Longest pipe run from combi?

[baker332]

Hi,

I need to run some new copper central heating pipe 15mm from the manifold to new radiators in an extension, the run to the first rad could be up to 8m and to the next possibly 12m - is it too far away? What is the longest the combi boiler could push around the system without degrading performance too much?

The pipes will be hooked up to two quite large radiators.

Thanks

 

[oldbuffer]

1. Heat carrying capacity of 15 mm is around 4Kw at a speed which doesn't cause noise.
2. The answer depends on:
2.1 The capacity of the existing system. Is it 22 mm to the manifold then 15 mm to the radiators. How many radiators on existing.
2.2 The heat losses in the existing pipework. What is the length? Is it insulated? Will the new lengths be insulated. You lose about 28 watts / metre if uninsulated and around 8 watts / metre if well insulated.
2.3 The heat requirements of the radiators.
2.4 Whether the radiators are fed independently from the manifold, or one feed from the manifold running to radiator 1 then to radiator 2.
3. If possible it would be better (rule of thumb) to run 22 mm from the input of the existing manifold to individual radiator feed in 15 mm.

 

[martygturner]

The question is not really one of length of pipe or what the boiler can pump a 27kw combil will pump 200 -300m of UFH all on its own and still maintain 50 degrees or whatever the most efficient temp point it can condense at.

So the question is does your boiler pump out enough BTU's to heat the rads at its most efficient setting and do you plan to insulate the pipes if you can.

 

[Madrab]

The question is not really one of length of pipe

Totally agree the boiler needs to have the extra capacity to have new rads added but if there isn't enough head in the pump then it may struggle to adequately supply rads that are on overly extended runs of 15mm pipe especially with big rads.

A system's water will always take the path of least resistance and an overly long length of 15mm will be less likely to flow well when there are other easier paths that the flow can take. One of the primary reasons a system needs to be balanced especially when there are overly long runs of different sized pipework. Even then though sometimes even that isn't always enough

I came across that very issue when a client had issue with a large K2 (2.2kw) in their extension fed by ~14m of 15mm, just couldn't get enough flow to it, even when the system was properly balanced and there was still an extra 8Kw of headroom left in the boiler. Extending the 22mm backbone wasn't an option either. Old centralised system pump had been removed when an other replaced the boiler for a system boiler. It simply came down to a lack of flow and resistance through that extended pipework, when all the rads were calling and the system pump wasn't able to produce enough head to feed everything. Secondary pump added back in centrally and system is now fine.

15mm from the manifold to new radiators in an extension

As suggested, is there enough headroom in the current boiler? What manifold is it and the current size of system (# of rads)? Is there an option to at least extend the 22mm somewhat? What size of rads are being added and please don't say they are columns.

 

[BlueLoo]

I've a manifold system (google it, a simple brass manifold which commons the supplies and returns).
Off this manifold i have rad runs of 5m, 10m, and 20m.

All run off a new std pump in PP mode.
My rads are all independently controlled so at any one time, they can all be open, or closed or somewhere in between.
All of the lockshields are fully open. The system is deliberately not "balanced" (Or rather, i'm not throttling radiator throughput).
All rads have TRVs.

System works a charm. No rads ever struggle and are quiet and well behaved.

I imagine you will be fine.

 

[Mike13]

Well mine has always been a pig to balance.
When I used this link a few days ago it was all in English, update, found the english version:

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

using that and the nominal figures of power for each rad I ended up with is as the pressure loss each way to drive the heat through the rads, I know that at max bypass pressure I can get 350mB from my combi pump, your figures may be different.

I have worked out there are two places where I could extend the 22mm spine of the system, both choices reduce the max pressure drop, so would help but will cause cost cost and upheaval to fit.
You could do your own version of the diagram.

this site goes into a lot of theory, and also discusses that occasionally you might need a 15mm out to stop it taking too long to heat up, but a 22m return helps keep the pressure and lifetime pumping costs down a bit.

Pipework Calculations - Heatweb Wiki

www.heatweb.co.uk

 

[baker332]

Thanks for the replies everyone, lots of info here and quite a something to think about.

There isn't really an option to use 22mm piping as I'd need to rip up lots of floor boards, this pipe is taken from a rad that was removed and not required being on it's own run from the manifold.

It will supply two large rads x2 1800 x 472 double vertical.

The rest of the property has 15mm runs but then gets reduced to microbore to feed off to the rads, with TRV's and is balanced nicely therefore everything heats up swiftly.

The boiler is a baxi 836 (36kw).

