28mm/15mm CH system to 22mm/10mm

[mikej2005]

Hi guys,
The 35 year-old Glow-Worm conventional boiler that heats my fairly large 4-bed house is situated about 4m away from the main body of the house, at the far end of a single-storey extension.

The flow and return pipes that exit and enter the boiler are 28mm (boiler has 28mm connections) and this size pipe continues at least as far as the utility room is long (ie. 4m), before disappearing into the house and, after the HW cylinder, eventually reduces to 15mm at the radiators. (There may be 22mm sections inbetween but I haven't taken the floor up yet)

The current system is costly to run and inefficient, so the idea is to replace the boiler and radiators and reduce the overall system volume (the circuit currently also runs under the floor downstairs in 22mm pipe and we're going to be removing this and connecting downstairs radiators from above)

Now - the new condensing combi will have 22mm flow and return. The plan is to take that 22mm into the house and come off that in 10mm microbore to the radiators via manifolds.

My question is - is this thinking 'sound' ?

I realise that you can't make specific comments without seeing the 'site' so it's general observations/rules of thumb/thoughts that I'm after here.

ie. Is 28mm used in domestic installations of this size these days ? Would there ever be a case for going straight to 28mm from a 22mm connection on a modern boiler and if so, what would be the reason for doing this - length of pipe runs/property size etc ?

Many thanks in advance for your thoughts.

 

[chrishutt]

I can't see why there should be any advantage in reducing the size of the existing pipework. Providing they're well insulated where they don't contribute to heating the house, the larger pipe sizes will have no disadvantage, apart from aesthetics.

 

[mikej2005]

I can't see why there should be any advantage in reducing the size of the existing pipework. Providing they're well insulated where they don't contribute to heating the house, the larger pipe sizes will have no disadvantage, apart from aesthetics.

There are a number of reasons why I'm replacing the original pipework - reducing the system volume isn't a driving factor, but merely a consequence of moving to a system like I described.

Seeing as how all the boilers I'm looking at come with 22mm outlets, I was presuming that there was no need for larger diameter pipes in the typical houses these boilers are designed to heat ?

Surely the radiators will heat up quicker with less cold water in the system to shift initially too ? Flow rates can also increase considerably when moving from 28mm to 22mm as far as I've read.

 

[chrishutt]

Surely the radiators will heat up quicker with less cold water in the system to shift initially too ? Flow rates can also increase considerably when moving from 28mm to 22mm as far as I've read.

Yes, the system will heat up more quickly with less water content, but I wouldn't expect that to have much significance in practice. As for flow rates, logic suggests the opposite. For a given pump pressure a bigger pipe (28mm) will present less resistance than a smaller (22mm) pipe, hence better flow rate with bigger pipe.

Pipe sizing can be quite complicated, but in essence the object is to use the smallest practical pipes (mainly for installation cost reasons) without having excessive resistance to flow that the pump would not be able to overcome. You need to take account of the total heating load (usually expressed in kW) and the design temperature drop (traditionally 11°C). If heating load is high you might be pushing at the limits of what your pump can handle.

 

[gas4you]

As a general rule if your heat requirement is over 60,000 btu (18Kw approx) you will need to use 28mm as far as the zone valves, once the heat load drops then you can start reducung the pipe size.

You need to check your heating requirements accurately.

 

[chrishutt]

A general rule that fails to take account of the length of the pipe run or the temperature drop. Short runs of 22mm can handle well above 18kW (look at boiler pipe connections), but very long runs may require 28mm, even below 18 kW load, depending on total circuit resistances. There are useful rules of thumb but they only apply to typical situations. No substitute for understanding the principles and knowing when you need to calculate.

 

[bengasman]

In order to achieve your ∆t 20K, as is required with every HE boiler I have ever installed, you have a very low flow per radiator. Unless you have a very large radiator, it is usually harder to get the flow low enough than to get enough water around. When installing a system from scratch in smaller properties, up to 4 bed, I started using 15 mil plastic after the very first split. I dare say that anyone promoting 22 mil or larger does not consistently check the 80/60 value when balancing the system. Not too hard to get a 20 degree drop with all rads on and the system warming up, but how about your standard winter day with only lounge, batroom and hallway on? I found it almost impossible with radiators of less then 1.5 kw. By the way, 95% of my work is from word of mouth; I don’t advertise so I guess my customers are happy with how their new system works.

 

[bengasman]

the gobledygook in first line should be 'delta'

 

[gas4you]

New condensing boilers should be running at 60/40.

 

[bengasman]

Generally, the lower the temperatue, the better the efficiency as far as I know. I basically only install worcesters and the manual of those gives you 3 different opions, but I don't think that the 60-40 is always a must. From what I understand, the main thing is that the return temp stays under 55. A bit theoratical anyway as most users tend to fiddle about anyway

 

[chrishutt]

New condensing boilers should be running at 60/40.

Not much use if fitted to an existing system with radiators sized to operate at delta t 55° (rad temp 76° and room temp 21°). You will get a rad temp of 50° and delta t of only 29°, so little more than half the required output of the rads in design conditions.

 

[gas4you]

As I've stated before (not in this post) the 60/40 is for new systems, but I've never fitted a new condensing boiler yet on an existing system that I've had to leave set higher than 65C.