it is really simple, your installer has the responisbility to do it right so if he says 28mm, then 28 mm it is.
unless of course you are willing to sign a waiver and if it turns out that 22 is not enough, you got to pay him for laying the 22, AND for 28 pipe, AND for removing the 22, AND for laying the 28.
my money says you are not willing to sign that.
and if you were my customer, you would be looking for a new installer.
Nobody's mentioned that the tables are wrong. All of them. Everyone gets higher pressure drops than the tables predict, especially on smaller sizes, though also on 28mm.
SO if someone said to me 30kw boiler I'd expect it to be 28mm unless the pipework was very short.
Have just piped a 37kW boiler up to 40m of 42mm pipe -pressure drop is more than the charts say on that too.
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