I require a under sink water heater and my plumber is recommending a
10L 3Kw pressurised water heater but when I have been searching for one it seems they are either vented or unvented and no mention of pressurised. It will have a cold water feed from the mains. Can anyone shed any light on this? Also, will I need special taps?
For the purposes of this discussion, pressurised = unvented.
On most models, you do not require special taps. It's just a mini-megaflo. Mains CW in, HW stored at pressure, mains pressure HW out.
There are some stranger models around. These are un-pressurised, yet provide mains-pressure HW, and work like this:
Mains cold comes in, to the tap in the first instance. The tap does not open directly to the spout, it opens to a return pipe to the cylinder. The tap then opens the CW supply to the cylinder. This then forces the stored HW out a seperate pipe to the spout, at mains pressure. In this config, the cylinder is not pressurised, it is open to the spout at all times. The pressure is only applied when the cold supply is turned on.
These require a special tap ( with a return pipe to the cylinder. )
Sounds like the plumber probably knows what he's doing, and you ought to let him get on with it.
That sounds like it's a unit that just escapes all the really serious regs. So you may fit it with out a G3 ticket, plug it into (a FCU on) a 32A ring circuit, no discharge or vent pipes required (it forces expansion upstream into the mains).
Umm.. In an effort to avoid this, I installed ours in the *hot* water (softened) circuit.
The instructions were pretty insistent about expansion issues but I really could not see any extra safety concerns provided there was ample feed pipe length and no local shut off valves.
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