UKC

Question for the UKC hive mind...

We have an un-vented hot water tank in the loft. It's broken and needs replacing but we don't need such a large tank (210 litres).

The difference in price between different size tanks is minimal, it'll be close to £1,500 regardless ☹️

Question - how much difference is there in the running costs of different sized tanks?

I realise that in a perfectly insulated system with no heat loses (theoretical) that it'll make no difference (the volume of cool water to bring back up to hot will be the same even though once mixed it'll be smaller volume starting at lower temperature v larger volume at higher temperature).

But in the real world with actual heat losses from the tank, will the difference in tank size make a significant difference or will it be trivial?

You can run a small 40-80 litre 'fast recovery' tank if your boiler can supply sufficient output. This means smaller stored mass of water and better efficiency in minimising heat loss. I'm planning on doing this when I move our ancient tank out of the bathroom and into the loft. I have 30 kW boiler which makes this a viable option. Such a system should almost work like a combi, with the water in the tank reheating even as you take it out.

The labelling systems for cylinders lists a value in W, that's the standing loss of the cylinder. It should vary pretty linearly with length of the cylinder, and different constructions can make a difference. The below answer assumes you're off gas and using an immersion heater style cylinder. If using a boiler its similar but you can cut the prices down by 3 to get a rough estimate for gas costs.

The W value on cylinder labels is the energy input required to keep water at 65 in a 20 degree room. Using this, you can work our running costs (roughly) by doing  0.05 * 8760 * 0.34 (kW drawn * hours per year * kWh unit price from October for electricity) = 150 quid. Looking at this places catalogue https://www.mrcentralheating.co.uk/pub/media/catalog/product/ERP2018-Everfl... going from 180l to 300l goes from £200 to £260. Comparatively, a high end, well insulated cylinder from OSO https://osohotwater.co.uk/wp-content/uploads/2022/01/Product-Brochure-UK-EN... is £150 a year for their 300l model. 

Thanks, I've also just looked at the brochure on the cylinders that one quote is using. Heat loss ranges from 43 to 57 watts (in the sizes we're considering).

By my calc that would be 14/1000 * 24 * 365 = 122 kWh

If using the immersion heater (and assuming 100% efficiency, must be close in an insulated water environment) that would be approx £44/year (using the Oct price cap), presumably using the gas boiler's hot water would be cheaper otherwise why would anyone ever bother with an indirect tank.

I would seek some other quotes if you are being asked £1500.

Yep, marginal difference - assuming the extra capacity doesn't mean you take longer showers and therefore spend more time heating cold water, which that calculation doesn't capture. Direct tanks are used when you don't have gas, central heating, or space for all the other kit, and are better than needing power showers and taps, or an electric boiler. Much easier to run new wire than gas lines.

Using an indirect tank with a boiler means you can feed multiple showers/hot taps etc, whilst also running radiators direct from the combi boiler, or through a hot water priority setup with a system boiler. This can let you get away with a smaller boiler as well and the benefits of gas being cheaper per kWh than electricity. This kind of system is also pretty much required if you intend to end up with a heat pump, they produce warm water very efficiently, but not in great volumes, hence needing the intermediate storage tank. 

That's including fitting (changes to pipework), dealing with the old tank, etc.

Also look at the recent thread on the subject (assuming it wasn't in the pub and auto-deleted)

I’m just about to fit a new tank and after seeing this post went looking for the W number. I couldn’t find it, but did find a kWh figure! My old tank used to lose about 40Wh. Perhaps the new one will be enough to heat the whole house.

What an odd label - a kWh number on it's own is pretty useless - it also needs a measure of time. 

2.74kWh/hour is a lot

2.74kWh/day is 24 times less

although arguably there will be a bunch of other assumptions in play too, like water temperature and surrounding temperature. I guess as long as all manufactures make the same assumptions then you can compare the figures and work out the relative efficiency of each cylinder, but if they don't then you are kind of in a hole trying to work out what is best.

Whatever way, I wouldn't put a hot water tank in the loft for many reasons - and certainly not the unheated part of the loft if that is what is being considered by the OP.

In the past (at my mum's house) after plumber installed a decent u value rated tank, I also added a hot water cylinder insulation (red padded "jacket" lagging kit) of the sort sold for people with ancient uninsulated tanks.

Maybe that's unnecessary belt and braces, but it was quick and easy to do, so I did...

Looking at a Teford data sheet for one their tanks it's kWh/24 hours.

That still doesn't make sense. If it's thermal loss it should be in kW/24h surely? kWh is power consumption.

No, kW is a unit of power, a kWh is a unit of energy.

Exactly. Why not just say power loss is 114 Watts on this Telford cylinder? The other funny thing about the label is the requirement for a portable water supply - I would have thought potable would have been good enough..

I'd say using kWh per day (if they'd actually labelled it properly) is more useful. Easy to find unit price of electricity is 34p/kWh and get running cost straight from that. 

Ah. Got it, thanks.