Distant Boiler House - advice, please.

[Adiethesailor]

Having sorted our PV panels and put on external insulation last summer, my project this year is to replace our old oil combi boiler.  We suffer with low water flow here and so it is always disappointing, is becoming unreliable and is, I suspect, considerably less efficient than I would like.  I am awaiting the RHI decisions before commiting to anything but am hoping to make use of an outhouse for a new wood pellet boiler and pellet storage.  This will link to the original pipework from the combi by underground pre-insulated pipe running about 25m along the front of the house.
First question is whether the new thermal store should be put in the position of the old combi boiler (a boarded out corner of a downstairs bathroom) or kept with the boiler (possibly with recirculating hot water in a circuit to the original pipework to reduce the time to get hot water?).  The advantage of keeping it near the boiler would be that the roof of this shed is south facing and could support up to 10 square metres of solar hot water panels to feed in to the thermal store.  I am unsure of the idea of the recirculating pump - we already wait a while to get hot water and the idea of prolonging that waiting for the water to come from the shed is not too attractive, whilst on the other hand I am concerned about heat losses and pump power usage in a recirculating underground circuit.
Any thoughts would be gratefully received.
Regards to all,
Adrian

[dhaslam]

What you need to avoid is  having  a lot of short pump runs to the house.   Even though insulation may be quite good the water will still cool down  in between heat transfers so each time you lose the  heat in the  25 metres of pipe each way.     What would be best is to divide  the heat store between the two locations and  transfer heat in  a  one or two long transfers each day.   

[KLD]

Kristen's old thread is probably a good reference regarding the heat-loss issues. As Dhaslam said, with or without re-circulation the long pipe runs will have a considerable amount of heat-loss. I'm not so certain, though, that two stores are necessary. If it were a log batch boiler, then yes, you'd need a very large primary store, and maybe have a smaller DHW store in the main house / nearer the taps. But a pellet boiler? Do they also gain substantially from being used like a batch boiler? And wouldn't the gain be offset by the losses of the large store?
It also depends a little on you heating pattern. Is your CH on most of the time, or only during more or less short periods of the day?

Klaus

PS Did you say, you've got a 25m long house front? 

[dhaslam]

There is the issue of the solar panels as well.    If they  could  be  located on the house  then the tank in the house would be  better  option and the pellet boiler could just run continuously until the  tank at the house is hot. 

[Dyslexicbloke]

I am no expert but it seems to me you may want to think a little out of the box, or in this case the house.

If you put the store in the house and the panels outside conventional wisdom dictates a separate 25m circuit to the lower coil in the store, which isn’t that practical and would cost a great deal in adiquatly insulated pipe.

However you have an insulated circuit from the boiler room to the house … SO …
How about using it for both, you would need a couple of valves and a variable speed pump would help but isn’t essential.

Plumb the Panels on a separate local circuit feeding a plate heat exchanger in the boiler return leg.

When the boiler is on your solar heat will be preheating the boiler return and your delivery circuit feeding the main coil in the store.

When the boiler is off divert the return to the feed, bypassing the boiler, and the delivery loop to the bottom solar coil in the store. Now your panels are feeding the store directly and you can either vary the pump speed according to the temperature differential or use a thermostatically controlled valve to feed the solar coil in the store only when the loop is hot.

As a final twist …. You could extract a great deal of the low grade heat during boiler cooldown, which is normally lost, by treating it as solar heat.

I can draw it for you if you are interested, of course I my be talking total nonsense but it seems logical and workable to me.

Alistair

[Adiethesailor]

Wow... thanks for all your thoughts.
I read Kristens old thread with interest and admired his set-up.  I think I was hoping for something less glamorous (and cheaper!).  We are a family of five in an old Devon longhouse with converted attached dairy and shippen so the house is indeed about 30m long but only 6m wide (this has caused problems with all sorts of previous projects).  We are generally out during the day and our major use of hot water is over-filled childrens baths in the evenings - though they are still young enough to use the same water without too many complaints at present.
Unfortunately the usable area of house roof has been taken by the PV panels so the shed roof is the only area available. 
Dyslixicbloke - I like your thinking! - I think I understand your idea and like it a lot. 
However, now I understand more about heat losses etc., I am pondering an alternative, more straightforward, idea of building a lean-to extension outside the present boiler area to house the boiler, putting the tank inside where the present boiler goes and using the outhouse for the pellet store with pellets blown along to the boiler.
All good stuff for lying in bed looking at the ceiling . . . . !
Thanks again.
Adrian