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PostPosted: Tue Nov 30, 2010 1:47 pm 
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Hello, I'm planning on fitting a combi boiler in my terraced house, as a replacement for an ancient Baxi back boiler. The gas supply will run from the meter at the front of the house to the planned boiler site on the back upstairs wall, a total run of about 16m with 6 elbows. I've found references to sizing charts, but not any actual charts. I'm planning to go for a boiler in the 28 - 31 KW range. The only other gas appliance will be a hob.
Obviously I'll get a pro to do the gas work, but I'd like to work it out myself first. Most of the existing pipe is under a newly laid oak floor, so I want to know whether 22mm pipe will be OK or if I need to think again.
Thanks, Brian


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PostPosted: Tue Nov 30, 2010 2:30 pm 
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The RGI will work it out onsite, but going on the information the length to the hob would need to be 28mm


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PostPosted: Tue Nov 30, 2010 2:52 pm 
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Thanks Scruff, but I'd like to try to work it out first as I might go for a smaller boiler rather than pull the floor up again. Surely I wouldn't need all 16m in 28mm pipe? Can you point me to a calculation chart/table?


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PostPosted: Tue Nov 30, 2010 3:19 pm 
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Depends where the hob is in relation to the boiler and the size of the hob.

A maximum length of 22mm would be around 9m plus a few bends for both.

As per your other post, you need to get the RGI involved from the start, the existing pipe will need testing anyway.


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PostPosted: Tue Nov 30, 2010 3:51 pm 
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OK, thanks for the advice


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PostPosted: Wed Dec 01, 2010 12:14 pm 
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You dont say what size the existing pipe is but if it needs to be upgraded under the floor then it does not matter if its done with 22mm or 28mm.

Of course it may already be in 22mm but as John says its probably going to need to be in 28mm.

Tony


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PostPosted: Wed Dec 01, 2010 3:04 pm 
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Hi Tony,

Existing pipe is 22mm reducing to 15mm: I'm hoping to retain the 22mm section. After a think I've found another site for the boiler in the kitchen that will need a shorter run of 11m. Flueing might be tricky though, will need some bends.
I found a pipe sizing chart here: http://www.ukcopperboard.co.uk/literature/pdfs/Installation-Tips/Domestic-gas-planning-and-sizing.pdf. I then looked up max gas flow rate for a typical 30kW boiler (Worcester) of 3.4 m3/hr. So this tells me that allowing for bends I should still be OK with 22mm pipe (3.6 m3/hr up to 15m). I might have to sacrifice a gas hob for electric as adding a hob makes it marginal.
I'll obviously get my RGI to check my sums when the time comes! I'm not in a rush to mess with the heating with a foot of snow outside.

Brian


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PostPosted: Wed Dec 01, 2010 3:39 pm 
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Its often possible to upgrade to 28mm at the meter where there are several elbows and in the kitchen where there are also elbows.

Since the effective length of ebows is high they make a very significant difference to the calculation.

By doing this expect you will beable to retain the gas hob as they are so much better.

Tony


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PostPosted: Wed Dec 01, 2010 5:05 pm 
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Thanks Tony, I'll consider this. Does it make any difference if the 28mm pipe is used AFTER 22mm? I've got about 6m of 22mm running from the meter outside the front wall to under the dining room floor. There are some elbows near the meter but not easily accessible. I could run 28mm for most of the other 5m from dining room to kitchen. I suppose logically it doesn't make any difference which size comes first? Brian.


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PostPosted: Wed Dec 01, 2010 6:01 pm 
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:roll:


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PostPosted: Wed Dec 01, 2010 6:16 pm 
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Brian/in/Haxby wrote:
I suppose logically it doesn't make any difference which size comes first? Brian.


err... Really???


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PostPosted: Wed Dec 01, 2010 6:35 pm 
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Quote:
I suppose logically it doesn't make any difference which size comes first?

Quote:
err... Really???

So I'm wrong?? It's a long time since I studied fluid dynamics, and not much of it sunk in then :-)


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PostPosted: Wed Dec 01, 2010 7:11 pm 
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Brian/in/Haxby wrote:
Quote:
I suppose logically it doesn't make any difference which size comes first?

Quote:
err... Really???

So I'm wrong?? It's a long time since I studied fluid dynamics, and not much of it sunk in then :-)


well if youre right, the gas board could save millions by only running 15mm pipes to everyones house! :lol:


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PostPosted: Wed Dec 01, 2010 7:27 pm 
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Actually the ops got a valid point although I haven't read the whole post.

It depends where the 28 mm is and whether the smaller section can take the load. We could do with a drawing showing the proposed run.

And some of the pipwork feeding houses is barely bigger than 15mm it's just under much greater pressure which gets reduced down at the meter :wink:

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PostPosted: Wed Dec 01, 2010 8:40 pm 
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I agree with Razor.

As long as you properly calculate the pressure drop in each part of the circuit, and it still all adds up to 1 mbar or less, you can sometimes do as the op asked and remain within regs.

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