Discussions in some other threads have touched on the possibility of installing an instantaneous electric water heater to ’top up’ a supply of stored hot water in circumstances where there are practical constraints on generating and storing a large-enough volume of hot-enough water using a heat pump.
I’ve previously flagged up some limitations which need to be taken into account when considering such a configuration but I can now add some further real-world experience of how (some of) these units operate.
My own use-case is a detached workshop building where it’s not appropriate to store any hot water, so the water for occasional handwashing and suchlike is heated (from cold) by instantaneous electric water heaters. I have two of them:
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A ZIP InLine ES4 (4.4 kW) feeding a single sink in the main workshop area, mostly used for handwashing
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A ZIP InLine CEX-U (8.8 kW) feeding both a kitchen sink and a washbasin, which are back-to-back either side of an internal wall so can share the same heater
The ES4 is fine for a single sink/basin where the water just needs heating to about 35C for handwashing. There’s an adjusting screw under the cover to control the outlet temperature target. Nominal flow rate is 2.5 litres/minute at ∆t = 25 K. If I was buying this again I’d probably opt for the higher-capacity ES6 (5.5 kW) which has a nominal flow rate of 3.3 litres/minute.
The CEX-U is physically larger and more sophisticated, with a digital display showing the target water temperature and buttons to adjust that (and some other settings). It’s intended for use with multiple water outlets, potentially including showers. My comments below relate to the CEX model specifically.
Key Points
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The documentation for these units very clearly states they are happy to take in already-hot water (up to 70C) and will ’top up’ the temperature as required to reach the specified outlet temperature
a. I suspect this ‘hot water inlet’ capability is primarily intended to cater for Solar Thermal water heating systems, where the stored water might vary from very-hot to not-at-all hot from one day to the next
b. They only actually draw as much power as is required to reach the target temperature
c. If 8.8 kW is insufficient they restrict the flow so that temperature can be achieved
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In the scenario where these units are acting as ‘insurance’ for stored hot water starting to run out, they might only be expected to heat water from e.g. 30C to 40C, to ensure it’s warm enough for showering
a. That’s not especially demanding from a power delivery standpoint. However…
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The external pipe fittings are threaded 1/2” BSP but some of the internal pipework is 10mm copper. There’s also a built-in flow restrictor rated for 5 litres/minute at 6 bar
a. My mains water installation is fitted with a 3 bar pressure regulator and with that in play I’m seeing 4 litres/minute from the hot tap
b. In my view, this is the biggest challenge with trying to use these units in any sort of ‘whole house temperature top-up’ scenario - they’re going to restrict the flow to (about) 4 litres/minute even when they’re not doing any top-up heating
Further Considerations
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With the electrical power switched off, these units still allow water to flow (and won’t attempt any ‘dynamic’ flow restriction) so they could be switched off at their electrical isolator and still remain plumbed-in
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These “ZIP” models are re-badged Clage units (and actually also show the Clage brand name). They’re made in Germany. Personally I’ve found the Clage website more informative than the ZIP website.
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The 8.8 kW maximum power rating can be restricted to 6.6 kW instead as part of the installation-time setup
a. This is to accommodate electrical installation constraints, where the wiring to the unit or the power available at the site cannot support 8.8 kW operation
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The CEX-U is intended for Under-sink installation, with the pipe connections pointing upwards. The CEX-O is intended for Over-sink installation, with the pipe connections pointing downwards.
a. These two variants are identical except for the orientation of the temperature display (and the printed logos). The orientation of the CEX-O is slightly better for preventing any water leaks entering the unit.
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There’s a gauze filter on the inlet pipe, about 10mm diameter, upstream of the flow restrictor. This is at risk of getting blocked and further reducing the flow. I’m currently in discussion with my plumber about adding a separate in-line Y-pattern strainer to protect (or potentially replace) the built-in gauze filter.
a. If the inlet is part of the ‘hot’ water pipework there would be more chance of scale and other debris clogging the built-in filter so I’d definitely recommend fitting a separate strainer that would have a larger surface area and would be easier to clean.
Conclusion
I suspect the 4 litres/minute flow rate is a deal-braker for using the CEX model as a ’top up’ for anything more than a single tap or shower (and even then it might be problematic). Installing multiple units would be possible but (even more) expensive.
An obvious question is whether different ZIP/Clage models accommodate a higher flow rate. Based on the ZIP and Clage websites it seems their 230V models max out at 11.5 kW (50 A!) and a 5.9 litres/minute flow rate. 3-phase (400V) models go higher; up to 27 kW and 8 litres/minute.
So while units like these could work in principle, the fact they’re primarily intended to heat water from stone-cold means they’re only expecting to see a modest flow rate (because in that scenario the limiting factor is the delivery of enough heat to the water).
Perhaps other manufacturers offer something more suitable for whole-house top-up.