DIY unvented water 'cylinder'

[Andy Hall]

Is that Shift F3?

[Ed Sirett]

According to David Hansen is OK. But unvented.

Well we are talking simply about a more modest mass of water to hold the energy.

You must follow the manufacturers instructions. If they say that you may use preheated water to supply a combi then you may. If they say you should connect it to a cold supply you may not. If they are unclear then you should contact them.

[dennis]

Are you insane? All this assertion cr@p. The static temperature it gets to has nothing at al to do with how much heat you need to remove. If you are going to try and argue can you at least try and be relevant.

What assertion? Do you know of a system that works like that without a variable flow, even when the clouds blow about? If they do as you say they already have variable flow and will work perfectly well without a "solar" coil.

[PCPaul]

Probably. I expect we'll see F4 next, 'Proof by intimidation'.

Alt-F4 gives a funnier one, though.

[The Natural Philosopher]

I wouldn't worry. in this country the solar pnaels wont get it much above freezing when you need the boiler anyway ;-)

[David Hansen]

On Wed, 16 Jul 2008 19:01:23 +0100 someone who may be "dennis@home" wrote this:-

Excellent, more personal attacks.

I will try once more.

The point is that reducing flow rate may mean that heat is not removed quickly enough from the collector. The stagnation temperature tubes can reach indicates that there is a lot of heat to be removed. Vacuum tubes are a very different proposition to an old radiator painted black, which is far more forgiving in terms of the system which needs to be attached to it. With vacuum tubes consideration needs to be given to what happens when the water goes above 100C and, eventually, a pocket of steam is formed in the system.

The one you made.

Yes.

You are confusing good design with having a control system which masks, to some extent, bad design.

[David Hansen]

On Wed, 16 Jul 2008 19:24:55 +0100 someone who may be The Natural Philosopher wrote this:-

A common fallacy. Solar panels work well in low temperatures, what matters is the amount of sunshine, not the external temperature.

In the UK the sunshine isn't brilliant in winter and is of short duration. How much a given panel raises the temperature of the storage depends on the volume of water. More panels will increase the temperature a given volume of water will reach, at the expense of having to do something from the excess heat generated in summer. That is why the recommendation for most people is simply to use solar on the hot water. If one wants to do more then it becomes more complicated.

[dennis]

I will try again.. its irrelevant.

Solar coils do not need to be longer than boiler coils! Anyone saying so doesn't understand heat transfer. Just look at 30 kW of heat going into the tank from the boiler and compare it to the few hundred watts from a large solar panel. If you think solar coils are expensive because they are long you would believe anything.

No it does not! Ask a firewalker how you can walk on hot coals, they are hot but they don't have any thermal mass.

Vacuum tubes have a lower thermal mass than an old radiator.

That's why you pressurise them. More interesting is what you do with the extra heat when the solar tank is hot, not everyone has a heat store to dump excess heat to.

You are making the assertions here! You don't appear to know what you are talking about either!

Go on then describe how it works?

Lets hear what you think is good design then?

[John Rumm]

The temperature of something is not an indication of its heat capacity. It would be equally possible for the tube to reach a high temperature and yet have little heat to be removed due to a low heat capacity.

If you have a flat panel collecting at a rate of 500W, a flow rate of as little as one lpm of water is only going to receive a temperature rise of about 7 degrees.

[David Hansen]

On Thu, 17 Jul 2008 12:21:22 +0100 someone who may be "dennis@home" wrote this:-

Nice try. However, I have not said that they do. What I have said is that there are advantages in doing so.

Do you always distort what others have said?

Two points:

1) I never said what you claimed

2) I do understand heat transfer reasonably well, it comes from studying it at university and then spending a decade of my working life on systems which transfer a fair bit of heat. However, I'm not one for mine's bigger than yours discussions so will not take that any further.

I note that the point I made about the different sorts of primary in calorifiers heated by various means remains unchallenged.

My observation is that people who use the word tank to describe a calorifier (called a hot water cylinder on a small scale or a shell and tube heat exchanger in more formal language) are at best somewhat out of date. The old iron hot water tanks must have mostly faded from the scene in the early 1960s at the latest.

I have no need to "look at" it, I understand it already. Even correcting your mistake about the output of a large solar panel, which may be several kW, both involve very different flow rates and amounts of heat to transfer, even if the temperatures are sometimes similar.

One of the reasons solar coils are more expensive is indeed that they are much longer, another factor is that they are much more complicated, due to the fins. There is no comparison between one and the typical coil of pipe.

Should you wish to, you may have the last word.

[David Hansen]

On Thu, 17 Jul 2008 12:35:34 +0100 someone who may be John Rumm wrote this:-

I was simplifying things, perhaps too much.

[dennis]

There is simplifying things and getting it wrong, you did the latter.

[dennis]

Do you really want me to challenge it? I suggest you stop digging.

This is a DIY group, what do you think most people think they are? Tanks, cylinders maybe, calorifier is not going to be meaningful here. Are you one of those "experts" that is going to resort to jargon in an attempt to hide your error?

You really should stop digging.

Not that any of the extra length or fins are needed on domestic systems. Which is where this argument started.

The real reason they cost a lot more is marketing and lack of market and nothing to do with special coils.

Fine. It sounds like you have stopped digging at last.