My grant 70 mk 2 heating pressure is constantly increasing in pressure. I have changed the heat exchanger

[SimonG]

Pressure increases when heating on, drops to zero when heating off

Then its either expansion vessel, expansion vessel connection, or where the expansion vessel is connected to boiler.

 

[Confusedsparkie]

Then its either expansion vessel, expansion vessel connection, or where the expansion vessel is connected to boiler.

I'll check them in the morning, thanks for replying

 

[John.g]

The E.vessel pre fill (air end) pressure should be 0.75bar with zero pressure at the water end, the filling valve should then be opened to give 1.25bar (cold system) , the pressure should then reach ~ 1.5 to 2 bar when system hot and will return to 1.25 bar when cold.

 

[exedon2]

I'm fairly certain that model of grant should have a cold system pressure of between 0.5 bar to a max of 1 bar.
Expansion vessel pre charge 0.8 though later on in instructions tells you it should be 1 bar !
Remember that because it seemed a bit odd.

 

[John.g]

Grant used never give a pre charge pressure which was a bit daft as you have no water reserve to make up for tiny leaks etc without constant top ups but nearly always specified 1.0 bar filling, their PRVs are all set to 2.5 bar.
I set up a few neighbors to 0.75/1.25 bar and no problems in exceeding 2ish bar on heat up/

 

[John.g]

Ambiguous to say the least, it says pre-charged to 1 bar in one place and that the heating system vol is based on a "vessel charge and cold fill pressure of 0.5bar" in another reference.
By having the precharge&fill pressures the same means a much smaller vessel is needed, see my calcs below based on a average (of flow&return) of 65C. for a 50/90 with a 10 litre EV.

Its quite clear that the "temptation" is to have the precharge&fill pressures the same to avoid having to install a external EV especially since the boiler+stored vol of combis alone can be 40 litres, not so much a problem with the non combis with a boiler volume of 20/25 litres.

precharge/fill/final/system volume
0.5/0.5/0.86/106
1.0/1.0/1.48/106
0.5/1.0/1.70/106
0.75/1.25/2.0/106

Allowing final pressure of 2.2bar:
0.5/0.5/2.2/290
1.0/1.0/2.2/205
0.5/1.0/2.2/154
0.75/1.25/126