Tankless water heater ( undersink )

Peter3 · May 31, 2020

Hello.
I notice several tankless water heater units being sold Ebay.
Some of the listings show these units fitted under sinks and connected directly to the hot side of a mixer tap.

I had been under the impression, that units such as these required an open vent on the outlet side of the heater
( as per a shower head ) to avoid overpressure due to heat expansion when shut off.

Could anyone please advise, if it is considered safe to use one of these units in this manner ?
If not, is there some form of safety valve or inline expansion device which could be used ?

It is my intention to use this unit for hand washing etc and not for boiling water.

Thank you, Pete.

moonlight · May 31, 2020

If they are directly under the sink they will need a pressure relief valve, which must go outside, along with an expansion vessel. Would need to be fitted by someone who knows what they are doing.

SJB060685 · May 31, 2020

They all need pressure reducing valve, double check valve and a pressure relief valve minimum. If memory serves me correct 15 litres and below only needs an expansion vessel if less than 2m of cold supply pipe after double check valve. Any heater with a capacity of over 15 litres needs an EV

Christopher buzzard · May 31, 2020

The open vented undersink water heater you are talking about vents through the tap. But you need the tap that goes with the water heater. When the water heater heats up, the increase of pressure inside the unit drips out of the tap. Like the tap i have linked.

HEATRAE 94970015 TXH02 VENTED PILLAR HOT TAP | Direct Heating Spares

Buy the Heatrae 94970015 Txh02 Vented Pillar Hot Tap from Direct Heating Spares. DHS Part Number: 1735165 - Manufacturer Code: 94970015. We have over 20,000 items in stock and available Next Day. Free Boiler Manuals also available to download.

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