Megaflow, oil and solar

[David Hansen]

On Sun, 17 Dec 2006 18:43:11 -0000 someone who may be "Suz" wrote this:-

It is a brand name for a type of "unvented" cylinder that stores hot water destined for taps in the shell. This water is under mains pressure. It is heated by a coil fed from the boiler circuit.

A "traditional" vented cylinder works in the same way, though the water in the shell is under gravity pressure.

A thermal store/heatbank does not store water destined for the taps. Rather that water is heated instantly as it passes through the heat exchanger. This water is normally under mains pressure, though it does not have to be. The water to heat the heat exchanger is in the shell of thermal store. It could be under mains pressure, but in domestic systems it is always under gravity pressure. Think of it as a hot water cylinder working backwards.

It takes a while to heat the thermal store, but when it is heated the boiler will go off until the store is depleted. When hot water for taps is drawn off it is heated instantly as it flows through the heat exchanger. This lowers the temperature of the store a little. Eventually the store temperature will be lowered enough for the boiler to come back on, it will then run efficiently for a long period as it charges up the store again.

One of the most useful features of a thermal store is that with the heating connected to it instant heat is available when one comes in. Flick a switch and the radiators will be filled with hot water in a minute or two. If one comes back an hour later one just turns the radiators on an hour later. This will start to deplete the store and the boiler will then come on to keep it up to temperature, but one doesn't have to wait for the boiler to heat the whole system up (or use a time clock to bring on the boiler on early).

[Doctor Drivel]

Hot water that you use. A heat bank has store water that stores heat that is transferred to incoming mains cold water and heats it instantly.

for understanding. Many companies make them.

[Doctor Drivel]

It isn't a religion. It is a matter of good engineering/economic analysis. Heat bank are hard to beat. I see few cases for an unvented cylinder. Combis are brilliant for the average house and many have high flow rates. High flow muti-point water heaters are superb too and beat unvented cylinders.

The FAQ put together by Ed Sirrett is 20 years out of date and bit of waste of time for modern solutions. Ed has limited experience.

[Doctor Drivel]

The boiler can be downsized. It only requires a boiler to heat average usage, not peak usage. Once the house is up to temperature the boiler then heat the thermal store faster than the heat is extracted from the store. This heat is stored and the boiler switches off. The house extracts this heat and when exhausted the boiler switches in. Overnight is stores heat, then the house is instantly heated in the morning.

[John Stumbles]

There's a general explanation of different types of systems here

[Doctor Drivel]

Recovery

"Conventional cylinders used with boilers and central heating systems are indirect types because the primary water must be kept free of oxygen and may be treated with corrosion inhibitor and other chemicals. However in thermal stores and heat banks the water in the cylinder heats DHW indirectly through a heat exchanger so the water in the store can be the primary water shared with the boiler etc. This allows the store to recover as fast as the boiler can generate heat. Also, by eliminating the temperature drop across the primary heat exchanger, the store can be kept at its desired temperature from a lower temperature of primary water. This results in better efficiency with high-efficiency condensing boilers. This arrangement does, however, require that the primary circuit be vented (since an unvented system would place the storage vessel under pressure, as in unvented DHW systems). In practice this means a small header tank is required above the highest point in the system i.e. above any radiators on the top floor. "

Has to be vented? Not quite. Pressurised heat banks and thermal stores are available. No needed for G3 when fitting one and no annual service needed. Some of the writing is very confusing.

[Doctor Drivel]

[John Stumbles]

Agreed, there are many more permutations and combinations I wanted to include (also e.g. thermal stores used for supplying central heating as well as DHW, also lists of & links to suppliers etc). What's on the wiki is as far as I'd got with it.

Really? I thought _any_ unvented store over (what is it?) 15 litres came under the regs.

I tried not to make it that way :-) If anyone can make it clearer please feel free to do so.

[John Stumbles]

Most of those were stolen from manufacturers' web sites and edited with the gimp. I've been playing with & trying to get up to speed with dia and another tool whose name escapes me. The openoffice version of visio is a crock'o'sh1t3 :-(

[Doctor Drivel]

Only if potable water is inside the cylinder. Primary water in the cylinder does not come under the G3 rule. The water stays inside the cylinder (and CH/boiler system) so no turning to steam (potential explosion) if over 100C when drawing off water, as can be the case with an unvented cylinder.

The Worcester-Bosch HighFlow has a pressurised thermal store inside and no G3 ticket needed and can be DIYed. A Powermax has an unvented cylinder inside and needs a G3 certificate to fit.

So, you can have a pressurised thermal store and only have one 3.5 bar pressure relief valve off it and that is it!!!!! Most makers recommend on having one on the boiler as well (which most boilers have anyway, even heating boilers), for belt and braces. Most insist on a boiler with a high temp cut off, which most have now anyway and must have if sealed and pressurised. You could have a normal cheap system boiler heat it directly switched off and on by the stores stat(s). It can be DIYed and no annual service. In short having the store between the boiler and rads is just having a big pipe and is treated as such and the same controls of any CH pressurised system. A bigger pressure vessel that is all, no high temp relief valve either.

You could buy your own Stainless "direct" cylinder with a high pressure rating (the Heatrea is 4 bar "working pressure", so well with the range of

3.4 bar of a relief valve, and DIY a heat bank with a boiler heating it directly and the rads off it directly; all yourself, using a 20 litre pressure vessel and a 3.5 bar relief valve off or very near the cylinder. Most makers will put in extra tappings and stat probe points in a stainless cylinder for you to DIY one. There are even some copper cylinders that can take 4 bar working pressures too. I would always have two relief valves and a high temp cylinder stat in case. So one relief on the cylinder, a high temp cylinder stat and a system boiler which has all it own controls. then a 20 litre vessel. If the boilers vessel packs up in time just cap that one off.

I wouldn't mention it in the FAQ that a pressurised heat bank can be DIYed as it may cause problems. DIYers playing with these is dangerous. But the dangers of an explosion is far less than a normal unvented cylinder as long a proper system boiler and two pressure relief valves are used with one being on the cylinder.

[Doctor Drivel]

What is the gimp?

Normal Visio is alright.

[Steve Firth]

You wouldn't want it, it's not written by Microsoft, so it works.

[Doctor Drivel]

I though they were questioning you? Did you get away?

[Steve Firth]

No you didn't, and you didn't think either.

I'd call you a moron, but I couldn't stand the bleating about how unfair I was being, from all the self-respecting morons.

[Doctor Drivel]

OK I knew. That picture of you on the web site - boy do you look like a plantpot.

[Suz]

between thermal stores and heat banks. This requires a cup of char.

[Steve Firth]

Oh bless look, Drivel needs to correct his typos when he quotes them.

[Doctor Drivel]

You are a plantpot.