Venting and feeding new install y plan

I am updating my hot water / central heating from an old gravity fed system to fully pumped. I will be installing the pump and 3 way valve in the loft next to the H/W cylinder.I am running a 22mm from the valve down to the central heating flow and installing a new boiler downstairs. Is it neccessary to vent and feed the system on the input side of the pump or is it OK to vent on the output side before it goes into the 3 way valve. Chris London
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Firstly, check if the boiler can support sealed pressurised operation. If so, abandon the feed/expansion tank and install a sealed operation kit. Sealed operation has considerable advantages over gravity fed.
Secondly, consider using an S-Plan system, rather than Y-Plan. Although it will cost very slightly more, there are considerable advantages to S-Plan. Firstly, the 2 port zones used by S-Plan are more reliable. However, the main advantage is that it is easy to subdivide into zones (technically S-Plan-Plus). You can have separately timed zones for upstairs and downstairs, for example, so you don't heat bedrooms during the day.
If you do this, use programmable thermostats so that each zone has individual temperature and timing settings, with conveniently located override controls in the principal rooms. Use TRVs on radiators on all radiators that are NOT in rooms with thermostats. In those rooms, fit lockshield valves both ends to physically prevent the radiators being turned off. Fit an automatic bypass valve, too.
The zone valves don't have to be in the same location either. The unzoned pipework from the boiler can branch off round the house and be valved at a suitable location. For example, you could have the zone valve for the hot water cylinder in the loft, whilst having the heating zone valve near the boiler, for example, meaning that you only need one pair of pipes to the loft. The exact positioning would depend on your subzoning requirements and your existing pipework layout.

If you really can't have a proper sealed operation kit fitted, then the order is:
Boiler flow -> Vent -> Feed -> Pump -> Valve -> Rads/Cylinder -> Boiler return
The positioning of the pump isn't that critical. Many boilers have internal pumps, for example. However, I would check with the manufacturer that it is permitted to have an external pump on the steam escape path.
In any case, though, the vent and feed MUST be taken off in that order, and no more than 15cm apart on the pipework. Some systems have a combined vent and feed pipe. If so, the boiler is required to have the same overheat cutouts that a pressurised system needs, so you may as well go pressurised.
Christian.
P.S.
This is all UK practice.
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