Combi and Open Vented Cylinder

[AndyHingston]

Hi All,

I am still deciding on the best replacement heating system and the DH flow rate is quite important. Whilst browsing many websites I cam across the Albion Water Heaters

site an in particular the Superduty tanks that provide a very quick recharge And the site has a schematic for a system using this tank with a combi I have attached this diagram.

I would really like to have your views on this solution, as if it i ok, it could provide me an answer and looks pretty easy to install although I am not sure what is different between this and "conventional" system!

Thanks

And

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[Andy Hall]

It's basically a conventional storage system with the CH part of the combi used to heat the radiators or the cylinder. Not an unreasonable solution. Flow rate for the bath can be what you want it to be if you have a large enough tank and cylinder and you do some of the pipework from tank to cylinder to tap in 28mm.

However, watch out for the wording.

The 45 litre CF45 will contain water at 60 degrees, but the amount stored could well be too little for filling a bath. The heat exchanger is quite efficient because it has a large surface area, but will be limited by what the boiler can produce.

The 80 litre model would be a far safer choice and is not substantially larger in terms of cupboard space requirements.

The combi part of the set up could feed a shower or kitchen taps and in effect the bath would not impact or be impacted.

[IMM]

A conventional system is using a system boiler and no mains pressure to the shower. All is off the cold tank in the loft.

I prefer a combination cylidner of tank/cylidner with a quick recovery coil to do all oulets at low pressure and a combi to do the high pressure shower only. The heating side of the combi re-heats the cylidner. Justy like the drawing on Albion. Look at:

The bottom one on the right, sized to suit. Far better setup than than an unvented cylidner, or silly power shower pumps.

Using a combination tank/cylinder eliminates tanks in the loft. In fact you could put it in the loft out of the way.

It gives you what you want. Instant high pressure showers from the combi. Fast flow for quick bath fillups.

[IMM]

The water could be stored at 80C and a blending valve on the hot water draw-off takes water down to 45C or so. This in effect makes the cylinder larger. Having a flow switch on the DHW to bring on the boiler immediately hot water id drawn off, also will extend the size of the cylinder. In effect using the two points above you can make the cylinder give as much water as an 80 litre cylinder. If space is a problem then this is the way. A 45 litre cylinder can go in a kitchen cupboard.

Also see: http://range-cylinders,co.uk

[Andy Hall]

Storing the water at 80 degrees increases the effective capacity of a

45 litre cylinder to *up to* 60 litres.

A flow switch may improve that a little, but may not be desirable, since without additional controls, the boiler would cycle each time a tap is turned on and off. Then the system becomes more complex and further cost is added.

Unless space is exceptionally tight, a larger cylinder is a more sensible option for a system like this.

[IMM]

yep!

Yes up to 80 litres, if the boiler is approx 25-28 kW

So, the boiler cycles on and off, so does a combi. An immersion probe stat can be fitted right near the bottom of the cylinder to detect cold water is entering and switches on the boiler on pronto. This prevents boiler cycling when taps are turned on.

Nope. see above.

Space must be tight as he is wanting a small cylinder. It may be best to get a square tank specifically made to size by Elson. That will store more water. A 12" high x 20" d x 23" w, square will go easily into a kitchen cupboard bottom and holds 90 litres. Put a blending valve on and store water at 80C and it will produce about 120 litres of hot water.

[Andy Hall]

It's impossible to say that because you don't know the flow rate through the cylinder. Without that information, you have no data to calculate the amount of energy that 25kW would deliver into the cylinder.

OK, so the flow switch was a bad idea.......

By reverting back to a thermostat.

Further misinformation.

A tank of the dimensions you give will only hold 90 litres if *all* of that space is used for water storage. Either this would imply no insulation, or the net storage capacity would be less.

If the tank has 50mm of insulation, the net dimensions would be

20.5cm x 40.8cm x 48.5cm and would have a capacity of 40.5 litres

With 30mm of insulation, net dimensions would be 24.5 x 45 x 52.5cm giving just under 58 litres.

Neither anywhere near 90 litres.

[IMM]

No. two ways of achieving the same.

[Roger]

The message from "IMM" contains these words:

As usual dIMM hasn't a clue about the use of figures and switches to denial mode. I think he must have been an ostrich in his previous existence.

[IMM]