Hot water system plan

[Learning Plumber]

Any experienced plumbers out there kind enough to take a look at my hot water system drawing? You can be critical - I'm learning!

It’s designed to supply PV solar preheated water to an electric water heater and then on to small bathroom (daily light use). At the end of an ‘average' day, I calculate the cylinder will likely be at 35c 95f . The instant water heater would then take this up to 50c/122f - working harder during cloudy or shorter days.

I’ve specified a dual-voltage immersion heater so that the cylinder can be pasteurised weekly (60c/140f) with mains 240v (UK) as a precaution against Legionella etc.

The system has to be as simple as possible as I will be installing/maintaining it myself. Hiring a pro-plumber would be a wise suggestion, but my budget only just covers the parts! I’m experienced with DIY, pipe runs etc. and have tools.

Any thoughts on the plumbing & parts list? Problems I’ve missed, or just general advice and suggestions would be welcome. I am also unsure what height the cistern should be for good pressure at the outlets - 3m/9.8ft?

 

[ShaunCorbs]

Have you got a big solar array?

 

[Learning Plumber]

No, there's only a small roof area available (south facing and unshaded) : 800w-1kw. Potentially another 500w could be added but that would involve a more difficult install. I've attached the circuit drawing which is incomplete but should give an idea.

 

[ShaunCorbs]

The problem your going to have is that the immersion will only heat up the first 11” and being 300w it’s not going to do much

It might be better to look at cheap night off peak and heat up the whole cylinder

 

[Chuck]

No, there's only a small roof area available (south facing and unshaded) : 800w-1kw. Potentially another 500w could be added but that would involve a more difficult install.

That means that a rather large fraction of the cost of the PV system will be the electronics/controller. You need to calculate your return on investment with this in mind. Also, the electronics are not likely to last as long as the PV panels so you need to factor in the cost of replacement(s).

Another issue arises with using PV to heat water is that when your HW power consumption is maximum (winter) your PV generation is at its minimum.

 

[Learning Plumber]

I have some spare panels already and installed brackets while re-roofing so that side of things is effectively cost-free at this stage. Some people are using panels direct to an immersion (no controller/battery) with effective results but I'm concerned on/off fluctuations in voltage will have effects down the line (relay, thermostat etc.). I need some more input there, however If the solar is inadequate, the water heater should take up the slack - it will heat from cold. As long as the plumbing design is sound that is...

 

[Learning Plumber]

The problem your going to have is that the immersion will only heat up the first 11” and being 300w it’s not going to do much

It might be better to look at cheap night off peak and heat up the whole cylinder

Thanks for the response. The twin voltage immersion is at the base of the cylinder - shouldn't the warmer water rise ?
Or would the input simply not keep pace with heat loss from the cylinder?
I'm not expecting fireworks - just enough to lessen the load on the instantaneous water heater in the bathroom. It's mostly evening and early morning use so the cylinder would have a full day of 300w input (as long as the sun shines).

 

[ShaunCorbs]

Thanks for the response. The twin voltage immersion is at the base of the cylinder - shouldn't the warmer water rise ?
Or would the input simply not keep pace with heat loss from the cylinder?
I'm not expecting fireworks - just enough to lessen the load on the instantaneous water heater in the bathroom. It's mostly evening and early morning use so the cylinder would have a full day of 300w input (as long as the sun shines).

The cylinder you have only has a top immersion unless you’ve spotted a different one ?

 

[Learning Plumber]

So I found one with twin immersion inlets - top and bottom. 'Direct vented' I believe is the type? Bottom one normally for econ 7 top one (I think) for heating small amounts during the day. I would blank the top and just use the bottom install.

 

[Learning Plumber]

The cylinder you have only has a top immersion unless you’ve spotted a different one ?

The immersion heater itself has both 240v (700w) and 12v (300w) elements in the same unit (typically used on yachts, motorhomes etc.) so they can run off a battery or shore power. I realise this is a pretty weird setup...!

 

[Devisham]

Run the vent pipe to tundish - drain.
Make sure the CWSV overflow is screened.
Put a screened breather on the CWSV.
Consider TMNT/TMV to reduce scalding risk at outlets.
Remember check valves for backflow.
Consider localised pipework isolations to outlets/HWSV/POU to aid potential future works/repairs.
Consider HWSV destrat and/or HWS return pipework to aid HWS circulation.
Consider immersion min. DD HWSV instead of D HWSV.

 

[Knappers]

Lost me at 'DD HWSV'!?
But yea I like the idea of mains element min, think I'd loose the instantaneous htr as high maintenance and run mains immersion at say 40 with solar 'topping up'. Could drop mains supplied temp in summer and raise in winter.
You'll want atleast 22mm pipework for good flow (I'd go all the way to bath) and you'll get 0.1bar for every meter above outlet. Bear in mind that taps min working pressure is often the absolute min to get any water out so look for ones with 0.1/0.2bar min WP.
Also not sure how effective scale inhibitor would be in that position? I'd be inclined to put before instantaneous heater if you use one

 

[Learning Plumber]

Thank you Devisham, that's really useful detail. I'm going to google some of those acronyms now! I do my own maintainance work and it's always easier when the original install has been done with that in mind.

Run the vent pipe to tundish - drain.
Make sure the CWSV overflow is screened.
Put a screened breather on the CWSV.
Consider TMNT/TMV to reduce scalding risk at outlets.
Remember check valves for backflow.
Consider localised pipework isolations to outlets/HWSV/POU to aid potential future works/repairs.
Consider HWSV destrat and/or HWS return pipework to aid HWS circulation.
Consider immersion min. DD HWSV instead of D HWSV.

 

[Learning Plumber]

Thanks Knappers, I'll extend the 22mm runs. Had wondered about the scale inhib. position. Sounds like I might get away with a 3 metre height on the cistern with the right outlets - loft space won't allow much more and I don't fancy the headache of adding a pump. I went for the inst. htr option for flexibility, as there are low demand days, but a sealed complex unit like that does have obvious drawbacks.

 

[Learning Plumber]

I'll start updating my drawing and post back. Responses really appreciated.