Recommend a DHW tank ( unvented)

[joe90]

Dear all, planning my DHW strategy and would appreciate recommendations for a pressurised ( after comments from crofters thread) DHW tank. It will be 200 to 300 litres, this for two people but with backup from an inline water heater when guests stay, we will have one bath ( our en suite only occasional use) and two showers ( one occasional use, guests). I will provide my own insulation in that I propose to build an airing cupboard with at least 150mm of insulation all round it. Probably no immersion ( they are a problem if surrounded in insulation are they not?) . Heat source is an ASHP with modulating in line heater to cope with guests/ running short. I was thinking of a tank with long pipes sticking out so they protrude through the deep insulation into the cupboard for plumbing into the system.

 

i hope that makes sense?, all comments gratefully received.

[TerryE]

I am using 2×SunAmp PV so no Thermal Store or UVC.  Maybe you might consider this (or 1× in your case) as an option?

[PeterW]

Telford Tempest 250L... HP model, single mid mount immersion. Mention the Welsh God @Nickfromwales to Trevor at Cylinders 2 Go...

 

You will need the immersion as you can't do a legionella cycle without one unless you want to crucify your COP

[SteamyTea]

The immersion is not a problem, just needs uprated wiring to it.  Easy to check:

http://myelectrical.com/tools/cable-sizing-calculator

 

As an example

[joe90]

Thanks guys, regarding the immersion I guess I could put a large "tube" through the insulation with a plug of insulation to gain access and uprated cable within the insulation. Do I need to specify type/size of coil, plate heat exchanger???

[SteamyTea]

3 minutes ago, joe90 said:

"tube" through the insulation with a plug of insulation to gain access

Basically what I did.

150mm of insulation and a 3 kW heater probably only needs 4mm² cable, and that is probably what you would use anyway to reduce voltage drop.

[Nickfromwales]

With a wet coil and an ASHP, and a 300ltr UVC, I seriously doubt if there is any need whatsoever for the instant water heater. 

Id just mount the cylinder stat lower and have the ashp kick in sooner to deal with higher DHW consumption. 

2 or more showers isn't going to deplete that imo, but as your storing at lower temp I'd be tempted to go slightly bigger to give better redundancy, IF you ever see yourself having guests frequently. 

Forgive my poor brain, but are you having PV?

[joe90]

Thanks Nick, I wanted the inline heater as belt and braces in case temps were a bit low or Christmas with the family caused depletion, perhaps I could just leave room for it and plumb it in afterwards if needed. With regard PV because of the low fits ( and the fact I hate the look of them especially on a " pretty cottage") we decided against it at the moment ( but cables are planned to be installed to future proof it). Can you recommend a supplier and do you think long pipes sticking through lots of insulation is a good idea.?

[Nickfromwales]

Long pipes & insulation etc is fine. The only thing you need access to is the Temp+PRV, which is tank mounted as it has a temp probe, and the immersion. 

At 250 / 300 Ltr, you'll be able to select dual 3kw immersions so why not go that route for occasional 'boost'? That'll add another 6kw of heat input which, along with the ashp, should give you shower after shower after shower and still have headroom for a bath. The ashp will quickly start replenishing the heat as it depletes, and a combination of the two, when under duress, should be ample imo IF you go for a 350ltr cylinder. I know that's pushing your Monday to Friday envelope a bit, but you must design for your worst case scenario I'm afraid, and I think this would be the best option. 

Remember that if you fit the 15mm Steibel after the UVC, it can only service one bathroom. If you connect both rooms to it, you'll be strangling the 22mm hot outlet of the UVC and subjecting it to the max flow rate the instant will allow ( through said 15mm internals ).   

Very difficult to see how you'd 'provision' for retro fitting the Steibel at a later date as that would require a whole different approach to what should be a straight-forward UVC install ;

UVC to manifolds in 22mm pipe. 

15mm feeds to each room or outlet. 

Job done.