Central heating in 15mm

[marsaday]

Is there a problem with using 15mm for the heating system ?

A house i have bought is currently in 15mm, but was burgled before i bought it and so a lot of the copper has been ripped out.

So this means i have to put in a new lot of pipe work. I will be using 22mm plastic, but wondered could i use 15mm again. The system is quite small, feeding 3 upstairs rads and 3 downstairs.

I Will be using 22mm i am sure, but would like to know what problems you can get with 15mm. Does it take longer to heat up for example ?

 

[kay-jay]

15 mm is standard, problems only with 8 and 10 mm, 22mm not needed (other than a couple of meters from boiler)

 

[mountainman]

It would be much better to use 22mm from the boiler to both upstairs and downstairs then reduce to 15mm for the last two rads on each floor.

 

[marsaday]

That's weird because i have been told i need to use 22mm for the main spine of the 22mm and i was going to do the drops in 22mm as well as there was little cost difference.

 

[kay-jay]

no! main pipe from boiler 22mm then step down to 15mm for drops, will look better and is more than adequate for your heating needs.

 

[mountainman]

Drops???
is the ground floor concrete then??

 

[dancinplumba]

i would 22mm to first rad and drop, then rest in 15mm, with kay-jay, dont need 22mm drops unless you have one drop to rads downstairs then this wants to be 22mm to IMHO ad dont forget the by-pass

 

[marsaday]

yes it is a concrete floor downstairs. So i will do all my main feed on the first floor in 22mm and then use 15mm for the 4 drops. Dont think it is that much cheaper using the plastic.

i thought combis have internal bypass's now ? I will leave one rad with no thermostat on it.

 

[dancinplumba]

yes it is a concrete floor downstairs. So i will do all my main feed on the first floor in 22mm and then use 15mm for the 4 drops. Dont think it is that much cheaper using the plastic.

i thought combis have internal bypass's now ? I will leave one rad with no thermostat on it.

they do and the rad may not be adequate unless its a big one, check your boiler MI's also put in room stat,build regs requirment and energy saving. imho

 

[doitmyself]

The correct sized pipe is determined by the amount of heat they are required to carry and the flow rate. See Small Bore heating Systems and Copper Tubes in Domestic Heating Systems

The flow rate will depend on the design temperature differential (20C for condensing boilers). However rads will then need to be "oversized" by about 20% as their output is reduced.

 

[kimbo]

Doitmyself is spot on 15mm pipe can only carry I think about 3.5kw although I may be wrong so if your rads require 4kw they wont reach their correct output. as a general rule of thumb no more than 3 rads on 15mm.

 

[Best]

As above, - 15mm should only do 3 or 4 small rads max & 1 or 2 larger rads max.
I would only do one large rads from 15mm.
Even a small house should have 22mm to each floor, or in the case of pipe drops, then 22mm to all tee offs, except perhaps the last couple upstair rads.
You never know if later a house gets a room or two added, & then you are glad to have 22mm to connect to.

 

[marsaday]

ok thanks for all replies.

It will be 22m in the pipe work feeding all the rads upstairs and then t off's down to the next floor in 15mm.

 

[P4ULT]

I personally would never run more than 3 rads off of 15mm pipe. if it is drops to individual radiators 15mm is your best bet looks much better than 22mm.

 

[Mike Jackson]

As a rule of thumb I generally say 2 rads, max 3 on 15 depending on size. With the cost of copper now you don't want to be running too much 22.

 

[CES]

I agree, I've got a landlord who has a system ran in 15mm. six rads all piped up in 15mm, first two rads are skelping, others are barely lukewarm.

 

[mountainman]

As a rule of thumb I generally say 2 rads, max 3 on 15 depending on size. With the cost of copper now you don't want to be running too much 22.

Too true, I bought 3m of 22, 3m of 15, a few 22 endfeed elbows and sockets the other day and spent £40.
When I was a lad I could have bought a Lambretta for that and had much more fun.

 

[doitmyself]

This table may help! Although it shows kW for 0.3 and 1.5m/s, the aim should be to size the pipe for a flow of about 1m/sec.

The greater the capacity of the pipe compared to the radiator attached to it, the more the LS valve has to be closed down.

 

[billybob150811]

i was told a rule of thumb 6kw 15mm 24kw 22mm above 28mm but looking at these 30+kw combi,s with 22mm f&r i wounder if that still holds good

 

[AWheating]

i would suggest 22mm off the boiler to your zone valves and then you should be fine reducing down to 15mm after the first rad, if not straight off the zone valves.

doitmyself has given you some very good information/links to help you decide whats suitable. dont just size off the max kw 15mm can supply but also consider length of runs as this has a major effect on output and noise.

 

[Best]

i was told a rule of thumb 6kw 15mm 24kw 22mm above 28mm but looking at these 30+kw combi,s with 22mm f&r i wounder if that still holds good

Sealed heating systems, (as a lot now are), & high head pumps will mean the pipes will do bigger output.
Even though the boiler may have only 22mm connections, it may be better to still have 28mm from boiler to first rads tees in some houses, especially with long pipe runs.

 

[desalman]

I did some research whilst designing my CH system and found :- 22mm up to 11,500 W, 15mm up to 4,250 W, 10mm up to 1,948 W.

I have done all my design sizing based on those limits - I hope I am right, if not then some expensive installation will have to be undone.

 

[Best]

I did some research whilst designing my CH system and found :- 22mm up to 11,500 W, 15mm up to 4,250 W, 10mm up to 1,948 W.

I have done all my design sizing based on those limits - I hope I am right, if not then some expensive installation will have to be undone.

Those limits are on the low side, even on an open system, so you are well on the right side.
My limits I use for an open heating system are approx 18kw for 22mm & 6kw for 15mm.
Never any harm in staying well within those maximum limits.

 

[doitmyself]

Those limits are on the low side, even on an open system, so you are well on the right side.
My limits I use for an open heating system are approx 18kw for 22mm & 6kw for 15mm.
Never any harm in staying well within those maximum limits.

Those limits are fine for systems with non-condensing boilers running with a 10C or 11C drop. However, for condensing boilers, the Domestic Buildings Compliance Guide 2010 recommends (see page 16) a return temperature of less than 55C.

If you just reduce the flow temperature to 65C, so the differential remains 10C, the output of each radiator reduces by 25%. Rads would then have to be "oversized" by 35% to give the required output. If the flow temperature is kept at 75C and return reduced to 55C (20C drop), the rad output reduces by only 15%, so 17% oversized rads would be required, with an obvious saving in cost and smaller rads in the house.

Condensing boilers are now designed for a 20C differential so you will meet problems, if you try to run the system with a 10C differential, due to the much higher flow resistance through the heat exchanger. This is caused by the higher flow rate (nearly double) of a 10C differential compared to a 20C differential.

Another advantage of a 20C differential is that the pump does not have to be so powerful as the flow rate and head in the system is lower. Pipework can also be smaller size due to the higher capacity the same size pipe can carry when running with a higher differential. (See the table I posted earlier.)

 

[Best]

All that really matters IMO, is that the pipes are not undersized, & only a matter of balancing properly.