Hello,
I have a few questions about gas supply pipe sizing calculations I was hoping for help with.
Q1. I am looking at replacing an existing combi boiler with a Worcester Bosch 38CDI. I've had a quote saying that the pipe may need upgrading to 28mm and that upgrading the first 5m should be enough. However, the existing gas supply pipe is 22mm diameter and I estimate has an effective (i.e. including allowance for bends) length of around 26m. Having looked into the calculations (for example using http://www.installeronline.co.uk/wp-content/uploads/2013/06/Size-matters.pdf ) it seems that for a WB 38CDI which requires a flow rate of 4.2m^3/h around 12m of 22mm diameter pipe would produce 1mbar pressure drop meaning that far more of the pipe would need replacing than quoted. Am I missing something here, are these calculations quite conservative?
Q2. The gas pipe runs from the meter on the ground floor, to the boiler which is also on the ground floor, via the upstairs (i.e. under the floor boards upstairs). If it is required to upgrade more of the pipe then it is going to be difficult to replace the portion of the pipe that runs from the ground floor to upstairs as it is behind kitchen units. One thought I had was to replace the pipe on the ground floor and the pipe that is upstairs and leave the difficult to access pipe that runs between the two floors. The guy didn't think you could do that (and like I say didn't think it was necessary anyway) and seemed to suggest that the upgraded portion would need to all be at the start of the pipe to have an effect. Obviously he does this for a living so I took his word for it, but given it's a pressure loss calculation I wasn't sure why that would be? What I mean is, are you not effectively just summing the frictional losses along the pipe for a given flow rate, so it wouldn't matter where each pipe was in the system.
Many thanks
I have a few questions about gas supply pipe sizing calculations I was hoping for help with.
Q1. I am looking at replacing an existing combi boiler with a Worcester Bosch 38CDI. I've had a quote saying that the pipe may need upgrading to 28mm and that upgrading the first 5m should be enough. However, the existing gas supply pipe is 22mm diameter and I estimate has an effective (i.e. including allowance for bends) length of around 26m. Having looked into the calculations (for example using http://www.installeronline.co.uk/wp-content/uploads/2013/06/Size-matters.pdf ) it seems that for a WB 38CDI which requires a flow rate of 4.2m^3/h around 12m of 22mm diameter pipe would produce 1mbar pressure drop meaning that far more of the pipe would need replacing than quoted. Am I missing something here, are these calculations quite conservative?
Q2. The gas pipe runs from the meter on the ground floor, to the boiler which is also on the ground floor, via the upstairs (i.e. under the floor boards upstairs). If it is required to upgrade more of the pipe then it is going to be difficult to replace the portion of the pipe that runs from the ground floor to upstairs as it is behind kitchen units. One thought I had was to replace the pipe on the ground floor and the pipe that is upstairs and leave the difficult to access pipe that runs between the two floors. The guy didn't think you could do that (and like I say didn't think it was necessary anyway) and seemed to suggest that the upgraded portion would need to all be at the start of the pipe to have an effect. Obviously he does this for a living so I took his word for it, but given it's a pressure loss calculation I wasn't sure why that would be? What I mean is, are you not effectively just summing the frictional losses along the pipe for a given flow rate, so it wouldn't matter where each pipe was in the system.
Many thanks
Last edited: