Replacing system boiler with combi? Do I keep the 22mm DHW pipes or replace with 15mm

[gazzarose]

Hi guys

Im in the process of changing my heating system from an old Baxi back boiler in the dining room to a Combi in the loft. Im going to do all the easy stuff, mounting, joist notches and water piping, and getting a gas safe engineer to do the gas flue and comissioning. Ive pulled up the floor boards upstairs today to start planning and making an order list and I'm unsure on what to do with the DHW pipes. Im planning on replacing everypipe in the house eventully, so want to make as few compromises as I can. Within reason I want things done the best way rather than the cheapest way hence my question about the pipe sizes. At the moment the DHW is fed from a cylinder in the airing cupboard in 22mm to the bathroom then downstairs to the kitchen an utility room. 18 months ago I fully gutted and started again in the kitchen/utility and replumbed with 22mm for the DHW because I still had my lower pressure hot, and at the time intended to keep with a system boiler. That leaves me with 5 metres of 22mm under a finished floor that I cant replace. The length of pipe from that point to the boiler will be about 15m with a couple of bends and tees in between. My mains water comes in with 15mm, and the boilder Im looking at getting exits with 15mm. Would I be right in saying that although I wont nesassarily gain any flow using 22mm after the boiler, I just wont lose as much, but I'd also have alot of waste cold water to flush through before I get hot out. And 20m of 22mm pipe holds alot of water. Given that apart from the 5m in the kitchen, I have a free reign with the pipe size, what would be the recommended approach. I don't mind if you come back and say put 22mm from the stopcock to the boiler (15m) if thats the best, I just dont want to be regretting something after I've finished. The boiler I'm looking at is a Worchester Bosch Junior 28i with a DHW flow rate of 11.4 litres per minute @ 35' temperature rise.

Any advice would be most appreciated.

Regards

Gareth

 

[Mike Jackson]

Re: Replacing system boiler with combi? Do I keep the 22mm DHW pipes or replace with

That's a long run for the hot water so you definitely want to reduce it to 15mm. Is there nowhere closer that you can get the boiler to save wasting water?

 

[gazzarose]

Re: Replacing system boiler with combi? Do I keep the 22mm DHW pipes or replace with

Hi,
Thanks for the reply.The only place closer is the airing cupboard but that will only save a meter.everywhere else hasn't got access to an exterior wall. I think I will go for 15mm then. The boiler I'm getting had got a small cylinder in it so if you turn the tap off then back on won't fill the pipe work with a lump of cold water.

Thanks again

Gareth

 

[Rob Sale]

Re: Replacing system boiler with combi? Do I keep the 22mm DHW pipes or replace with

you might be able to locate it elsewhere closer by using a vertical flue through the ceiling into the loft and out through the roof. Might be cheaper that way saving all those length's off copper.

if you know what you are doing there is no need to replace every pipe in the house, you proplably could use 60-80% of the copper pipe. Just t into the 22mm pipe where you need to and use reducers where needed onto new pipe at rads and boiler.

 

[Avatar]

Re: Replacing system boiler with combi? Do I keep the 22mm DHW pipes or replace with

I thought it was fairly standard to keep the 22mm pipe as is . . .

Put the boiler in first, fire it up and test your hot water pressure at all the outlets. It should still get up to pressure in 22mm.

if it doesn't output ok - THEN replace with 15mm. The price of copper at the moment means that doing this will probably cost more than your boiler easily!!! Best not to do it unless you have to.

The 28K boiler you suggest should be up to the task.

 

[gazzarose]

Re: Replacing system boiler with combi? Do I keep the 22mm DHW pipes or replace with

The main reason for the long lengths is because the house is L shaped with the kitchen sticking out from the back of the house.the bathroom taps will only be about 8 meters from the boiler.

The reason I want to replace all the pipe is a because they're quite old, and b the old guy that had the house before me was A good old fashioned botcher.parts of it had compression fittings, nothing was clipped and lots of the pipes touch each other.I think in total I only have to replace 15m of pipe, the rest being either redundant when I take the cylinder out or the new bits to extend up to the loft.

Thanks for the replies

Gareth