Heat bank - one or two plates and other detailed decisions

[Peter]

I had originally posted this to an existing thread I started, but as it apperas to be 'just off the end' of the standard 25 message display .. perhaps people missed it.

Anyway - I'm reporting in the hope people can help me.

Situation in summary is that I have a 45kW boiler currently connected to an aging unvented copper cylinder. I'm looking to install a new heat bank which when directly connected to the boiler should provide (as somebody put it) "blindingly fast" recovery. I am looking to add rads to the attic at the same time.

The question is where to run the rads from and how large the store should be:

1. With blindingly fast recovery .. how do I figure out how small the heat bank can be? I mean normally, you'd use the heatweb calculator for #baths, #sinks, showers etc. But with my arrangement what I've got is more like a very large water content combi! I'd want to run a bath and shower and a tap at the same time, and would want to run 2 baths and two showers within an hour.

1. I already have, existing, 4 pipe runs to the attic. These will become available for use with the mains pressure DHW provided by the heat store. It is difficult - though not impossible to add more.

If I go for a fully direct system (Rads off heat store as well), then I need a header and expanstion tank in attic. So that means I need pipes: Flow, returm, Cold feed, Vent, Mains Cold. Thats five! So - my quesiton is .... what are the dos and donts about the header tank feeding into the return line? [ If I did this, I get back to 4 pipes again ] I know normally you do it just after an air seperator and just before the pump - but I dont understand why.

1. Alternative to the idea in (2) is to run the rads off a seperate heat exchanger off the heatbank and pressurise just the rads. Anybody got any thoughts about the pros and cons. Ones I can think of are: Yet another pump (would now be up to 4) Not quite as fast heatup separate boiler/heatstore water from rad water (a good thing i think)

Again - very grateful for all the ideas and discussion.

Peter

PS Amazing how ones thoughts change - I realsie that none of the above options leaves me with my rads running off the clever modulating controller - something which earlier on in the disucssion I said was a must-have. Now it just doesn't seem so important.

[TheOldFellow]

Google groups isn't the usual way of looking at Usenet lists, so no, very few of us missed it for that reason!

R.

[John Stumbles]

If it weren't for the stratification factor it should be fairly straightforward O-level physics (though it's so long since I did mine it's now rocket science to me :-)). In practical and economic terms it probably comes down to a standard cylinder from Screwfix etc being the best bangs per buck.

Is that a problem? They're not expensive. The PHE would cost more (but needn't be as big or as expensive as a DHW one).