35kW combi, 15mm CH pipes, 22kW rads - problem?

[SJRSJR]

I’m feeling despondent so please go easy on me.
I’ve swapped 15mm hot and cold pipes for 22mm and where I could get access replaced the old 15mm CH pipe with new 15mm Hep20.

I’ve got a 35kw combi boiler that feeds a 5 bed house including loft conversion. There’s a main bathroom with separate shower, two en suite shower rooms and a separate shower in the loft (which will probably only get used by visitors)

The hot and cold mains is 22mm but the CH pipes are 15mm.

Mindful that the boiler might not be up to feeding the showers I got a G3 plumber round to test the flow and pressure and advise on adding an indirect hot water cylinder.

He thought the 35kw boiler would be okay for two showers at the same time (which is probably the most usual) and thought a cylinder may not be necessary. But he wondered, with 15mm CH pipes, whether the boiler would be up to the heating demand. (I’ve since estimated approx 22kW at delta50 across 21 rads.)

If I did get a cylinder he said the central heating needs 22mm pipes at its backbone (which is something I only discovered after the 15mm was replaced.)

So now I have three things getting me down.

1. Do nothing and regret it later.
2. Have inadequate heating.
3. Go for an indirect unvented cylinder and if so how far do I need to go to achieve the required 22mm CH ‘backbone’? And what is considered the backbone - does it mean all but the final metre to the rads? Or could it be 22mm in the rooms that we live in most? It would really hurt to undo all the new 15mm Hep pipe.

Are there any alternatives that don’t require completely re-doing the CH pipework?

Thanks

 

[cross thread]

Heating part of the system would definitely benefit from 22 mm flow and return fit the main part of the system of till it possibly split into different runs.
35 kw boiler will give around 14 litres per minute at @ 35 degrees rise so if your incoming supply is good enough I think it will be fine for 2 showers.
Regarding unvented cylinder has the installer measured the incoming supply flow/ pressure ?

 

[gas112]

22MM is far better but If it worked beforehand then you have added quite a restriction by using 15mm hep instead of 15mm copper

 

[Mister Banks]

What is the heat loss of your property? I can't believe it is 22kW more likely to be around 8-10kW. A combi boiler in a 5 bedroom house with 4 bathrooms is less than ideal.

As @gas112 has said swapping 15mm copper for 15mm hep is going to add some restriction to the flow.

 

[Razor900]

Is it really 15mm from the boiler or does it reduce later. Pictures would help

 

[SJRSJR]

Thanks for the replies.

Regarding unvented cylinder has the installer measured the incoming supply flow/ pressure ?

Yes, he said flow and pressure was okay but didn’t give me any readings.

It’s a mix of plastic and copper pipe as an extension has been added.. I replaced the 15mm copper hot and cold supply with 22mm Hep. For the CH the extension was in old 15mm Hep which has been replaced by new Hep The shower rooms have 15mm hep to the towel radiators but the rest of the CH is still 15mm copper.

I’ve been looking under the ground floorboards to see where I can get access. There’s a 4m run 22mm copper at the back of the house but otherwise it’s all 15mm. There’s a tiled floor between the front and the back where I’ll never get to, and somewhere there it drops to 15mm. So if I swapped the front and back to 22mm but there was still 15mm in between would that be a problem - it all or nothing?

 

[SJRSJR]

Is it really 15mm from the boiler or does it reduce later. Pictures would help

There’s a 22mm to 15mm reducer under the floorboards below the boiler. I’m tracing it all now and will sketch it out.

Thanks for everyone’s input.

 

[Madrab]

If you have 21 rads of average size then a 15mm backbone whether copper or plastic is not going to cut it TBH - that will take an age to heat up properly and could be a real PIA to balance properly. It also would be a must to have 22mm primaries if a cylinder is to be fitted. I have no idea what the installers were thinking about using 15mm on a system that size, almost sounds like it's a hangover from a much smaller system

TBH no size of combi boiler will feed more than 1 good HW outlet at a time unless there is some pretty extensive water saving appliance being used, it's the primary draw back of any combi boiler.

What the dynamic flow and pressure is will define whether a large combi or unvented could be catered for. Do you have a note of that and how was it tested ( where and how many outlets running) If there is the potential for at least 3 showers being used at once or even 2 and other hot cold outlets at the same and worst case 4 on rare occasion then a combi is a non starter.

 

[SJRSJR]

Is there any heating system that will work off 15mm pipes? If so might be cheaper to change the boiler than the upheaval of changing the pipes?

 

[SJRSJR]

What the dynamic flow and pressure is will define whether a large combi or unvented could be catered for. Do you have a note of that and how was it tested

No info, sorry. He ran two taps and used a container that was overflowing as it filled. I didn’t see him do the pressure but he mentioned about it being okay, and said something about it probably having a limiter on it, so no problem.

 

[SJRSJR]

What is the heat loss of your property?

Sorry, would you mind explaining what that means?

 

[Madrab]

Is there any heating system that will work off 15mm pipes? If so might be cheaper to change the boiler than the upheaval of changing the pipes?

It's not the boiler, it's getting the heat from the boiler out to the system in general and getting the radiators to all heat up quickly, efficiently and as much as possible, all at the same time.

15mm will work, eventually, the system will be slow to heat up and the rads furthest away will take longer unless everything is very tightly controlled. TBH I would suggest that the prices of putting in a new boiler should cover the cost of changing all the main system pipework to 22mm and then some

Not a lot we can recommend without knowing how well the cold mains performs, that's key to suggesting what the HW system may be capable of and what may work best, without it it's all just guesswork I'm afraid.

 

[SJRSJR]

Thanks to all, it’s really helped and I understand more now. I will bite the bullet and get 22mm pipes.

The plumber asked me to send him a schematic diagram (which I’d like to do for myself anyway). Does anyone know if there’s a decent (free) drawing tool available? I looked at MS Paint but I don’t seem to be able to format a line to put an arrow on it! Thanks