Boiler gas pipe inlet size

[badluc]

Hello,

The old Baxi Potterton Promax 24HE heat only, installed when the house was built in 2003, is on its last legs.

I've decided to replace it and after some research I've bought a Viessmann Vitodens WB1B316 (part no. 7416307) 13kW heat only boiler. The downsizing of the boiler is not a problem as 24kW was way oversized given it's a small 2 beds with just 7 small rads and a megaflo.

The current gas pipe into the Baxi is just 15mm. I presume it delivers sufficient pressure as the boiler has been working fine for the 5 years since we've moved in, and according to their respective manuals, the Baxi has a min. inlet pressure at gas valve of 18.1 mbar and the Vitodens 17.4 mbar. Two of the heating engineers who came to provide a quote thought the size was probably fine as it is, although a third one was not so sure and pointed out that we will only know once the boiler is installed and tested if we need to dug up the floor and upgrade the piping.

But in addition to making sure that sufficient pressure reaches the boiler, should I also worry about whether a 15mm pipe is enough for a gas inlet of R 1/2, which is what the manual says the WB1B316 has?

From what I understand, as long as the minimum pressure is delivered to the boiler, and/or the pressure drop from the meter to the boiler is smaller than 1 mbar, then the size of both the piping and the gas inlet of the boiler is irrelevant, is that correct?

Thanks

 

[Chuck]

Two of the heating engineers who came to provide a quote thought the size was probably fine as it is, although a third one was not so sure and pointed out that we will only know once the boiler is installed and tested if we need to dug up the floor and upgrade the piping.

It will depend on the length, path and number of joints in the pipe between the meter and boiler. Also whether it's 15mm for the whole way. (Sometimes you find 22mm to the kitchen where it splits down to 15mm for the appliances.) Are there any other gas appliances (hobs, ovens, heaters, etc.) in the house? In the absence of these details you can hope you won't have to change it but make sure that the quotes include a figure for the upgrade as a contingency item, i.e. something you pay if it proves necessary so you know where you stand.

With a 13kW boiler you'll probably want to schedule your Megaflo to heat the water during the night as it will take about an hour to come up to temperature and it will probably starve the CH while this is happening.

Also, do the installers know you want them to fit a boiler you have bought yourself? This is not how things are normally done.

 

[Brambles]

The only (and best) advice you need is from the Engineer who is going to register the boiler installation with Gas Safe.

As an order of magnitude if the existing gas pipe is 15mm all the way from the meter to the boiler the maximum length (with an allowance for fittings - (4 bends and a tee)) will be less than 4m. It will be less if there are a plethora of fittings.

 

[badluc]

Thank you both.

Yes the heating engineer knows I'm providing the boiler myself, and is quoting for labour only. I realised it's not common practice when I was struggling to find somebody to quote for labour only... Finger crossed I won't need to upgrade the pipe but I'll have to wait until after he has installed the boiler and tested the pressure before knowing that.

The distance between the meter and the boiler is about 6.5, of which approx 4m under the concrete floor, and then the last 1.5m or so up the wall to the boiler.

Before reaching the boiler, the pipe also supplies the kitchen hobs (7kW or so I should think, the oven is electric). I suspect the gas pipe is at least 22mm for the length of its underfloor run, and then becomes 15mm for the two vertical tees to the hobs and boiler respectively.

Using an online pipe size calculator as a very rough guide, and including fittings and tee, I got less than 0.6 mbar drop from the meter to the boiler in that scenario (i.e. if underfloor pipe is 22mm from meter, and then only 15mm for the last stretch from floor to boiler in kitchen cupboard, as you suggest is common).

If it were 15mm all the way from the meter, the drop would be roughly 1.75mbar I believe, so I can't see how they could have built it that way to begin with back in 2003, unless they were cowboy builders.

"With a 13kW boiler you'll probably want to schedule your Megaflo to heat the water during the night as it will take about an hour to come up to temperature and it will probably starve the CH while this is happening."

Thanks for the tip. I'm having Tado with HW controls installed, so will play around with it a bit and see how the boiler copes... It seems to me the old Baxi 24kW was basically working like a combi, as every time I would open a hot water tap it would come to life full blast, and I always felt it was delivering hot water pretty much on demand, rather defeating the purpose of having a megaflo...

So am I right that as long as a 15mm pipe brings enough pressure it can legally be fitted into the gas inlet of the boiler even if it is bigger than 15mm?

 

[Brambles]

If you want total piece of mind, once the new boiler is installed and registered with Gas Safe, Gas Safe will send you the registration / building control certificate and ask you if you want Gas Safe to consider your new boiler for a free compliance check of the installation

 

[Scott_D]

1mb between meter and appliance with all appliances at maximum.