Conventional to combi conversion. Inlet size reassurance.

I've searched the forum and there are a few similar threads, but I was wondering if I could ask the question before I commit to spending quite a bit of money. I've got a conventional potterton boiler gravity system that's about 17 years old. I have a loud shower pump upstairs in the airing cupboard. Boiler is starting to show its age and I can no longer bleed most of the Rads as the bleed screws have rounded. So I'm planning to replace the system with a combi and new rads in the spring. Have had three quotes so far, all from really professional gas safe fitters who I'd gladly give the job to. The only thing that's stopping me is the possible work involved with the supply to the boiler. 2 fitters have suggested an Ideal logic plus 30kw and the other a WB (32kw I think). I know the WB requires a 22mm connection so quite rightly that guy has quoted me with a new feed in 22mm from the meter to the boiler. One other suggesting an ideal has also said the same. The problem is that will mean I'll need an external copper pipe snaking from the front to the back of the house which I'm really not keen on. The MI for the logic plus States it can take a 15mm intake and I've asked the third fitter who has said he thinks this will be sufficient for this boiler. I've taken a picture of where the intake comes up through the concrete floor under current boiler and I've read that 15mm may be ok if the 15mm portion is not too long. What I don't know is if the pipe coming out of the floor is the 22mm feed from the meter or not. I'd be very grateful for people's thoughts. I'd really like to know in advance if the required 1mb drop can be met before I agree to the job before I commit to the external pipe work. I've attached the picture if it helps. The only other gas appliance I have is a regular 4 burner gas hob.

That's 3/4 barel comming out of the floor, which will pass a bit more gas than 22mm copper.

As long as you have no issues with black dust in the barel I would connect on from the 3/4 in 22mm copper upto boiler and test.

Whether you achieve the 1 mbar or less drop will be dependent on the length and number of elbows etc ( resistance) in the pipe run.

There are no guarantees until it's fitted and tested

I thought that looked like 1/2", with a 1/2 male iron to 15mm compression elbow with a copper feed of it. Or do my eyes deceive me.

It looked like 3/4 to me, but my eyes could be deceived.

They're not what they were

If it is 1/2 inch it would be obvious to your installer whilst in front of it.

Then it would most likely need upgrading

Looking again I think Michelle is right

michele5041 said:

I thought that looked like 1/2", with a 1/2 male iron to 15mm compression elbow with a copper feed of it. Or do my eyes deceive me.

Click to expand...

+1 there.

Alright Dan, no need to rub it in

The circumference of the pipe coming out of the floor is 90mm, which I work out to be 28mm diameter, presuming that's steel? But I don't know. It looks like a straight run from the gas meter into the house to the gas hob first then to the boiler. I've worked out approximate run lengths:

5.5M from meter to first connection - hob
1.5M from cooker to second connection - boiler
Then about a 1M up from the floor (second connection) to current boiler in 15mm as in the pic.

I don't know if that helps at all. I really appreciate people taking the time, I realise it's difficult if you don't actually have the pipes in front of you.

I guess the only guaranteed option would be to have new external pipe work, but I'm not keen on how that will look, or the added cost.

If it is 1/2" Steel then you are undersized for the first run from the meter.

You're looking to pull 3.7 cubic meters through that run and that is limited to about 3 meters. As it stands you're going to have double the permissible pressure drop to the split to the hob alone - let alone the extra runs to the appliances from the split.


Pipe upgrade time I'm afraid.

If it is 3/4" then you might just scrape in.

If the pipe is 28mm diameter then it is 3/4 barel which gives you a fighting chance


Straight back at ya Dan

I know nothing about pipes so don't know by looking, but a taking a 90mm circumference assuming a 28mm diameter gives a 3/4" pipe size I think.

jeez how do you guys keep sane with all the metric/imperial mix and match??

Very much looks like 1/2" and 15mm from the pic.... btu it is hard.

OP, you get used to it after a while.

I've got dodgy eyes as well but, that could be 3/4" male iron elbow with a 15mm reducer in it. I'm only comparing the pipe/nut ratio mind. Did all three quotees see this pipe £

I mean ? told you i had dodgy eyes

It's 1/2 iron out the floor by my calcs...

90mm circumference giving a 28mm outer diameter...

1/2 iron pipe is 21mm diameter, stick a socket on the end with say a wall thickness of 3.5mm and you're down to 28mm