Newbie - Which cylinder?

[Tarik]

I currently have a dead oil fired combi boiler, so I am starting from scratch and I am baffled by the number of choices.  I am looking for a new boiler/cyclinder/solar etc.

Before I start please forgive me and be gentle if I use the wrong terms.

Our heating circuit consists of radiators on a two pipe sealed system.

My thoughts were replace the combi with a system boiler and to have some kind of heat bank (either a cyclinder from newark or DPS), which  I think I would need a coil for the solar, a coil for the boiler (indirect?) and then either a coil for DHW or tappings to fit an external PHE.  Is there an off the shelf product that meets these needs or do I need to get into specifying a custom cyclinder?

Have I got this right so far?

The next question would be how big.  We have a four bed house (not relevant I guess) with one bath with shower over, one shower in ensuite, cloackroom, utility sink and kitchen sink.  I would like to base it on 2 adults and two children, obviously as they grow they will use more water. 

I am worried that we will run out of hot water and the annual costs will be higher than with the combi.

Please help!

[dhaslam]

It looks like  you need a heat store of about 300 litres, very well insulated and   at least  a 60 tube (47mm tubes) solar system.   The heat store should have  good internal coil for DHW and solar coil.  Radiators can be fed directly from the water in the store  and the boiler can heat the store directly. The take off for the radiators  should be  below the DHW coil.   You can also add a wood stove  if the store is in a suitable place for gravity feed.             

[Tarik]

Thanks for the quick reply.

Since posting I have been reading further and came to the conclusion I would need around 300l and 30x58mm tubes, which kind of matches with the suggestion.

Still confused about how I can connect the boiler and radiators directly to the store as they are currently on a sealed pressurised system and I assume the store would be vented.  Also is there a danger of a conflict between heating and hot water demand?

Ideally I don't want to reinstate the header tank in the loft to run a vented heating system, unless there is a clear benefit.  I think I was looking at a coil to heat the store indirectly from the boiler.  Comments welcome.

Also you mention a coil for DHW.  Would that be in prefernce to a take of to a plate heat exchanger?  We live in a hardwater area.

[dhaslam]

The direct connection of the radiator circuit  is simpler but the header tank for the store needs to be higher than upstairs radiators.  The solar system needs to be pressurised because it is normally higher up and should have antifreeze.      It is normal to have  a preheat coil for DHW lower down in the cylinder and an  upper coil in its own space at the top of the store.   The advantage of the radiators connected to the store is that   they heat immediately  when turned on and the boiler can  be smaller and run for longer periods.  By having the radiator circuit outlet below the top DHW coil  avoids  interference with the  final temperature of DHW.  Also   in summer selected radiators can be used briefly to cool  the water if overheated by the solar panels.   It is advisable to over specify solar panels with this sort of system because  solar can contribute to space heating  in sunny/frosty  weather, like now and it  is good to be able to have the boiler permanently off in summer.      It  would be advisable to  soften the hot water  feed to save the DHW coil.    Although the coil is more expensive than a  PHE it should be more efficient and  should produce water within about six degrees of the store temperature  at a good flow rate.  There are some disadvantages of the  thermal stores, one being that  they may not able to supply two showers at once, they may need a temperature boost sometimes in summer  and need a dependable mains supply.  However they give  a good water flow without the need for pumps.

[Brandon]

you could use a pressurised store

A coil from Newark is FAR cheaper than a PHE, when you consider the flow switch, non ferrous pump, and will not require power to give you hot water

You will struggle to get a meaningful DHW preheat lower coil into a 300l store once you have got the solar coil in, and the DHW coil (that will run the bulk of the height of the store) especially if you want a boiler coil, (which i would not advocate).