Gas flow for replacement boiler

[DaveMatt0709]

H guys, this is my first post on here and would be grateful for some advice. I will try and give as much detail as I think relevant. We want to upgrade our boiler and have run into a few issues that has confused us.

We currently have a Worcester Highflow 3.5 Open Flue floor standing boiler that was installed in 1993 in a cupboard in the centre of the house upstairs. The gas supply pipe that feeds the boiler is approx 16m in length, which runs from the meter at the front of the house to the back of the house (goes right through the centre of the house and so no access), up to the first floor then to the cupboard. I have calculated that there will be 6 90 degree bends. The supply pipe is 22mm throughout. We have a 4 bedroom semi-detached house, with one bathroom and a shower room. We have no other gas appliances in the house.

We want to upgrade to another Worcester boiler but need to locate the new one in the same location. British gas have given us a quotation and have recommended us to replace with the Worcester Greenstar 440CDi, while another Gas-safe installer has said we cannot go above the Greenstar 34 CDi as the gas supply pipe run is too long for the other boilers. We are now wondering why British Gas did not raise this as an issue. We were hoping to go for the 38 or 42 CDi as these have a higher DHW flow rate similar to our existing boiler. The Worcester-Bosch website also states that for a 4 to 5 bedroom house with 1 bathroom and 1 shower room, they recommend the 38CDi or 42CDi.

Can anyone please advise us as to whether you think the 34CDi would be adequate, or whether we could indeed go with the 38 or 42CDi? Is the 22mm gas pipe sufficient or not?

Any help appreciated. Thanks, Dave

 

[bolshy]

With 6 elbows (including the ones at the meter I assume), you won't achieve the 1mb drop required from meter to appliance at full rate for the larger boilers. The 34CDi uses approx 3 cubic metres of gas per hour (30Kw) at full rate? I haven't checked the Manu's specs. You may just scrape in with your current gas pipework, commissioning checks will tell. Has anybody checked the working pressure at your old boiler at full rate?
You should upgrade the first few metres of pipework from the meter to 28mm if possible. If you manage to increase the first 6 metres or so, then you should be able to fit a 40KW boiler, or near to. Every little helps.
As for the boiler sizing, how many rads have you got? Large combi boilers are generally selected for improved DHW flow rates. I have a Baxi Neta-tec 33 KW combi running 15 radiators over 2 zones totalling 33 KW of heating. They are more efficient when working hard.

 

[bolshy]

Just checked the specs of the 440 CDi. It's maximum gas rate is 3.1 cubic metres per hour. You aren't a million miles away, but not quite there.

 

[DaveMatt0709]

With 6 elbows (including the ones at the meter I assume), you won't achieve the 1mb drop required from meter to appliance at full rate for the larger boilers. The 34CDi uses approx 3 cubic metres of gas per hour (30Kw) at full rate? I haven't checked the Manu's specs. You may just scrape in with your current gas pipework, commissioning checks will tell. Has anybody checked the working pressure at your old boiler at full rate?
You should upgrade the first few metres of pipework from the meter to 28mm if possible. If you manage to increase the first 6 metres or so, then you should be able to fit a 40KW boiler, or near to. Every little helps.
As for the boiler sizing, how many rads have you got? Large combi boilers are generally selected for improved DHW flow rates. I have a Baxi Neta-tec 33 KW combi running 15 radiators over 2 zones totalling 33 KW of heating. They are more efficient when working hard.

Thanks bolshy for your reply. Neither British Gas or the other installer have suggested checking the working pressure at the present boiler.
We cannot upgrade the pipework from the meter as it goes directly under the house from front to back. There is no access. Only other option is to use new piping on outside of house, but that would be a last resort and would look pretty ugly on front of house so wife not keen. Regardless of this we are probably OK with a 30kW boiler, was not really looking for a 40 one. The boiler services 11 radiators and 2 towel radiators.
But if the gas rate for the 34 is 3cum3ph and the 440 is 3.1, I don't understand what difference it would make which we went for! So now confused again as to why the second guy has told us we can't go any higher than a 34CDi.

