Have you done this? Instant warm / hot water at sinks (not boiling taps)

[Great_scot_selfbuild]

I’m looking at an options of installing an instant water heater at the sink taps at the end of long runs. I have had more than one person suggest such an idea to me and I’m trying to understand the options out there.

 

I’m not talking about instant boiling taps, but rather running only cold supply to the sinks, with an instant water heater at the sink to heat it at the point of use.

 

Willis heater https://www.unventedcomponentseurope.com/willis-jacket-external-immersion-heater-complete-pre-wired-and-fully-assembled.html/

Stiebel Eltron Instant water heater https://www.stiebel-eltron.co.uk/en/products-solutions/Hotwater/instantaneous_waterheater.html

We only have single phase, so are looking at the smallest options

 

Has anyone done this, and what does the cost to the electricity bill look like? Noticeable?

[Russdl]

There is a fair bit on the forum going back a good while regarding instant water heaters, I don’t know if you’ve searched yet but something there may answer your question.
 

I have an instant water heater in the garage, puny little thing but produces nice tepid water at the end of a 20m cold run. 

As for the Willis heater option, that won’t automatically switch on when you run the tap so I’d suggest it would not be a straight forward solution for your problem (great for UFH though!)

Edited February 11 by Russdl

[MikeSharp01]

6 hours ago, Great_scot_selfbuild said:

Stiebel Eltron Instant water heater

@Jeremy Harris played with those although I think he just used them to boost the hot water at the end of the HW run. I dimly recall he gave it his best shot but in the end removed them as they didn't meet his need.  

[ProDave]

3 hours ago, MikeSharp01 said:

@Jeremy Harris played with those although I think he just used them to boost the hot water at the end of the HW run. I dimly recall he gave it his best shot but in the end removed them as they didn't meet his need.  

I have Jeremy's old Stiebel Eltron 10kW instant water heater.  It is on the hot feed out from the UVC to the bathroom pipework.  It was fitted as a comfort blanket when only heating hot water in the tank to a low temperature, so a given size tank would not deliver as much as most people would be used to as dilution with cold would be a lot less.  The theory was if you used up all the hot water part way through a shower you could carry on using it just like a 10kW electric shower.

 

It is still there but I suspect it never turns on.

[Nickfromwales]

Electric instants are dire at best. The 3ph ones are amazing, but single phase are pretty shit and completely driven by cold water temp; in winter, they get soooo much worse performing.👎

 

Add the cost of a huge cable to supply it, electrician costs to supply / fit, and the eye-watering amount of electricity it will gulp whilst you are using it.

 

Fit a hot return, and enjoy the benefits. 

[Mr Punter]

I have one in a student HMO.  I have also had the undersink type with storage in an office suite.  Neither property has gas.  I don't think I would want one at home.

 

[Big Jimbo]

I have got One in my posh shed. 10kW. Instant with no stored water. A bit rubbish, but my only source of hot water in there. I do have posh filtered drinking water though. Must check when i last changed the filter😅

[sharpener]

I have got one in a garage separate from the main house and one in an outside loo, both 3kW Triton. They are fine for hand washing, and the occasional sink full for cleaning bigger things if you are prepared to wait. There is a diaphragm which I have had to replace because it no longer sealed properly, the replacement was ~£30 IIRC which I thought outrageous for what it was but at least was available.

 

If you wanted a hot supply e.g. for regular washing up then the 7kW is the minimum viable size IMO but still not an adequate replacement for a proper hot supply from a cylinder.

[Nickfromwales]

11 hours ago, sharpener said:

I have got one in a garage separate from the main house and one in an outside loo, both 3kW Triton. They are fine for hand washing, and the occasional sink full for cleaning bigger things if you are prepared to wait. There is a diaphragm which I have had to replace because it no longer sealed properly, the replacement was ~£30 IIRC which I thought outrageous for what it was but at least was available.

 

If you wanted a hot supply e.g. for regular washing up then the 7kW is the minimum viable size IMO but still not an adequate replacement for a proper hot supply from a cylinder.

Hot water from an ASHP is cheap enough, and if you've invested in all that infrastructure, have already plumbed the house for hot water to all bathrooms etc, then it would be madness to not use it??

[JohnMo]

On 10/02/2026 at 23:42, Great_scot_selfbuild said:

looking at an options of installing an instant water heater at the sink taps at

Really would not bother, run hot and cold to manifolds central in the house and out to each wet room from there. If you are concerned about distance and time to get warm water, add a secondary loop and pump or simpler run 10mm pipe sink. Hot water will be there pretty quickly 40m of 10mm Hep2O only holds a couple of litres. If you need notes than 40m you must have a HUGE house