unvented hot water cylinders in tandem

[dav54321]

Hi All, customer wants unvented hot water, large house, large family, ideally 210-250 litre capacity. problem is room limited to site such a large cylinder. Rooms already in loft, have eaves, thought of horizontal unit, but think could be an access problem, ie small hatchway, ok diameter but length possibly too long to manoeuvre around corner. So, think I will be able to get 2 cylinders in there, however, cant get head around how these could work independently of each other, as in the flexibility of only heating one cylinder when demand is low. Thought of putting motorized valve to primary flow to one cylinder so would only heat when programed to, however this would not stop cold water being drawn off when hot tap is run, is there such a thing as a temperature controlled valve to open when the water reaches a certain temperature and would this be pointless as when the cold water replaces the hot the valve would close. I feel it may be that both cylinders will have to be heated as it would have been the case had there been just one large one.

 

[Mike Jackson]

You could use one cylinder to preheat the water to the second one. At periods of low demand just heat cylinder two and at peak times heat both cylinders.

 

[Ray Stafford]

I would be surprised if its worth it. Standing losses from modern unvented cylinders are pretty low.

Looking at the Worcester SC range, the potential difference in standing losses per 24hr period using 2 x 120 litre instead of 1 x 210 litre, ASSUMING you could find a 100% efficient way of controlling for high/low demand would be:

In periods of low demand - a saving of 0.61 kw/24hrs
In periods of high demand - a loss of 0.45 kw/24hrs

Given that you will not be able to find a 100% efficient way of doing it, and anything you try to do will cost money and energy, I would not worry about it, and run them together.

Edit. When we say "low demad" - what do we mean? less people living in the house? Water heating isn't like space heating - if some selfish barsteward uses 120 litres of hot water, you suddenly have a high demand situation, but its hard to detect in advance.

 

[Killy Bing]

Acv can go in tandem, give them a ring.
also the way the ACV set up works they give out far more hot water than they store.

 

[Ermintrude]

It's the Goliath (jumbo) tanks that usually go in tandem 800l plus.
But a super will give 20% more water than standard due to it holding water at 85c and blending with cold at outlet to give 65c or below. You could link two in series but for small volumes it's a bit rubbish.