So.. Further to my last large topic regarding pipe sizes I am in a bit of a quandary.. what ever one of those is..Basically I am renovating a 1930's largish 3 bed semi. I am intending to install a 300 ltr thermal store, gas boiler and solar thermal collector on the roof. For some reason and I am more than happy to listen to arguments for and against here, I want the Thermal store to provide heat for the ground floor under floor heating, the rads on the first floor and DHW. I do however want the DHW to be mains pressure. Now this either means (I think) an un-vented cylinder with the water in the tank being used as the DHW or that water being used to heat the mains pressure water via heat exchangers. I would like to do the latter. I have had quotes ranging from 3.5k to about 1.7k including VAT. The Un-vented system look cheaper, (and if anyone has any good steers on one of these they would be much appreciated) but for some reason, (legionella) I am unwilling to have the DHW sat in the tank. The way I am thinking about it now and I am sorry if I am being less than succinct here, is that the tank will be connected to the boiler. That connection will be by a coil. the underfloor heating will be a coil in and the rads will be a coil in. I will need a boiler heating coil, a solar coil and a DHW either external plate heat exchanger or a coil. I think I can get an AKVAIR SOLAR 300ltr for about £1500 but would still need to pay for all the plumbing, pumps, and associated pipe-work. McDonalds and Newarke cylinders are I think offering something along the same lines for a similar price, both in copper. I think these can be vented or un-vented. All the steel un-vented cylinders seem to be much more 'main stream' and don't seem to do what I want. I am speaking to RM cylinders through H2solar and they are being helpful but I think I may be hard of understanding as they are saying use the water in the tank as DHW. Am I worrying over nothing? How much is all the associated paraphernalia of pumps and valves etc going to cost. Should I be using a regular boiler for this? Are Regular boilers Ok for vented and un-vented. As you may well be aware by now, I am no plumber, I am however confused and running out of funds.I have bought and installed by insulated flexible steel pipe in readiness for my solar panels. I have all drainage and pipe-work ready in the allotted cupboard for the tank and to the wall where the boiler goes. Anyone feel like saying 'this is what you need, this is why you need it and this is how much it is going to cost you and yes you will get change from 3k!

Sorry to be so meandering. Its late-ish and this is beginning to stump me as I need to get things ordered and in quite soon before the money really does run out.

HI,
I have been reading through that yes, and the connection between the boiler and the tank was going to be via a coil in making a closed circuit. What I am concerned about is that the tank water will then be used to heat a coil to do the rads, heat a coil doing the UFH and either be used as the actual UFH or to heat a coil or heat exchanger externally for the DHW. There will also be a coil in for the solar panel. That makes 5 coils at a maximum and four at a minimum. I am concerned about having the tank water as DHW for Legionella reasons and also because I live in an area of very hard water. If the tank is a closed system too, it can be dosed up with inhibitor which will stop the coils calcyfying up. The only one to do so should be the DHW coil. If this is an external heat exchanger then it is easily flushed or replaced without disturbing the rest of the set up too much... I don't know if I am worrying un-necessarily though.

IIRC the risk of calcification in pipes is strongly temperature dependent as it comes primarily from thermal decomposition of bicarbonates to carbonates. If you keep the DHW temperature below around 50 to 55 deg C then the pipes take a long time to build up any appreciable scale deposits.

Posted By: Phil.Chaddah-Duke........................We are in very hard water area as well and we have had a nightmare with our current copper unvented cylinder after 10 years its falling apart. I was planning a good efficient water softener with the hope that that would avoid scaling problems, I am assuming it will solve the problem?

Polyphosphate dosing the incoming mains may help, without the expense of a water softener.

I don't know why I have been fixated with the number of coils. It does of course make complete sense to just have the tank as a closed system that runs to the rads, and UFH and from and back to the boiler. Would not the Solar thermal need to be in a coil though due to the different make up (Glycol etc) of the fluid within the solar system? The DHW through a removable coil does sound like the most sensible option.. Now for suppliers! Anyone?

Posted By: MarkBennett.........."I also don't like the plate heat exchanger route. and the flow rate of the primary loop is likely to destroy stratification in the tank. It will also take time to heat the exchanger to provide hot water to the taps, leading to wasted water and inconvenience."

Not a problem in my experience.

"I strongly prefer the coil in tank. With the right size of coil it can act as a compromise between tank-in-tank and a plate heat exchanger. For our intended store the coil holds something like 70 litres, so there is ready hot water sat there when it is needed. All the water through the coil is changed when the tap is run, so it doesn't sit around for long (or can be easily flushed). It also continues to deliver hat water until the store is depleted, not just the tank in tank. Finally, with a well designed coil it can pre-heat from low temperature water at the bottom of the store before being topped up with the hot water at the top. It is worthwhile making sure that the plumbing allows disconnection from the coil in case the coil needs to be flushed to remove limescale etc. but this should be relatively simple to do."

The best coils for DHW heat exchange are corrugated in construction and fairly large bore running up the centre of the tank. I read somewhere that in Germany there have been problems with these coils. Their comparatively rough interior and complex construction can lead to weaknesses and an increase in calcification especially as you could be storing water at a high temp, think Summer and your solar. That high temp water has to be reduced at the taps as Mark suggests and not all TMVs can cope with that and the blending process sort of equals out any perceived time lag on a PHx. Whatever you decide keep it simple and you won't be dissapointed.