Solar Hot Water heating

[RogerM]

Roger,

I'm slightly confused by the logic in your direct system, Am I right in assuming that you are intending running the hot water supply via the solar collector? If that is the case surely you will only get 8-10 litres of hot water at a time.

Not at all Les. The concept of a direct system is not so very different from an indirect system that uses a 2nd coil as a heat exchanger. However, you are heating the water that you will actually use directly. Cool water is pumped from the bottom of the tank and returned hot to the top of the tank, which is where you need it and where it will stay until drawn off by the user. The pump will continue to draw water from the bottom of the tank and return it to the top hot until the whole tank is full of hot water. In contrast, as I understand it, when you use a heat exchanger you are heating the water at the bottom of the tank which then rises to the top, with a certain degree of mixing on the way, so initially I would expect it to be cooler than water from a direct system.

Also from the research I've done one of the problems is preventing the water freezing in the solar collector in winter.

This is prevented in both the direct and indirect systems by starting the pump again if the outside temperature falls below zero (admittedly taking a little heat from your store of hot water). Also as the heat exchanger on the roof is well lagged freezing is only a problem in extreme conditions which we usually avoid in S.Devon.

I've spent the last 6 months looking at these things, for me the current front runner is:
http://www.energycabin.com
which comes in at €25,000 installed (I will get a grant of 6000 off this) and a payback of 6 years approximatly (assuming all sorts of things obviously)

Looks interesting but apart from the cost it looks like a really bulky system and wouldn't fit in a sunny position on our plot. Great for a big plot with the right aspect tho'

 

[ike]

I might be missing something but to me the cost advantage of a direct system is far outweighed by the danger posed to t by frost. So although you can run the pump to stop any possibility of the collector manifold freezing, in fact you then effectively have a heat pump depleting your heat store overnight!
:?

Or have I got it completely round my neck??.

Ike

 

[RogerM]

I might be missing something but to me the cost advantage of a direct system is far outweighed by the danger posed to t by frost. So although you can run the pump to stop any possibility of the collector manifold freezing, in fact you then effectively have a heat pump depleting your heat store overnight!
:?

Or have I got it completely round my neck??.

Ike

I don't think there is a significant difference in risk between the direct and indirect system. In each case you have the same collector and heat exchanger on the roof with connecting pipework, full of water which shouldn't be allowed to freeze. The indirect system will probably contain some form of antifreeze, but I think it has to be something pretty innocuous just in case there is leakage between the direct and indirect systems inside the hot water tank. This actually happened to me in a previous property - the CH header tank was higher than the domestic hot water header tank which started to overflow. I turned off the supply at the main, but it continued to overflow - I thought I had found a system that was making water out of nothing! :?: :?: :idea:

I admit that in extremely cold conditions with the direct system it might be necessary to briefly run the pump to ensure that it doesn't freeze in the heat exchanger on the roof, but a minute or so very occasionally ought to be enough I would think. Hey - this is Devon! We only get a frost 3 or 4 times each winter!

 

[Anonymous]

Roger

I loved that book and told my brother-in-law about it (he is just completing building his house). He bought the book and is currently installing solar water heating. I'll let you know how he gets on

 

[Les Mahon]

Roger,

Now it makes sense. Interesting, you have explained it far better than any of the documentation I had read on it!.

The indirect system will probably contain some form of antifreeze, but I think it has to be something pretty innocuous

The antifreeze used is marked as "food grade", though why anyone would want to eat anti-freeze is beyond me :shock:

it looks like a really bulky system

That's sort of true, but any system bassed on wood pellets for heating will require a large storage capacity anyway. In fact the storage in the energy cabin is only 1.5 tonnes which equates to 750 litres of oil. The advantage for me is that the boiler house / wood pellet storage also houses the solar panels, and the fact that it is all pre-commisioned means less labour costs on site. Alos, I am lucky enough to have a large enough plot with an absolutly clear southerly view.

Les

 

[Jake]

The indirect system will probably contain some form of antifreeze, but I think it has to be something pretty innocuous just in case there is leakage between the direct and indirect systems inside the hot water tank.

It is interesting that all the diagrams seem to show rather old-fashioned thermal stores with internal coil exchangers rather than the more modern (and much more efficient) type with external pumped heat exchangers.

With the latter, there is virtually nil chance of interchange between primary and secondary circuits, and the manufacturers recommend dosing the water with fernox-style anti-corrosion stuff which is pretty noxious stuff. I.e. you could add any anti-freeze you liked.