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

C

Confusedsparkie

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6

• Oct 30, 2021

• #1

My grant 70 mk 2 heating pressure is constantly increasing in pressure. I have changed the heat exchanger as I believe that was the problem. Any ideas

 

exedon2

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• Oct 30, 2021

• #2

I assume you mean you have changed the plate to plate?
Have you totally disconnected the filling loop?

 

C

Confusedsparkie

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6

• Oct 30, 2021

• #3

exedon2 said:

I assume you mean you have changed the plate to plate?
Have you totally disconnected the filling loop?

Click to expand...

The filling loop is disconnected. Also the pressure drops to zero when off, and the expansion vessel is new

 

SimonG

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• Oct 30, 2021

• #4

The cost of plate heat exchangers I'd want more than a belief it's at fault.

Checked the filling loops not passing?

Is it rising when the boilers off?

Pressure vessel ok?

Pressure vessel hose not blocked?

 

C

Confusedsparkie

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6

• Oct 30, 2021

• #5

SimonG said:

The cost of plate heat exchangers I'd want more than a belief it's at fault.

Checked the filling loops not passing?

Is it rising when the boilers off?

Pressure vessel ok?

Pressure vessel hose not blocked?

Click to expand...

Filling loop disconnected, pressure drops when boiler off, new expansion vessel, hose disconnected and blown through

 

SimonG

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• Oct 30, 2021

• #6

Thought pressure was increasing?

 

C

Confusedsparkie

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• Oct 30, 2021

• #7

SimonG said:

Thought pressure was increasing?

Click to expand...

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

 

exedon2

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• Oct 30, 2021

• #8

Pressure ok when just on for hot water?

 

snowhead

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• Oct 30, 2021

• #9

What's the pressure when the boiler first fires up?
How fast does the pressure increase?
What's the pressure at it's highest when the boiler is on?
Is the Pressure relief discharging as the boiler is heating up or when the pressure is at it's max?
How fast does the pressure reduce?

Is the Expansion vessel suitably sized for the system and set to the correct precharge?

 

C

Confusedsparkie

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6

• Oct 30, 2021

• #10

exedon2 said:

Pressure ok when just on for hot water?

Click to expand...

Hot water is fine, even when the heating is on

 

SimonG

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• Oct 30, 2021

• #11

Confusedsparkie said:

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

Click to expand...

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

 

C

Confusedsparkie

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6

• Oct 30, 2021

• #12

SimonG said:

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

Click to expand...

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

 

J

John.g

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• Oct 30, 2021

• #13

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

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• Oct 31, 2021

• #14

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.

 

J

John.g

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• Oct 31, 2021

• #15

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/

 

J

John.g

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• Oct 31, 2021

• #16

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

 

Last edited: Oct 31, 2021