Grant 36kW Combi

[Johntheo5]

A plumbing friend of mine has been asked to look at installing a Grant 36kW Combi in a system with 12 double rads and asked me to have a look at the MIs and give my opinion of whether the installed 14L EV is big enough to accomodate these 12 rads.
I don't know the rad sizes but probably safe enough to all0w say 10L/rad? for a total of 120L.
The stated boiler+store contents are 53.5L so a total system vol of 175L.
The Mis state for this combi boiler, precharge&filling pressures of 0.5bar sufficient for system contents of only 148L.
My calcs, below show that this results in a final pressure of only 0.95bar, the PRV on all the Grants is set to 2.5bar so I can't see any reason why a final pressure of 2.0bar isn't perfectly acceptable.
Again, by my calcs, a total system contents of 322L, 269L to rads will still only result in a final pressure of 2bar.
IMO, the filling pressure should be 0.25/0.5bar higher than the precharge pressure to allow for any tiny leakages or collapsing air bubbles without the need for constant top ups, precharge/filling pressures of 0.5bar/1.0bar, water reserve 3.5L, would result in slightly higher final pressure of 2.13bar with 120L of rads+pipework and precharge/filling pressures of 0.75bar/1.0bar, water reserve 1.75L, will accomodate 135L of rads+pipework at a final pressure of 2bar.

Grant 36kW Combi EV sizing & pressures. (Recommended)
Precharge/Filling pressures 0.5bar/0.5bar
System Capacity (from Grant Mis)	148.00​	Litres
Boiler+store	53.50​	Litres
Net to rads	94.50​	Litres
Final Pressure	0.95​	Bar
Grant 36kW Combi EV sizing & pressures
Precharge/Filling pressures 0.5bar/0.5bar
System Capacity	322.00​	Litres
Boiler+store	53.50​	Litres
Net to rads	268.50​	Litres
Final Pressure	2.01​	Bar
Grant 36kW Combi EV sizing & pressures
Precharge/Filling pressures 1.0bar/1.0bar
System Capacity	215.00​	Litres
Boiler+store	53.50​	Litres
Net to rads	161.50​	Litres
Final Pressure	2.01​	Bar
Grant 36kW Combi EV sizing & pressures
Precharge/Filling pressures 0.5bar/1.0bar
System Capacity	162.00​	Litres
Boiler+store	53.50​	Litres
Net to rads	108.50​	Litres
Final Pressure	2.01​	Bar
Grant 36kW Combi EV sizing & pressures
Precharge/Filling pressures 0.75bar/1.0bar
System Capacity	188.00​	Litres
Boiler+store	53.50​	Litres
Net to rads	134.50​	Litres
Final Pressure	2.01​	Bar

 

[Exedon]

Obviously some differences in specification to boiler supplied in UK
Grant combi's here have used a 3 bar prv for some time,around the same time they stopped fitting low limit pressure switches.
Also all there combi's use 24L expansion vessel
We run them at .5 bar

 

[Johntheo5]

A bit surprised that you run them at only 0.5bar even though this will just about satisfy the rads static head in a 2 storey house, also that (if you do) you run with the same precharge and filling pressures, I look after a few family Firebird boilers all with a rad in the attic convertd bedroom, 10 double rads, a 18L external EV (no internal) results in a final pressure of < 2.0bar, from precharge/filling pressures of 1.0bar/1.5bar, I only have to give a very small top up of ~ 0.2bar, 1.2/1.5L, to top up the EV 3.5L water reserve, every 15 months or so.