Hi,
There seems a lack of god information on this so I thought I'd put together this post.
As hot water expands then in a sealed CH system you need an expansion vessel to allow for this.
C Factor
85 0.0324
90 0.0359
95 0.0396
100 0.0434
85C is the hottest the water should be but to play safe lets take the fault condition of 95C, 0.0396. You next need to know the total volume, either by draining it into a bucket if you are converting a vented CH, or from the sizes of the radiators, boiler, and pipe runs if it is a new system, or guessing. Let's say CH volume is 100l, so expansion is 3.96l. BUT, that does NOT mean you need a 4l expansion vessel.
Next we need to look at static pressure. If we assume 2 storey house, 3m per storey then total head is say 6m. 10m of water = 1bar so this would be 0.6bar. We want the highest radiator to be under positive pressure since negative pressure would draw in air. Typically this is rounded up to 1bar. The expansion vessel which is best sited at return near the pump, as that way the pump increases the pressure in the whole CH loop. As you move away from the pump return then you see a drop in pressure from vessel to pump return due to the water flow. Let's assume boiler is on ground floor so here we want 1bar. This is the precharge pressure we want in the expansion tank. If the boiler, pump and expansion tank was in the loft then it would be at the minimum pressure point so 0.5bar precharge would be all we'd need, just enough to ensure system has a positive pressure so doesn't draw in air.
But let's stick with ground floor boiler, pump, expansion, and 1bar. We cold fill the CH system to 1bar. Let's say we have an 8l EV. When water gets hot we have an extra 4l. That goes into the expansion tank and 8l air gets squashed to 4l. Pressure is the inverse of volume so 1/2 volume double pressure, 1bar -> 2bar. The boiler PRV is usually 3bar (when converting open vented to sealed you need to fit PRV BTW). Boiler spec usual is to say max pressure if 2.5bar so 8l EV works ok for 100l CH. If we fit 12l then volume is 2/3 so 3/2 on pressure, i.e. 1bar -> 1.5bar.
Some people say set cold pressure 10% over precharge in which case you need to allow for that, i.e. initial pressure is 1.1bar, 110/100 initial volume is 100/110, ~90.9%, or roughly 10% of EV is already used up, so a 12l becomes 10.8l at initial 1.1bar. 4l from hot CH gives 6.8/10.8 volume change, 10.8/6.8 pressure change, 1.1bar -> 1.75bar.
Setting a lower initial pressure works better of course so for a loft mount the 4l change means 0.5bar -> 1bar with the 8l EV.
The other debate is how to mount the EV, air or water at the top? The advice so potable water is air up, water at the bottom, otherwise crap can fall into the EV and it can't get out. Same applies to CH IMO so water pipe work should come up to EV.
There seems a lack of god information on this so I thought I'd put together this post.
As hot water expands then in a sealed CH system you need an expansion vessel to allow for this.
C Factor
85 0.0324
90 0.0359
95 0.0396
100 0.0434
85C is the hottest the water should be but to play safe lets take the fault condition of 95C, 0.0396. You next need to know the total volume, either by draining it into a bucket if you are converting a vented CH, or from the sizes of the radiators, boiler, and pipe runs if it is a new system, or guessing. Let's say CH volume is 100l, so expansion is 3.96l. BUT, that does NOT mean you need a 4l expansion vessel.
Next we need to look at static pressure. If we assume 2 storey house, 3m per storey then total head is say 6m. 10m of water = 1bar so this would be 0.6bar. We want the highest radiator to be under positive pressure since negative pressure would draw in air. Typically this is rounded up to 1bar. The expansion vessel which is best sited at return near the pump, as that way the pump increases the pressure in the whole CH loop. As you move away from the pump return then you see a drop in pressure from vessel to pump return due to the water flow. Let's assume boiler is on ground floor so here we want 1bar. This is the precharge pressure we want in the expansion tank. If the boiler, pump and expansion tank was in the loft then it would be at the minimum pressure point so 0.5bar precharge would be all we'd need, just enough to ensure system has a positive pressure so doesn't draw in air.
But let's stick with ground floor boiler, pump, expansion, and 1bar. We cold fill the CH system to 1bar. Let's say we have an 8l EV. When water gets hot we have an extra 4l. That goes into the expansion tank and 8l air gets squashed to 4l. Pressure is the inverse of volume so 1/2 volume double pressure, 1bar -> 2bar. The boiler PRV is usually 3bar (when converting open vented to sealed you need to fit PRV BTW). Boiler spec usual is to say max pressure if 2.5bar so 8l EV works ok for 100l CH. If we fit 12l then volume is 2/3 so 3/2 on pressure, i.e. 1bar -> 1.5bar.
Some people say set cold pressure 10% over precharge in which case you need to allow for that, i.e. initial pressure is 1.1bar, 110/100 initial volume is 100/110, ~90.9%, or roughly 10% of EV is already used up, so a 12l becomes 10.8l at initial 1.1bar. 4l from hot CH gives 6.8/10.8 volume change, 10.8/6.8 pressure change, 1.1bar -> 1.75bar.
Setting a lower initial pressure works better of course so for a loft mount the 4l change means 0.5bar -> 1bar with the 8l EV.
The other debate is how to mount the EV, air or water at the top? The advice so potable water is air up, water at the bottom, otherwise crap can fall into the EV and it can't get out. Same applies to CH IMO so water pipe work should come up to EV.
Its not diy.
