How to use a 3.5kW instant water heater
Updated: 17.10.2026
You have opened the tap and the water is tepid, or you are about to fit a small instantaneous heater. How to use a 3.5kW instant water heater comes down to flow. The display temperature is a target. Outlet temperature falls when the flow rises, and a cold winter inlet makes that worse. Turn the flow down before you decide the heater is broken. If it still will not heat, or the plug is hot, stop and get an electrician. The product table is the [tankless water heater 3.5kW comparison](/hub/hot-water/tankless-water-heater-3-5kw). We did not lab-test these units. A qualifying purchase can earn a commission at no extra cost to you.
You have opened the tap and the water is tepid, or you are about to fit a small instantaneous heater. How to use a 3.5kW instant water heater comes down to flow. The display temperature is a target.
You have opened the tap and the water is tepid, or you are about to fit a small instantaneous heater. How to use a 3.5kW instant water heater comes down to flow. The display temperature is a target. Outlet temperature falls when the flow rises, and a cold winter inlet makes that worse. Turn the flow down before you decide the heater is broken. If it still will not heat, or the plug is hot, stop and get an electrician. The product table is the tankless water heater 3.5kW comparison. We did not lab-test these units. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Stated power | Flow claim | Temperature control | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Axelrivet Axelrivet 3.0 kW under-sink | 3,000 W | 2.9 L/min bullet | LED touch display | 5.2 | about £44.99 | View offer |
| Alternative |  Builrand Builrand 3.0 kW under-sink | 3,000 W | Not stated | LED display | 4.8 | about £44.99 | View offer |
 MRNIU MRNIU 3.5 kW under-sink | 3,500 W | 4 L/min | Display and remote | 6.8 | about £42.00 | View offer |
On this page
What happens when you open the tap
Cold water already sitting in the spout comes out first. On a short pipe that is a few seconds. Heat then continues only while water is flowing. There is no tank in this class. Shelley’s review of the MRNIU describes warmth for hand washing in about 5–10 seconds once the flow was reduced. That is one review, not a specification, and it was not a shower.
The MRNIU listing says outlet temperature depends on inlet temperature and flow. A Builrand bullet claims activation in 3–5 seconds. None of the five manuals is in the files we used, so this page will not invent button presses. If the booklet disagrees with a bullet, follow the booklet. Close the tap and the element should stop. If it keeps heating with no flow, stop using it.
Why the setpoint and the tap disagree
Set 55°C on the MRNIU and you have set a target. Hyco’s formula is temperature rise equals kilowatts times 14.3, divided by litres per minute. At 3.5 kW and 2 L/min the rise is about 25°C. At 4 L/min, the flow the MRNIU states, the rise is about 12.5°C. From a 10°C inlet that is still in the low twenties. Label that as arithmetic, not as a test of ASIN B0F8HT6V6D.
Winter mains often sit nearer 10°C than 15°C, so the same tap can feel thinner in January. The element adds a rise. It does not hold an absolute outlet temperature. Reviews report about 18°C to 30°C with the control at 55°C. If the outlet is cool, the first suspect is too much flow.
A simple flow check
Time how long a one-litre jug takes to fill at the tap position you actually use. Fifteen seconds is 4 L/min. Thirty seconds is 2 L/min. Forty seconds is 1.5 L/min. If a litre fills in about 15 seconds, you are at the MRNIU’s stated 4 L/min, and the formula says the rise will be about 12.5°C.
Narrow the tap, or fit the spray head or restrictor this class expects, and time the jug again. You want something near 1.5–2 L/min for the Ascot-shaped result, around 40°C from a 15°C inlet at 2 L/min. Where the heater sits is the 3.5kW under sink water heater page. None of these listings states a descaling interval. A blocked spray hole is the first thing you can see without opening the case.
When it is not a flow problem
Some units in this set never heat, and turning the tap down will not fix that. Builrand reviews include no heat, a leak from the bottom, and a return. john davies says an Axelrivet ran for about 10 seconds and then stopped on a live socket. A leak, a burning smell, a hot plug, or water on the electrics is a stop. Isolate it.
Shelley and Rose report a three-pin plug on the MRNIU. The spec table does not. 3,500 W at 230 V is about 15.2 A. The circuit side is the 3.5kW instant water heater buying guide. Which listing states 3,500 W is on the best 3.5kW tankless water heater page.
Common questions
Why is the display at 55°C when the water is tepid?
The 55°C figure is a setpoint, not the temperature at a high flow. Hyco’s formula puts the rise from 3.5 kW at about 25°C at 2 L/min and about 12.5°C at 4 L/min. From a 10°C inlet, 4 L/min still leaves the outlet in the low twenties. The MRNIU states both figures. They cannot both hold from a cold inlet.
How long should I wait after opening the tap?
Expect a few seconds of cold water that was already in the spout, then heat only while the tap stays open. Shelley’s MRNIU review describes hand-wash warmth in about 5–10 seconds after the flow was reduced. A Builrand bullet claims activation in 3–5 seconds. Neither figure is a standard for every unit here. If the water is still cold after that short run, check the flow before you assume the element has failed.
Should I open the tap fully?
No. A fully open tap is how this class goes lukewarm. At 3.5 kW, Hyco’s formula gives about 12.5°C of rise at 4 L/min, against about 25°C at 2 L/min. A spray head or restrictor holds the flow down. A single-lever mixer can also blend cold water past the heater. If the instructions are silent, narrow the stream and time a one-litre jug: about 30 seconds is 2 L/min, and about 15 seconds is 4 L/min.
Does winter mains water change the result?
Yes. The heater adds a rise to whatever arrives. A Delux 3.5 kW example at 1.5 L/min is about 49°C from a 15°C inlet and about 44°C from a 10°C inlet. Ascot’s chart is drawn at 15°C. If your mains are nearer 10°C in January, the same tap position will feel cooler. You cannot cancel a cold inlet by turning the dial once you are already at the top. Reduce the flow instead.
What does a 4 L/min claim imply at 3.5 kW?
It implies a small rise, not 55°C water. Using Hyco’s formula, 3.5 times 14.3 divided by 4 is about 12.5°C. Add a 10°C inlet and the outlet is about 22°C. Ascot’s chart has no 4 L/min cell for this class. Its 3 L/min cell, at a 15°C inlet, is about 32°C. The MRNIU’s 4 L/min line and its 55°C line describe different operating points.
The unit runs for a few seconds and then stops. What next?
Stop using it if the water stays cold, or if you see a leak, smell burning, or find a hot plug. An Axelrivet review describes a unit that died after about 10 seconds on a live socket. Builrand reviews include no heat and a leak from the bottom. Those are not a flow setting you can tidy up with a jug. Isolate the supply. Do not open the case to look for the element.
Related pages
Sources
Public UK sources used with the amazon.co.uk listings in this set; shelf prices from 15 October 2026.
- Approved Document G, sanitation, hot water safety and water efficiency (retrieved 15 October 2026)
- Water Supply (Water Fittings) Regulations 1999, Schedule 2 (retrieved 15 October 2026)
- IET Part P frequently asked questions, England and Wales (retrieved 15 October 2026)
- SELECT guidance on bathrooms and BS 7671 section 701 (retrieved 15 October 2026)
- IET Wiring Matters, BS 7671:2018 bathroom socket questions (retrieved 15 October 2026)
- Plugs and Sockets etc. (Safety) Regulations 1994 (retrieved 15 October 2026)
- Hyco instantaneous heater charts (retrieved 15 October 2026)
- Ascot technical chart for instantaneous heaters (retrieved 15 October 2026)