How many BTU's will 15 and 10mm pipe supply?

[mikesdiy]

The central heating in my house has evolved over the years; there is a 22mm backbone to the boiler with 15mm and 10mm branches. It is an open vented system and at a guess several radiators have been added onto the branches over time.

There must be a limit to the heat supply capability of 15 and 10mm pipe, can anybody help me with some rough indication as to what the limits are? Obviously it is no good having the boiler big enough if it cant get the heat through to all the radiators because they predominantly hang off a 15mm pipe.

 

[oilman]

15mm can supply about 4kW, or twice that if you double the flow/return temperature difference. 22mm is about 13.5kW which is usually the real heating limiter. However, the way to get round it is to have the heating on for a longer time, so even a 3kW boiler will give you 72kWh over a day.

 

[DP]

I had to get the book out for this one as I rarely change boilers let alone full instals, therefore while aware of pipe sizing, rarely have need for this information

28mm will supply up to 68000 BTU per hour over 140 foot line. This bore will carry 340 gallons of water perhour

22mm- 38000 btu per hour over 100 foot line. capacity 190 gallons per hour

15mm 15000 btu/ hour on 80 foot line. Capacity 75 gallons per hour

One gallon of water weighs 10 pounds.

22mm pipe will carry 190 gallons. Follows pipe capacity is 1900 pounds per hour

If 20 degree F is sunk by the load, heat released is 30000 BTU per hour.

Information sourced from book by Trevor Crabtree

 

[mikesdiy]

Thanks guys, thats brilliant. Confirms what I was comming to suspect; there are too many radiators hung off a 15mm pipe! Some of this pipe needs to be upgraded to 22mm. Thanks again.

 

[Agile]

You cannot come to that conclusion quite so quickly.

As Danny's figures should have alerted you, the salient aspect is the distance involved on each pipe length.

In a very rough rule of thumb, if your 10 mm is within 3 m of a 1 kW rad ( 6 M total length ) and your 15 mm within 3 m of 4 kW of rads then it is likely to be OK.

You can check it by measuring the temperature that each room reaches, its thats Ok then the pipe sizes are OK.

To investigate one step further measure the flow and return temperature at each rad and rebalance if necessary. Thats more likely to be the crux of any problem. Balancing becomes much more important with several rads on the end of a high resistance circuit.

Does anybody have time to convert Danny's figures into modern units?

Tony

 

[oilman]

Not necessarily. Are the radiators always on full tilt?

 

[corgiman]

if they are they better level them out it will look rubbish, tilty rads

 

[croydoncorgi]

With a condensing boiler (if you have one) it's a Bad Idea to provide an 'easy' path for the water to get back to the boiler. It will raise the Return temperature and the boiler will be less efficient as a result. If the system gives adequate warm-up time and holds the comfort level you want in all areas, it's not broken, so don't fix it.
If you put in larger pipes to serve one area you may also starve other areas still served by undersized pipes and will then have to restrict some flows by more 'balancing' of rads, etc.

 

[croydoncorgi]

PS If you want a good reference site for pipe capacities, pressure losses, etc. see http://www.ukcopperboard.co.uk/