Thermal store?

[jb236]

Hi all. I am a newbie self builder trying to specify a heating/ hot water system with woodburning boiler stove, solar thermal panels (Velux) to run UFH (90 sq m) and provide for 3 bed home , 2 showers and bath.
I have had conflicting advice regarding material type for the 300ltr thermal store with two reputable heating suppliers swearing that the best type is stainless steel for durability or copper for efficiency.?? HELP

 

[gasbanni]

I've fitted both go for the one that gives the longest warranty on the cylinder itself. to be honest not really had a problem with either.

Just found on some models the integral top up tank on top of the store loses heat through inadequate insulation.

 

[dcawkwell]

Forget the wood burning stove.

Messy and you'll soon get bored filling it with wood and lighting
it. Cleaning it out. Lighting it again.

Suggest air source heat pump. Solar thermal panels for hot water maybe
although I still think it is unlikely they would ever pay for themselves.

Go for lots and lots of insulation. Passive building I remember seeing on
grand designs required very little heat to keep warm.

You could consider air con. If you insulation enough there wouldn't be
any need for the underfloor heating at all.

 

[D_Hailsham]

reputable heating suppliers swearing that the best type is stainless steel for durability or copper for efficiency.

I would be interested to know how they define "efficiency" in this context.

 

[Hufty]

Forget the wood burning stove.

Messy and you'll soon get bored filling it with wood and lighting
it. Cleaning it out. Lighting it again.

Wood burning stoves are great, and if you're a bit of a skip monkey you'll never be short of stuff to burn. Best thing I ever did in my house.

Surely the efficiency of the thermal store has much more to do with how much insulation it's got round it rather than the material used to construct it?
Also correct installation of the pipes going into it makes a difference to heat loss.

 

[mysteryman]

Henry Ford said: "Chop your own wood, and it warms you twice."

 

[SimonH2]

Surely the efficiency of the thermal store has much more to do with how much insulation it's got round it rather than the material used to construct it?

If it's a thermal store rather than heat bank then the DHW is heated by a coil inside rather than an external PHE. Thus there is in fact a coil efficiency to consider - a poor efficiency coil will result in poor DHW flow rate and/or temp.

Similarly, the solar input will usually be via a coil so the solar circuit can be pressurised (collectors on roof above F&E tank) and dosed with antifreeze.

 

[ChrisR]

Stainless steel every time, "Duplex" is good becaue it's particularly strong and corrosion resistant.
The coil material doesn't matter, its transfer rate does ( kW). If you do the sums on conductivity of the metal in the coil you find it's negligible, it's the lack of tubulence in the water that's the limiting factor.

 

[simond]

The ability of a stainless steel primary coil to transfer heat into a store has been found to be less than copper.

When on the Mitsubishi Ecodan heat pump courses a few years back, it was explained that they had trialled all manner of stainless coils against copper, and at 45C the copper turned in significantly superior performance.

Copper of course, is more expensive, and less strong than SS. So they R&D'ed various options such as making the SS ridged, finned etc but each time the copper was demonstrably better.

Of course, in the example posted by the OP, the heatsources are all hotter than 45C, so the issue will be less marked.

There are advantages in Stainless, such as strength and in some circumstances, durability. It depends which question you ask.

 

[D_Hailsham]

The ability of a stainless steel primary coil to transfer heat into a store has been found to be less than copper.

Not surprising as the thermal conductivity of copper is 25 times greater than that of stainless steel; and this is one time when you want the heat to escape from inside the coil - unlike the pipes under your ground floor.

 

[simond]

Well, you did ask how you define copper as improving efficiency!

Whilst I understood the copper was a better heat conductor, it was surprisig that the design of the stainless heat coil could not be enhanced to match the performance - but apparently, copper was the only choice that would make the heat pump work efficiently.

Reading between the lines, I think this is why the Megaflo renewable compatible cylinder was such a long time coming, it required some new engineering solutions. The heat pump manufacturers were one step ahead.

 

[D_Hailsham]

Well, you did ask how you define copper as improving efficiency!

I was going to use this argument originally, but I'm not sure it's a valid one. It all depends on how you define "efficiency".

I was very surprised to find that the coils in many stainless steel HW cylinders are made of stainless steel; and the reheat times are comparable to those of a copper cylinder with a copper coil!

Many stainless steel saucepans have a copper base as the heat is transferred better into the water.

 

[SimonH2]

Of course, you can make a SS coil the same performance as a copper coil - you just have to make it bigger (or whatever) to compensate for the poorer heat conductivity. Of course, bigger = more expensive, and you are having to fit quite a bit of coil inside a confined space which means packaging gets progressively harder.

Not to mention of course, that SS is somewhat harder that copper, so it takes more effort to make a long tightly coiled bit of pipe.

 

[D_Hailsham]

Of course, you can make a SS coil the same performance as a copper coil - you just have to make it bigger (or whatever) to compensate for the poorer heat conductivity.

The more space taken up by the coil, the less space there is for usable water!

 

[simond]

Of course, you can make a SS coil the same performance as a copper coil - you just have to make it bigger (or whatever) to compensate for the poorer heat conductivity. Of course, bigger = more expensive, and you are having to fit quite a bit of coil inside a confined space which means packaging gets progressively harder.

I made those same points when I was debating the coil type with Mitsub... they saids that stainless was not going to work as well and they had tried all permutations.

As a result, their cylinder has a 2.1Bar PRV setting.