 

[Dan Robinson]

3.1 for the 440.

3.7 for the 34CDi.

Think you should look for a 3rd guy.

 

[DaveMatt0709]

3.1 for the 440.

3.7 for the 34CDi.

Think you should look for a 3rd guy.

I have a 3rd guy coming on Tuesday, but just wanted to get my facts sorted first! But you are saying the 34CDi is 3.7m3/h NOT the 3 that bolshy mentioned?

 

[Dan Robinson]

Bolshy was using an approximation. My figures are specific to the boilers.

 

[bolshy]

3.7 , wow! I thought they were roughly a 30 kW unit, seems I was a bit out. Thanks for the input, Dan

 

[Dan Robinson]

No worries.

 

[DaveMatt0709]

Bolshy was using an approximation. My figures are specific to the boilers.

Ok thanks very much, then I really do not understand why the local installer, who is on the Worcester-Bosch list of approved installers, would say that one is the best. Really confused now. Will see what the guys on Tuesday come up with! Can you tell me where you find this info, as would be keen to see what the figures are for the 38CDi and 42CDi? Have looked around but not found anything so far. Thanks for your help.

Edit: Don't worry, just found it finally!
34CDi 3.7m3/h 31 kW
38CDi 4.2m3/h 31kW
42CDi 4.4m3/h 31kW
440CDi 3.1m3/h 29.5 kW

 

[Dan Robinson]

You missed out your decimal points.

The WB list of approved installers means little when it comes to quality. Just an easier route to shift more boilers.

 

[DaveMatt0709]

You missed out your decimal points.

The WB list of approved installers means little when it comes to quality. Just an easier route to shift more boilers.

Yes realised and corrected thanks! I am beginning to understand exactly what you mean! It is looking like the British Gas recommendation of the 440CDi would be the best option, assuming that the pipe length is OK. I need to get someone to measure the pressure at the boiler, will ask the guys coming on Tuesday.

 

[bolshy]

You haven't looked at the 34, 38 and 42 CDi DHW rated inputs properly. When in DHW mode these boilers are 35kW, 40kW and 42kW respectively. They need a lot of gas, as you and Dan have posted to produce their impressive hot water flow rates @ 35 ° C rise.
I hope this will clear up your confusion.
The 440 CDi highflow has the lowest rating of them all at 30 kW. It will only maintain a high flow of DHW for a limited time, as it uses a small thermal store.
Read the specs thoroughly to make sure this boiler fits your needs. If you opt for one of the other boilers, you will definitely need a gas pipe upgrade, and as we pointed out, could possibly need one anyway.
If they tell you they can fit one of the higher rated ones on your existing supply, then they've got their sums wrong.

Perhaps you should consider a decent sized thermal store yourself as an alternative to a large combi boiler, if you have room to install one.

 

[Agile]

Sometimes its possible to feed larger pipe in from one end!

Tony

 

[DaveMatt0709]

You haven't looked at the 34, 38 and 42 CDi DHW rated inputs properly. When in DHW mode these boilers are 35kW, 40kW and 42kW respectively. They need a lot of gas, as you and Dan have posted to produce their impressive hot water flow rates @ 35 ° C rise.
I hope this will clear up your confusion.
The 440 CDi highflow has the lowest rating of them all at 30 kW. It will only maintain a high flow of DHW for a limited time, as it uses a small thermal store.
Read the specs thoroughly to make sure this boiler fits your needs. If you opt for one of the other boilers, you will definitely need a gas pipe upgrade, and as we pointed out, could possibly need one anyway.
If they tell you they can fit one of the higher rated ones on your existing supply, then they've got their sums wrong.

Perhaps you should consider a decent sized thermal store yourself as an alternative to a large combi boiler, if you have room to install one.

Wow, OK I will read through the installation manuals properly. Thanks for your help. Can't upgrade the pipework as stated so now not sure what to do. May need to rethink location, could put a wallhanging boiler downstairs and connect up to the water supply and CH pipes in the cupboard upstairs using new piping, that would put the boiler much closer to the meter, around 8.5m instead of 16m.