The pipes will be insulated but they will go into the screed, I assume this could mean some heat loss (3m will be buried) - there is the option to not run the pipes in the floor and mount the rads onto another wall (ruins the designed look) but also reduces the pipe run.

 

[BlueLoo]

Thanks for the replies everyone, lots of info here and quite a something to think about.

There isn't really an option to use 22mm piping as I'd need to rip up lots of floor boards, this pipe is taken from a rad that was removed and not required being on it's own run from the manifold.

It will supply two large rads x2 1800 x 472 double vertical.

The rest of the property has 15mm runs but then gets reduced to microbore to feed off to the rads, with TRV's and is balanced nicely therefore everything heats up swiftly.

The boiler is a baxi 836 (36kw).

The pipes will be insulated but they will go into the screed, I assume this could mean some heat loss (3m will be buried) - there is the option to not run the pipes in the floor and mount the rads onto another wall (ruins the designed look) but also reduces the pipe run.

Two rads off one feed?
Do the splits in parallel, not series.
Insulate under the screen and everywhere else.
Assume you've done basic heat calc checks for the additional load on the boiler.

 

[baker332]

Two rads off one feed?
Do the splits in parallel, not series.
Insulate under the screen and everywhere else.
Assume you've done basic heat calc checks for the additional load on the boiler.

Originally thought the house had a manifold but now looks to be a trunk and branch system, from 22mm to 8mm microbore to the radiators.

Assume this is effectively the same thing? Tapping into the 22mm pipe then running 15mm to the two radiators?

 

[baker332]

Come to think of it, I could take another branch off elsewhere to run the rads independently.

 

[BlueLoo]

Come to think of it, I could take another branch off elsewhere to run the rads independently.

Do that

 

[baker332]

Yeah cheers, as you can tell, I didn't think running in a series was an issue, thanks for bringing it to my attention.

 

[Madrab]

That adds another level of complexity - 22mm > 15mm > 8mm microbore - if all the rads are fed the same way then there's a level of self balancing as there's microbore involved, the length of the feeds not withstanding. Once a different setup is added, then it can easily upset a system like that.

two large rads x2 1800 x 472 double vertical

@ that size then the presumption is that they are 2 columns? How large is the extension? I can honestly say that if it's a half decent size that you maybe want to think twice about them, they just don't heat the space up properly. That and unless they do have a good flow, they will really struggle to warm up properly, their design means that they work at odds to how a normal rad would heat up.

 

[baker332]

That adds another level of complexity - 22mm > 15mm > 8mm microbore - if all the rads are fed the same way then there's a level of self balancing as there's microbore involved, the length of the feeds not withstanding. Once a different setup is added, then it can easily upset a system like that.


@ that size then the presumption is that they are 2 columns? How large is the extension? I can honestly say that if it's a half decent size that you maybe want to think twice about them, they just don't heat the space up properly. That and unless they do have a good flow, they will really struggle to warm up properly, their design means that they work at odds to how a normal rad would heat up.

Cheers Madrab - can you expand on vertical/column rads and why they don't tend to heat up as quickly as conventional rads?

I've taken into account the 22 > 15 > 8mm exisiting pipework and as you say I don't want to upset the system, however there is not alot else I can do to get around this? Can I not just rebalance on the new pipework and rads being added to the system?

Extension area is 8*3m 24m2 it's knocked into existing kitchen that is 3x3 with a small radiator that I could expand on. Therefore instead of adding two new radiators, I could add a large one as the pipework is already run and then modify the existing radiator into a larger one.

According to BTU calcs I'd need around 8000 to heat the 24m2 area space.

I'm here for everyone's experience as I want to obviously take the best approach.

 

[Madrab]

why they don't tend to heat up as quickly as conventional rads

The way they are constructed means that they actually work counter intuitively to the way convection currents would heat a normal rad. The HW is forced up the first couple of columns and then relies on the flow to force the HW down the others, the HW doesn't really want to flow downwards into all the columns, so it heads down the outside column heading out the return and more slowly down the other columns. Therefore if the flow into the rad isn't the best then it will struggle to move the HW down the other columns, that coupled with the fact that they aren't the most efficient at releasing their heat, again because of the way they are designed.

You could run 22mm > 15mm to them though but finely balance the others.

I could add a large one as the pipework

unfortunately you need to be careful with that too as 8mm pipe is very limited as to how large a rad it will supply properly. I used to have a 1400x600mm K2 on 10mm and no matter what I did I could never get it to warm up properly.

The ideal for that space would have been UFH IMO.