combi gas rates

[londongas]

although not in the acs that im taking next week im curious about one thing
1)combi has poor dhw flow hence the gas rate will be lower then the max specifed. in this case would you note the gas rate for the central heating instead?

2)in dhw mode on combi's does it matter if gas rate on dhw mode is lower then specified max? the guy i did my portfolio with said that as long as inlet is correct , that because combi's modulate hot water at various gas rates according to variables like water flow, thermistors/pcb voltage, temp stat, that it doesnt really matter if dhw gas rate is not up to max as long as its operating between mini- and max burner pressures

is he right? many thanks

 

[Corgigrouch]

The choice of combi should be sized to suit the flow of water that you have... If you cant get enough water to flow through even the smallest sized boiler, then a combi is the wrong boiler to fit and should look at some other system

If the boiler cannot be made to run at full output by either running the DHW or using the service mode provided by some manufacturers, then the boiler is too big for the application and therefore not the right boiler... If the gas rate works out less than the output of the boiler, then it could indicate that your pipework is too small or too many bends or restricted in some other way

 

[londongas]

i meant to say inlet pressure ok so pipe sizing os ok but poor dhw flow is due to mains reduced, scale in exchanger, blocked filters. thus gas rate affected but as long as its operating within min and max its ok ?

 

[andytheplumber]

meant to say inlet pressure ok so pipe sizing os ok but poor dhw flow is due to mains reduced, scale in exchanger, blocked filters. thus gas rate affected but as long as its operating within min and max its ok ?

Inlet pressure might be 20mbr but working pressure may not. You need to calculate the working pressure from the meter to see if it corresponds to the manufacturers recomendations. And then turn on the gas cooker if you have one to see if this has an affect on the gas rate. Then you can calculate your pipe sizing.