UVC to supply hot water & UFH; Octopus agile tariff

[DIWhy]

Allo, hopefully you'll be able to help with this.

 

Currently renovating a house with no existing heating system. Water underfloor heating is being installed; peak demand estimated at 5-6kw (across 80m²).

 

The plan is to make use of the Octopus Agile tariff which offers very low electricity cost at night. Overnight (cheap) energy would be stored in a large (potentially very large) unvented hot water cylinder. 

 

Essentially use this as a thermal battery and charge it up with cheap electricity at night.

 

The cylinder can be as large as it needs to be; the main requirement is to have enough stored thermal energy to survive through parts of the day that have much higher energy prices.

I can find plenty of options for this if I want separate cylinders for the hot water and UFH supply.

 

I'm finding it tricky to find an option that allows a single, large cylinder to supply both hot water AND an underfloor heating system.

 

In the future it might also be useful to have a spare (redundant) coil in the cylinder for heat input from an  external heat source, whether this be a boiler/ASHP/solar thermal.

 

Does anyone know of a product or supplier that allows for a single cylinder for the above?

[ProDave]

Thermal Store is what you need, not UVC, that might make your search easier.  You can get those very large.

[DIWhy]

I've managed to find one (potential) supplier via a local heating engineer. Are there any particular ones you might recommend or know of?

[Iceverge]

 

Be cautious of thermal stores as well combining your DHW and heating. You'll inevitably end up with complex,  expensive system and the prospect of the UFH stealing all your bathwater. 

Using water to store heat isn't very efficient as it typically needs to be done at high temperatures and even the most pricey cylinders leak lots of energy. 

 

Your heating requirement also seems high. Is the house old and poorly insulated/draft-proofed/ventilated? This is your first point of call.

 

I'd stick with an UVC for DHW.  The larger the better, as a large cylinder will lose less energy than a smaller one for a given amount of energy stored.

 

If you need to "bank" cheap overnight heat just dump it into the slab. With a slab thickness of 100mm you'll have about 20,000kg of concrete. This will store 5.5 Kwh for every degree you raise it's temperature. If you had a large 500L buffer tank you'd have to raise it's temperature by 10deg to get the same effect.

 

This is dependant on decent insulation below your floor however. 

 

Search @TerryE blog to have a look as how to achieve this.  

[PeterW]

How thick is the slab going to be..?? Make it 150mm and you’ll get a serious storage heater for your overnight energy. Add in a 500 litre UVC and you should I never run out of water. 
 

You’re not making the most of the tariff by using direct electric heating so consider the heat pump now as you will 2-3 times the heat per kW of electric. 

[ProDave]

What are you going to use for cooking?

 

The bit issue with most of these tariffs are you get a really expensive slot between 4pm and 7pm  30p per kWh not being uncommon.  Are you happy paying that much to cook your dinner?  are you going to shift your eating times?  Or are you going to install a gas hob and oven so it does not bother you?

[SteamyTea]

DHW and Space Heating are different things.  They do things at different temperatures, at different times and for different reasons.

Separate them.

 

I am also concerned that people may spend a lot on capital equipment to take advantage of one energy suppliers latest tariff.

Is there a guarantee that this tariff will remain in place for a decade?

Edited January 28, 2021 by SteamyTea

[DIWhy]

Wow, great advice all, thank you.

 

So for further information the house walls & floor are currently completely uninsulated. Double brick leaf with thin cavity & outside render. Loft is insulated with fibreglass. Concrete slab floor (unknown depth) with thin screed.

 

On the plus side (as PeterW & Iceverge mentioned) the building has a tremendous thermal mass. On the negative side retrofitting insulation is going to be tricky. After looking at options any additional insulation will need to be external unfortunately; the initial thought is to insulate by extension over the coming years, then externally insulate whatever is left over.

 

Because of this, as you mention, the estimated heating load is high. However even in winter the building is retaining heat well over several days; I suspect the base U-value isn't telling the full story here. Trying to spec the heating system for the worst case scenario.

 

The thermal storage company I contacted responded to some figures about sizing the thermal store with "we're not heat engineers", so my current thought is to follow your advice and go for two separate unvented cylinders.

 

My main thought with setting out was to keep this simple; two cylinders definitely does that.

 

My main concern about ASHP was installation cost, unit lifetime, and therefore payback period. I'll make sure the cylinders have a redundant loop/coil that can fit one, and then run the sums again once everything else is up and running.

 

ProDave; as far as I'm aware there is only one tariff like this. Average peak price per kWh (between 4pm and 8pm) over the past year is 20p/kWh to 25p/kWh. Overnight prices 5-6p/kWh, rest of the day prices 8-10p/kWh. Varies by region.