Indirect thermal store?

[stonehouse]

I've just been reading a book titled "Central Heating" written by Patrick Mitchell. It's a really good book with lots of information I must say.

On page 48 and 49, an example is mentioned:

"the water in the mass storage tank is heated indirectly by hot water from the boiler passing through a heat exchanger at the bottom of the tank as is found in a conventional indirect hot water cylinder. The hot water for the taps is taken directly from the cold water mains supply and heated by passing it through a second heat exchanger external to the thermal store tank. This arrangement involves a fairly complex control and pumping system but has the advantage that lime bearing water from the mains is not heated to as high a temperature as the water in the tank. This avoids lime scale deposition inside the piping carrying mains water."

So, does anyone here have a such a thermal store? Is it any good? What are the pros and cons?

 

[bernardgreen]

Yes I do to supply mains pressure water to the shower. Basins and bath are fed from the water in the cylinder.

This arrangement involves a fairly complex control

The control is the same as the control for a standard unvented hot water cylinder

PROs.
Mains pressure shower with low pressure ( 2 metre head ) supply to basins and bath ( no splashing of water out of the basin ).
Shower can be used during a power cut.
Airing cupboard


CONs
Cylinder is slightly more expensive than a standard single coil
Cylinder has to be sized to provide adequate heat storage for the number of showers required. This could mean a larger cylinder

 

[Dan Robinson]

This arrangement involves a fairly complex control and pumping system but has the advantage that lime bearing water from the mains is not heated to as high a temperature as the water in the tank. This avoids lime scale deposition inside the piping carrying mains water."

Makes one wonder where they think the water in the tank came from.

 

[stonehouse]

Yes I do to supply mains pressure water to the shower. Basins and bath are fed from the water in the cylinder.

Are you saying that your arrangement is a conventional indirect hot water cylinder but with the addition of a secondary heat exchanger (coil internal) for the shower?

What I was talking about is an external heat exchanger (see diagram below)

It is said that the principal of thermal stores is that a large volume of water is kept hot; not to act as a hot water store but rather to act as a heat store. Thermal stores are normally kept at 70 deg C or more. This is too hot to pipe straight into the hot water taps.

 

[stonehouse]

Makes one wonder where they think the water in the tank came from.

See the diagram above. It's fed from the cold water cistern above.

But where does the old water in the cylinder drain to? Manually? How often?

 

[Dan Robinson]

Never ideally.

 

[bernardgreen]

Are you saying that your arrangement is a conventional indirect hot water cylinder but with the addition of a secondary heat exchanger (coil internal) for the shower?

Yes, a secomd coil inside the cylinder at the top

But where does the old water in the cylinder drain to? Manually? How often?

Never ideally.

The "old" water is used in the hot taps of two basins and the bath.

 

[Dan Robinson]

The "old" water is used in the hot taps of two basins and the bath

Not talking about you.

 

[stonehouse]

Never ideally.

Why never?

 

[Dan Robinson]

Because it's the primary system water. Unless you are actually talking about Bernie's contraption, which is nothing like the one you displayed m

 

[echoes]

Yes, I've got that arrangement. I used my existing indirect cylinder, a plate heat exchanger off eBay, a spare central heating pump, a flow sensor and a motor speed controller I built myself.

Pros:
Mains pressure hot water at all taps
Very good shower
No cold water storage cistern on loft (just a small header tank)

Cons.
Parts to go wrong
No hot water in the event of a power cut

 

[stonehouse]

Yes, I've got that arrangement. I used my existing indirect cylinder, a plate heat exchanger off eBay, a spare central heating pump, a flow sensor and a motor speed controller I built myself.

Pros:
Mains pressure hot water at all taps
Very good shower
No cold water storage cistern on loft (just a small header tank)

Thanks for sharing. Would you say that arrangement is the best there is? Better than conventional open-vent? Better than un-vented?

Cons:
No hot water in the event of a power cut

Did you mean no hot water in the event of a boiler breakdown? If so, an immersion heater can be installed as back-up.

By the way, do you ever change the water in the cylinder and if so, how often?

 

[echoes]

Best is subjective. It certainly does the job of supplying hot water, and does it well. The shower in particular is fantastic. Going unvented would have certain advantages.
For my needs, it is a vast improvement on the tank-fed hot water.

One downside of an external heat exchanger and pump, is that without power, no water can be pumped through the heat exchanger.
I do have an immersion in case of boiler problems.

No plans to change the water in the cylinder, why would I? In any case it has inhibitor in it. The heat ex and pump can all be isolated for maintenance without requiring a drain off.

 

[stonehouse]

Never ideally.

No plans to change the water in the cylinder, why would I? In any case it has inhibitor in it. The heat ex and pump can all be isolated for maintenance without requiring a drain off.

So, what is the purpose of the cistern if you never change the water in the cylinder?


Another question - Can such a thermal store cylinder be located in the cellar?

There's a stone table in my cellar and I thought I could use it for an horizontal cylinder like this one:

http://www.newarkcoppercylinder.co.uk/images/photo 07-03-2014 13 56 53.png?crc=4028376825

Will the pressure from the mains cold water drive the water through the external plate heat exchanger and into the hot water taps? Even vertically upwards?

 

[wwwebber]

So, what is the purpose of the cistern if you never change the water in the cylinder?

He doesn't have a cistern.