How Many BTUs Or KWatts Can A 15mm Heating Circuit Take?

[2point8ghiax]

I have just bought a house that has a botched heating system. The Glow Worm 30CXi combi is downstairs running four 730mm high X 800mm wide single rads, on a 15mm flow and return heating circuit.

I estimate the total BTUs of these four rads at 13,000 (3.8 KW). Are the 15mm F and R pipes sufficient for the load?

If I wanted to add four extra rads upstairs, can I tee off the F and R pipes on the short run of 22mm F and R underneath the boiler? The new rads for upstairs will be approx 50 percent more output than the existing ones. Should I run a new 22mm F and R circuit for these, or is 15mm OK?

 

[Deleted member 105431]

as a rule of thumb 15mm is good for two rads with pipework no greater than 5kw.

 

[gas4you]

Usually one would take 14,000 btu's as the maximum efficient heat carrying capacity for 15mm, or approx 4Kw.

This will also depend on the temp difference required between flow and return and length of pipe run.

 

[bengasman]

Isn't the 30 cxi a steamer? If so you can run quite a bit of 15mm as it requires a lot less flow due to the 20 degree drop.

 

[snb]

with 20 degree drop and 1.5m/sec flow you can have up to 18kw on 15mm

with 20 degree drop and 0.3m/sec flow you can have up to 3.4kw on 15mm


with 11 degree drop and 1.5m/sec flow only up to 10kw on 15mm

with 11 degree drop and0.3m/sec flow only up to 2kw on 15mm

 

[2point8ghiax]

Thanks for the replies so far. Reading between the lines, it sounds like the 4 rads are OK on the 15mm pipes?

Yes, the 30 CXi is a condensing boiler.

What does 20 degree drop and 11degree drop mean? Drop between what, flow and return at boiler?? How do you change this?

Also the different flow rates, how do I know what the flows are, and how do you change them?

Sorry, its a bit confusing!

 

[bengasman]

The drop mentioned is between the flow and return. You change it by slowing down the pump and/or squeezing the lockshield valve

 

[glasgowgas]

Thanks for the replies so far. Reading between the lines, it sounds like the 4 rads are OK on the 15mm pipes?

Yes, the 30 CXi is a condensing boiler.

What does 20 degree drop and 11degree drop mean? Drop between what, flow and return at boiler?? How do you change this?

Also the different flow rates, how do I know what the flows are, and how do you change them?

Sorry, its a bit confusing!

as Mr Tony Glazier would say.........................

if you are confused, sir, may i suggest that you have neither the knowledge or, as is probable, the required certification (from a notaried certification pre requisite body).
Why don`t you post this in the Combustion Chamber?

 

[2point8ghiax]

Yes, his reputation preceeds him. Unfortunately, I will not be offered the sanctuary of the combustion chamber, as I am not of The Worshipful Order Of Plumbers And Gas Engineers.

As a humble 'lay person', he may assume that I know better than qualified people on this site, that I will ignore their advice and am liable to blow myself and my neighbours to smithereens!

In another life, he may have said "you can't park there!".

I've said too much, I'm going to cop it now from the 'RGI Registered Engineer'

 

[glasgowgas]

cool, you are online

the temp diff is between flow and return measured @ boiler

20 deg C for steamers

11 deg C for standard efficiency

because return is coming back at 50 deg C boiler is in "condensing"
mode longer
for steamers



because you cant fit SE boilers





cos you are English.

 

[2point8ghiax]

Cheers Glasgow. This makes it all clear now. So, as my boiler is a condenser, its a 20 degree drop. I see now.

So even if its pumping at only 0.3 m/sec, its just about enough to feed the 4 rads that are on the system.

Thanks all

 

[D_Hailsham]

So even if its pumping at only 0.3 m/sec, its just about enough to feed the 4 rads that are on the system.

You are getting slightly confused. It's not the speed of the pump which is important but the speed of the water flowing through the radiator and its connecting pipes. If the speed is too low, sludge can build up; if too fast, you will hear noises in the rad and pipework and TRVs may bang and not work properly.

You adjust the pump to give the correct differential - or as near as possible -at the boiler. This then sets the flow rate (litres/sec) of the water through the boiler The speed of the water (m/sec), for a given flow rate will change vary depending on pipe diameter - large pipe, slower speed; small pipe faster speed.

When it comes to the radiators and associated pipework there are two problems to contend with: sludge and noise. A speed of less than 0.3m/sec will cause sludge problems and above 1.5m/sec will cause noise problems. A speed of about 1m/sec is normally aimed for, but compromises have to be made as there isn't an infinite number of pipe sizes to choose from!

Apart from the speed, another factor to consider is the pressure loss due to friction. A 15mm pipe may be adequate for the heat travelling through it but, if the pipe is very long, the friction in the pipe may be more than the pump can overcome. A larger pipe would correct this.

You say you have 3.8kW downstairs and the upstairs will be about 50% larger, i.e 5.7kW. The flow rate through a 15mm pipe would be about 0.5 m/sec, which is OK. If you used 22mm the speed would be about 0.2m/sec, which means you could get sludge building up in the pipe.

 

[doitall]

Must be a Bath thing

 

[bengasman]

Methinks the sludge problem can be ignored if the system is installed correctly and maintained properly.

 

[D_Hailsham]

Methinks the sludge problem can be ignored IF the system is installed correctly and maintained properly.

But the OP said he has a botched heating installation

What nobody had mentioned, and I have just realized, is that this is another case of a combi possibly not being appropriate. The OP currently has 3.8kW of rads, while the 30cxi can deliver 4.95-22.4kW for heating. So the boiler would not be able to modulate low enough and would be running in on/off mode virtually all the time. Adding the extra 5.7kw of rads would bring the total up to 9.5kW, which is better but still less than half the boiler's max output. OK the boiler can be set to give a lower max output on the menu, which would be a good idea in the OP's case.