Justifying the 'Rules of thumb' for 15mm and 22mm CH pipes

[DIYpozer]

Hi All,

I'm trying to work out why the 'rule of thumb' is ~3 radiators on a single 15mm run...

I've calculated the heat demand of my house at ~14kW:

• 3 bed 1930s solid wall, end terrace, no wall insulation
• 7 radiators
• 2 air change per hour for all rooms
• 21deg in all rooms

When I do the mass flow calcs for a deltaT of 20 degrees, it needs 612 L/hr to pass through the pipes.
Then working out the linear flow velocity required to deliver that volume in a 15mm pipe (13.6mm ID), it comes out as ~1.1 m/s, which is ok.

So what am I missing? Why couldn't 15mm pipe be used in this case for the main line and the radiator branches?

Thanks!

 

[Timmy D]

I’ve not done and won’t do the calcs but have you taken in to account frictional losses (fittings etc)?

An increase in pump energy usage to overcome the frictional losses.
Also increased wear/cavitation and noise?

I wouldn’t run more than 2 rads on a 15mm branch.

 

[Enthalpy]

A quick pressure drop calculation for a flow of 0.612 m3/h water in 10 m of pipe gives:

• 15 mm pipe (13.6 mm ID): 138 mbar
• 22 mm pipe (20.2 mm ID): 21 mbar

Pressure Drop Online-Calculator - http://www.pressure-drop.com/Online-Calculator/

138 mbar is a significant part of a typical pump’s head.

A full pressure drop calculation needs to consider everything: arrangement of piping, actual pipe lengths, bends, valves (motorised, lock shield and TRV), boiler, filter, radiators.

Even if you can use 15 mm pipe it might be more economical to use 22 mm pipe and run the pump on a lower speed to reduce the consumption of electricity.

 

[ShaunCorbs]

Also new part L rules mean your rads need to be bigger / sized correctly for a flow of 55 eg dt25 instead of going off the dt 50 on the rad documents also main runs in 28mm etc due to later on upgrading to a heat pump etc

 

[DIYpozer]

A quick pressure drop calculation for a flow of 0.612 m3/h water in 10 m of pipe gives:

• 15 mm pipe (13.6 mm ID): 138 mbar
• 22 mm pipe (20.2 mm ID): 21 mbar

...

138 mbar is a significant part of a typical pump’s head.

A full pressure drop calculation needs to consider everything: arrangement of piping, actual pipe lengths, bends, valves (motorised, lock shield and TRV), boiler, filter, radiators.

Even if you can use 15 mm pipe it might be more economical to use 22 mm pipe and run the pump on a lower speed to reduce the consumption of electricity.

Yes I spent yesterday coming to the conclusion that while you can, it doesn't mean you should, but it is not because of having in-sufficient heat flow potential.

I have a Greenstar 28i Junior, it apparently has a 3 speed pump, but the only thing I can find about the pump head is this one line in the installers manual:
"Available pump head at 21°C system temperature rise: 2m"

If I've understood correctly, that is roughly enough pump power to overcome losses of up to 20kPa. Does that sound right?

 

[DIYpozer]

Also new part L rules mean your rads need to be bigger / sized correctly for a flow of 55 eg dt25 instead of going off the dt 50 on the rad documents also main runs in 28mm etc due to later on upgrading to a heat pump etc

I've seen this, but from what I have read, it only applies for:

• New Boiler Install (where if pipes are inaccessible you 'do your best' to get the lowest flow temp)
• New Boiler + Pipes + Rads

If you aren't changing the boiler, it doesn't sound like you need to achieve this 55deg flow or size for it?

 

[ShaunCorbs]

If your replacing the heating system yes you do need to comply with regs

 

[DIYpozer]

If your replacing the heating system yes you do need to comply with regs

But what constitutes the 'system'?
The document doesn't make it clear that changing pipes alone counts, as that alone doesn't enable a 55deg flow?

 

[ShaunCorbs]

Part L

To need to make sure the pipework can handle the new flow temp / dt even if the rads are old, tbh I can’t see why your not changing / upgrading the rads as well

 

[DIYpozer]

Part L

To need to make sure the pipework can handle the new flow temp / dt even if the rads are old, tbh I can’t see why your not changing / upgrading the rads as well

Oh I will, but it may not be at the same time.
I am retrofitting a very cold house, and the heat demand per room is too high for 55 deg flow without other retrofits to bring the fabric and air change losses down (mainly solid wall insulation and some extra loft insulation), which I also can't do at the same time...

It could be a year or more before the rest of the building catches up with the heating system changes, and if I size 'correctly' for the current heat loss, by the time I upgrade the rest, I'll probably then be overheating the house

So what do people do in this situation?

 

[ShaunCorbs]

I can’t see how a leaky house has anything to do with pipework sizing / upgrading your rads your heat loss decides this ?

Also isn’t it more cost effective to sort the leaky / more insulation before you sort the heating out as this will remove load from the system and might make it work properly?

 

[DIYpozer]

I can’t see how a leaky house has anything to do with pipework sizing / upgrading your rads your heat loss decides this ?

Also isn’t it more cost effective to sort the leaky / more insulation before you sort the heating out as this will remove load from the system and might make it work properly?

For sure its chicken and egg in a way...

But it is linked as the fabric heat loss / air changes determines your heat loss per room (air changes can be 50% of fabric losses for old houses), which gives your heating demand that the system needs to deliver, per room. As I understand it, the new regs say that the installer needs to assess the whole property's heat demands (room by room, which I've done), ironically to avoid oversizing the system.

So for my case, for a single room, I have 2.2kW demand (2 solid walls, big bay window). That needs to be a 1400*600 Type 22, which has some margin, for a 75/65/20 system design.
If I have to oversize to 'allow' compatibility with 55/45/20 system design, I'd need a radiator with an output 244% bigger... or a radiator 3000*700... completely ridiculous sizing.

The problem is cost.

• External wall insulation will cost £15k for my property, and may need the garage to be removed or removing the hallway stairs to properly allow internal insulation to be installed (or justify deviations with building control etc).
• Keeping the boiler I already have, and upgrading the 15mm single pipe system to a 22mm two branch layout, and replacing only the radiators which are needed now (the house is mostly empty), will cost me <£1500...

I assume this is why the new regs say that if it's not possible / "technically, functionally or economically feasible", the installer needs to design the system to the lowest flow temp possible, while still delivering the heating needs of the building.

I'd rather be warm when contemplating how to solve the wall insulation dilemma!

 

[ShaunCorbs]

It’s one of them as you but if you oversize depending on the type of boiler you have you can turn the flow temp down eg combi down to maybe 40 dc flow so the rads won’t over heat the room