the boiler produces is able to carry a certain amount of heat, therefore the greater the flow from the boiler the greater the amount of heat is available to the system.
Now, if you have a circulation pump trying to force water through 2 lengths of 22mm pipe, 1 is less than a metre long and the other is maybe 20 metres or more, then you would get far more water flowing through the short pipe than the long one. This is due to the resistance that the internal surface of the pipe offers to the water traveling inside it. Add to that all the formed bends, elbows, tees, burrs where the pipe has been cut, 15mm pipe, valves, blobs of solder in the pipes etc and the flow of water is effectively being choked and is vastly reduced.
However, when compared to a typical domestic system, the boiler has a negligible amount of 22mm pipe with maybe 1 or 2 bends. This means that the internal resistances are minimal and therefore the boiler is quite capable of doing exactly what it says on the box.
The 13kW/22mm and 6kW/15mm is just a rule of thumb (albeit quite a reliable one) for the whole system in general and bears no relation to the science of Fluid Dynamics. Hope this helps :-)

a&akay

We've a largish house with 22 radiators. When I installed the pipework I used an old British Coal pamplet to work out pipe sizes. All measurments were in BTUs and Imperial pipe sizes. This produced a requirement for a shortish run of 10m in 28mm flow (F) and return (R) to where the system split for downstairs, upstairs and downstairs granny flat sub loops in F&R 22mm and radiator feeds off them in 15mm. These 3 looops and the hot water tank loop all have their own zone valves and thermostats. The 22mm boiler F&R feed into these pipes via a 28/22mm reducer immediately below the boiler. There's never been a problem with heating even in the coldest Winters, like we had recently, and that's with no double glazing and the granny flat run at a constant 85 degrees F all year round. The shower was on constantly for 8 people at Christmas and the hot water recovered each time OK as well. The boiler's a 25 kW/85000 BTU.

mymedi

So, if I understand it right, it's not so much about the pipe size, as the ability of the boiler to fill them? If you run 28mm from the boiler with branching off into 22mm later, it's Ok for the boiler to have a 22mm outlet as long as the flow is sufficient for it to fill the 28mm fully? Presumably, this wouldn't work well in a low mains pressure situation?
Thanks very much.

mymedi

a&akay wrote: »

These 3 looops and the hot water tank loop all have their own zone valves and thermostats.

May I ask why you have separate thermostats on each loop? To set different temperatures on each level of the house?
Wouldn't the different thermostats conflict with each other on the boiler?
Or do they only control the zone valve?
Thanks!