Running two instant heaters together?

[Toby]

Hello!

I have (almost) acquired two 9.5KW "Redring Powerstream" unvented instant water heaters that have previously been tripping out.

I am guessing they are going to be scaled up, so will try and remove the scale, to re use them.

I would like to install them both near my boiler (which is also near the consumer unit) as backup water heating, in case the combi has problems (not that I am expecting that to happen, it is just these two heaters are spare)

Would the be any issues with connecting them between the cold feed to the boiler, and the DHW feed out of the boiler (Via isolation valves)

*in parallel* with each other (so that the cold feed enters both electric heaters, and the hot output is combined and then connects to the DHW pipe that feeds the house), so I can get maximum water flow?

Would I need to install a one way valve on the outlet of the boiler, so hot water can't flow back into it? (A Vaillant ecoTec 837)

I realise the hot water capacity of the two units is not going to batch the boiler, but it will be better than nothing in the event of a failure!

I would connect a separate consumer unit for these (Via a henley block after my main separate isolator), with each protected at 40A - this would be close to them (

[robgraham]

I have difficulties seeing why scaling up would cause the tripping out. No, actually to put it the other way round, I can't see why removing the scaling would remove the tripping out. If there is significant scaling then that might well have lead to overheating of the elements with the corresponding deterioration of the insulation, hence the tripping out.

Your first problem as I see it is to satisfy yourself as to what is causing the electrical failure - you certainly do not want to go down the route of extensive plumbing, on which I can't pass comment, and then find that the units are BER.

My first port of call would be a megger test on the electrics to confirm the insulation, and even then I would find some way to implement a jury rig to fully test the units before going anywhere. Rob

[Andy Wade]

That seems a lot of trouble to go to. Most combi problems can be easily and quickly fixed, provided parts are available.

Three come to mind:

- significant flow resistance, even with the two units in parallel, will permanently reduce the performance of the HW system (it might also violate the boiler manufacturer's installation instructions);

- you'd have to ensure equal flow sharing between the two units, symmetrical pipework required;

- 19 kVA / 80 amps electrical load - is your supply up to it? Connection of additional loads of that sort of size should be notified to your supplier or DNO.

[Toby]

Sorry, I was a little ambiguous with the "tripping out" part - they are both tripping their internal overheat thermostats randomly, I assumed because a build-up of scale is acting like a (heat) insulator between the heater element and the water.

[Toby]

But that will mean downtime, that could be avoided, and being at the mercy of having to make a distress purchase from a fast, but usually expensive supplier, or calling out an "emergency plumber" at vast cost.

I am not sure what you mean about this, the installation of these new heaters won't affect the flow of the boiler, as they won't be used at the same time, and the heaters switch in and out automatically when there is enough flow through them.

That should not be a problem, however, I was thinking it would be better to have the opposite, so if you are drawing a small amount of water, only one heater would switch in, then the other one would switch in at a higher flow rate.

Okay, I didn't know that - Does than mean when installing more then one electric shower to the same supply, the DNO has to be notified too?

[Andy Wade]

Sorry, I misread your article. I assumed the electric heaters were to be in series with the cold feed to the combi, which is not what you said at all.

The one "not switched in" will still pass cold water which will mix with and cool the water coming via the other one. If the units are like typical electric showers they have their own internal regulation - a flow regulator/stabiliser valve, division of the element into two halves and an o/p temperature sensor which switches out one half-element when the output is getting too hot.

Possibly, but people tend not to bother in practice. I wouldn't install a second shower in an installation with only a 60 A main fuse though, without getting the supply upgraded to 100 A. Also there will be diversity in practice with two showers which won't apply in this case.

[Andrew Gabriel]

Just one of them will be good enough as a back-up. Keep the other for spares.

[Harry Bloomfield]

Then why not, at your leisure, source a full set of replacement parts so that what ever might fail can be quickly replaced?

Our three port actuator fails regularly, so I have a spare always ready to be installed when the running unit fails.

[Toby]

That is a valid point, but I haven't had anything fail yet, and a replacement of even the most common parts, which may never get used by me, is going to cost me a lot more than the heaters, which will be free!

I have all the other parts to make the backup electric heater work you see.

[Harry Bloomfield]

That is apart from the upgraded incoming supply, which might well cost rather a lot.

[Toby]

Not going to be needed, I have a 100A cut-out already ;-)

[John Rumm]

Depends on why they are tripping...

If doing this, you would be better arranging that all three are in parallel with valves to control which are in circuit (i.e. the combi or pair of inline heaters). Note many water heaters will have a maximum inlet temperature that you might exceed with a series heater. Also each will add flow resistance.

Lever operated service valves would do.

They should have their own isolation after the MCBs really. Note also that you are talking about a massive electrical load - is your supply up to it?

Check the sums:

[Harry Bloomfield]

Just one more thought...

Are these water heaters intended for mains water pressure?

[Mike Tomlinson]

In article , Grimly Curmudgeon writes

He is Dribble in disguise AICMFP.

[John Rumm]

He is more like your anti-dribble - same tune, different words!

[geoff]

OIC

He uses a plastic pipe cutter on copper pipes ?