Direct unvented cylinder as hot water supply

[d000hg]

We are all electric in our log-cabin nursery and had planned to use on-demand electric water heaters - we have a total of 3 bathroom sinks and 3 kitchen sinks, no showers, etc.

We've been recommended to go for a cylinder instead, unvented using an immersion heater. It was suggested something at the smaller end like 120L would be suitable.
These appear to go for £400-500 and I believe the actual immersion heater is peanuts?

I'm unclear how it works in terms of timers, thermostats, etc. Do you normally just set it on a timer and that's that, or would one typically have a temperature probe as well?
I'm trying to get an idea on the parts list independently to the plumber just to a)check I understand b)check they aren't padding unreasonably.

So what would this setup typically look like? Any other thoughts welcome too.

 

[JohnD]

It has a thermostat, so it will stop using electricity when up to temperature.

An immersion heater warms water at a rate of about 1 litre per minute, so you need to hold enough in store to supply at least as much as you will use in a peak hour, pre-heated and ready for use.

Unless you have cheap night rate electricity, or close down for a week's holiday, there is not much point in a timer. Heat losses during a working day are small due to good insulation. Remember to insulate the pipes as well.

Is this a children's nursery, or plants?

 

[d000hg]

Kids. I guess it's not as simple as the tank needs to be capacity for a peak hour since temp will start to drop as it empties, any ballpark on what % of a tank you can take out before it starts to cool off?

Interesting point re: timers, so in normal use we might as well just leave it turned on and it will run as needed?

 

[JohnD]

No it won't.

The hot water rises to the top of the cylinder.

The cold water enters at the bottom

The taps draw from the top.

It stratifies. It doesn't mix.

But 120 litres sounds small to me.

You only need a couple of soiled underwear incidents to use gallons of hot water. A thermostatic shower head and mixer beside a large sink can be useful at such times.

Nowhere near the kitchen or food prep.

 

[d000hg]

That's a good point. We had initially considered exactly the sort of thing you mention... well probably a small electric shower separate to the hot water supply which would only be used by an adult.

Thanks.

 

[d000hg]

As I understand it we need:

• A tank - £4-500
• An immersion heater element - £20(?)
• A thermostat/control box £???
• A TMV for each sink/outlet that needs it - say £60 each

Have I missed something or grossly mis-estimated anything? In terms of thermostat are we talking a bog-standard Honeywell type basic unit?

 

[oldbuffer]

1. I'd expect the biggest cost to be installation. The cylinder has to be installed and commissioned by a plumber with a G3 qualification, and he / she has to register it with your local Building Control.
2. Have you checked that you have the flow rate (20 litres / minute plus) and pressure (2 to 3 bar) to make an unvented cylinder viable.
3. Unvented cylinder immersion heater are usually proprietary items, not the sort they sell at Screwfix. Often cost £60 each or more.
4. The thermostatic control will be built into the cylinder, and I would expect the immersion heater to be supplied with it as well.
5. Unvented cylinders are meant to be serviced annually. Depending on where you are this could cost £50 to £100, plus any parts / work to correct faults.
6. I'd have thought you would need more like 200 to 300 litres, but that is a guess, not a calculated estimate.

 

[d000hg]

Thanks. We don't yet have the new supply connected but it is a brand new pipe to the mains so we expect good pressure.
If not realised there were different sorts of immersion so that's handy.

What would a typical house tank size be for comparison? We only need it for hand washing but it will be a lot of hands to wash.