I've done a trawl of the archive and I've found some answers, but in some ways some more questions.
I've a 12 metre plus run to the kitchen sink from the HW tank with only a 3 - 4 metre head, and have compounded the problem by fitting a pull out mixer tap which has a high insertion loss. Various solutions have been played with but a local heat source off the mains seems the best option.
Just how good are under-sink heaters ? what is the storage types ? why do they vary in power from 2.2 to 9.5Kw? What is the problem with expansion - I would have just a short pipe run to the mixer tap ? Would I be best to arrange a longer run ? I've seen something about vented taps - how do these things fit in with an ordinary mixer?
Undersink heaters come in two power ranges. 2-3kW with storage which will deliver a reasonable volume of water to fill a sink, wash hands etc. They will do this at good flow rate, but once the store is empty that's it. They can be run from a spur on the ring circuit.
The 8-10kW models are "instantaneous". They deliver water at a reasonable rate although very slowly in the winter. They need a special circuit delivering about 45A to the heater - rather like one of the pathetic electric showers.
Certain types of heater are OK with a standard tap, others need a special tap or have installation rules.
The low power ones heat up a small tank of water -- if you won't be running off more than this amount more frequently than the reheat period, these can be good, but I doubt that would apply at most kitchen sinks. They are hot all the time and lose heat -- in sci.engr.electrical someone recently worked out the heat loss was equivalent to burning a 60W lamp continuously IIRC (although this was a US one and UK ones may be better insulated).
The higher power ones heat the water on demand -- they are very similar to electric showers. The hot water supply is unlimited, but the flow rate will be more limited at higher temperatures. They will require a heafty mains cable and high current fuseway.
When the water is heated it expands and forces a small amount of water back into the water main. This is permitted only to a limited extent.
Vented taps look like a regular singular tap, but the tap knob actually controls flow in a separate pipe from the one feeding the tap, which is always 'on'. They work by cutting off the input to the storage tank and leave the output permanently 'on' (vented). As the water expands in the heater, the excess drips from the tap. People try to stop this dripping by turning off the tap harder, and IME the washer is quickly destroyed and the tap really does start to leak. Also there's a sort of 5 second delayed action when the tap is opened or closed. This is not how people expect a tap to behave and also causes people to turn it off harder, knackering it. I don't see how this could be used with a mixer.
Vented taps won't help you. You need to get a storage type heater with enough storage to do what you need. It will probably have a 3kW element or thereabouts. You need to get the type with a built in expansion vessel. This will contain the pressure increase as the water is heated without requiring vented taps or long pipe runs.
Any hints on how to change the washer on a vented tap - replacements are £100... and I cannot see how to take the existing tap apart. Would it be better to fit a pressure release valve, discharging outside, and then fit an ordinary tap.
I've got one of the £90 screwfix ones, has it has 10L capacity and a
1.2kW element, comes with separate pressure release valve and is suitable for mains pressures.
This will take about 20 minutes to heat up to 60°C from room temperature, and I would guess about 40W/h to keep it there.
Since the pipe to the boiler contains only about 2L of water if you're using 15mm, there is not much to worry about running out of hot water.
Another option could be to use heating tape along the pipe, this would take about 70W/h to keep the pipe at 60°C if the pipe has 1/2 inch of insulation.
The electricity used by both of the above would be reduced by putting them on a time switch, adding more insulation is also a help.
Thanks Pete - that's quite useful. Ref. the pressure release valve - does that avoid the use of an expansion vessel if there is inadequate run to the hot tap. Althought there is a long run from my HW tank, the mains feed runs passed the kitchen sink, so I was thinking if fitting the heater quite close by. It seems that such an expansion vessel is required if the pipe length to the tap is short (that at least is what the Sadia web page says - see the link in an earlier posting)
No idea. I notice the maintenance people at work just gave up on trying to keep the vented tap working, and have replaced the whole heater with one which mounts on the wall, with a regular tap on the input.
That won't work -- the tank is not designed for mains water pressure. (It probably does already have a pressure release valve.)
The distributors say in their info that they are suitable for use without an expansion vessel, where the inlet water pressure does not exceed 6bar/88psi, and the expanded water can be accommodated by the pipework.
They then specify in turn that there should be a minimum of 9' of pipework for the 10L model between the inlet and any non return valve on the supply, to accomodate expanded water.
The pressure release valve is meant as a safety valve and must be installed into a tee on the cold water supply pipework. The one I had supplied is 6 bar and with 2 1/2" BSP female connections, one as an overflow.
It's pretty easy to install, it comes with a mounting plate it slots onto. The connections can do with some boss white or similar goo as PTFE tape doesn't quite cut it.
I bought it from Screwfix but it's distributed by Harlequin Products, contact details as follows:
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