Unbiased appraisal of pressure/flow/capacity of thermal store vs unvented

[mike42]

Hi

I've seen quite a few threads about these technologies around but can't quite get to understand something.

Pressure wise, both would seem to operate at mains pressure (minus domestic pipework, valves and connectors) although the UV system has a large body of water as part of the pressurised system and the thermal store just has regular (mains pressured) pipework running through it on the way to outlets. So roughly speaking, don't both offer broadly a similar pressure (and indeed flow) given the same domestic envionrment (incoming flow/pressure mains) ?

Capacity wise, it seems intuitive to me (who has little common sense) that a thermal store could produce more water of a given temperature because the thing that is doing the heating is much larger (i.e. much of the tank) than for UV which is being heated by a smaller volume coil of some kind.

Then finally to practicalities. I'm struggling to understand the figures. The thermal stores I've read about claim a flow rate of 35 l/min assuming a rise of temperature of 32 degrees. Is that plausible? What are the figures for a smilar sized megaflow or other UV tank I wonder?

Not sure its relevant to the above but I'm assuming the heat source is a single gas boiler.

 

[Dan Robinson]

Too many variables, btu a decent thermal store could have a heat transfer capacity of 100kW... but then the store has to hold enough heat energy to make the most of it, and the boiler has to be able to keep up too.

 

[mike42]

I wonder if anyone else might have anything to say on this?

 

[Agile]

Certainly!

An UV cylinder heats itself up to about 60 C. So you get, say, 300 li of water ready to use.

But a thermal store uses its heat to warm up mains water! They are only heated up to about 70 C by gas boilers because that's the maximum temperature they can reach economically while condensing. Electrically heated versions heat to about 85 C. So more heat is held in the store.

BUT the problem is that when used to heat mains water from say 10 C virtually twice as much water from a store is needed as from a cylinder because the store only gives useful heat from its temperature of 70 C to 85 C down to about 40 C. So a 300 li store can only heat about HALF its volume of mains water. That is only 150 li.

So a storage cylinder is a more efficient use of space for providing hot water as the whole heated volume is available for use.

Tony

 

[hazetimesfive]

a 300l store at 60 degrees wont give you 300l of 60 degree water.
a thermal store at max temperature will give you nearly it's entire capacity at 55 degrees, depending on manufacturer.