ThermalStore/HeatBank - Internal coil vs. External heat exchanger

My ever-so delayed project to install a thermal store has been kicked off again by the current DIY thermal store thread...

It's all good stuff - but I'm wondering... It would seem that there is a choice between having an intenal high surface area coil to put cold water in one side and get hot water out the other, OR have a plate heat exchanger which you pump store water through one side, and plumb a cold feed in the other to get hot water output...

The first solution (internal coil) sould seem to save a whole lot of plumbing and expense, but do they really work?

Gordon

Reply to
Gordon Henderson
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I think that it's worth starting by looking at the objectives.

One is to be able to get a good flow rate of HW from the system.

The obvious and traditional way is to have a storage cylinder. You can get HW at whatever rate you like, if plumbing is of wide enough bore until the cylinder is out of HW or the loft tank is out of cold.

Another is to get more pressure than you can get from a roof tank, hence the attraction of mains supplied HW.

However, there is a basic principle that governs the instant heating of water by any means and that is how much energy can be transferred to it in a given time. The temperature of the mains is not generally under one's control by that much, so to achieve a certain temperature at the tap, the ways are to transfer enough heat to it quickly enough as it flows through the heat exchanger to maintain the flow wanted ot to reduce the flow rate if the exchanger is the limiting factor.

The sums, in terms of what is needed are easily done using the equation of

energy = mass x specific heat x temperature rise

You may have seen what a typical 25-30kW combi boiler can do and it's not that impressive. To make a heat bank interesting in comparison one would want to be able to double that otherwise it seems hardly worth the effort.

Certainly one can get plate heat exchangers that will transfer

100-200kW in a very small size and it's easy enough to pump enough water through it from the store.

If you want to use the idea of an exchanger inside the store, it is simpler, but I would check the specs very carefully

The other factor is that an external exchanger can be removed for descaling or replacement. There may be more plumbing, but servicability is improved as well.

Reply to
Andy Hall

I would be surprised if you can get enough heat transfer from the store to the mains water to make it effective using just the coil. Even a fast recover cylinder may not be able to transfer heat at more than 25kW, you would have to check the manufacturers specs. You really need something that can exchange in excess of 50kW to make for a significant improvement over the bigger combis.

Reply to
John Rumm

The objectives are two-fold: One is to capture heat from 3 sources: A conventioal back boiler, a range cooker with a small back boiler and (eventually) a solar panel. The existing (non condensing) back boiler will be replaced with a wood stove & boiler in the near future. My initial thoughts are to directly heat the tank from the boilers & have a pressuresed circuit & coil for the solar. (it will be retro fitted)

The 2nd is to provide hot water (obviously :) for shower, washing, etc. and the occasional bath.

It won't be mains going into the heat exchange system - mains pressure is high (8 bar!) but flow rate is really slow - < 10lpm caused by a crushed plastic pipe/stopcock burried under too much concrete & tarmac.

The plan is to fit a 2nd loft tank to feed the hot water supply, the existing one feeding just the cold - waters going via a dual impellor (1.8 bar) stuart turner pump. (We can occasionally drain the existing tank when filling a bath as it doesn't fill up fast enough)

This is what we have - but it's a pittifully small tank (100l) and barely fills the bath. (Yes, it's a big 2-person bubble bath sort of thing!) It's currently heated via one of those stupid top-loading coils from the back boiler which cycles all the time when trying to heat the water up. I can't fit a bgger tank in the current space, (and SWIMBO wants to recover that space - cupboard in the spare room) which is why I'm looking at a complete new system elsewhere in the house. (The range cooker goes into a traditional coil at the bottom, and I could swap them over, but it's a lot of work in a confined space and if I'm going to do that work, then I might as well do something new!)

Or pumped :)

Fortunately scale isn't an issue here. Quite the opposite - copper pipes are always shiny here... (It wories the local steam railway people...)

So I'll see if I can get the specs off the cylinders I'm looking at (Off the Navitron site right now)

Thanks,

Gordon

Reply to
Gordon Henderson

This becomes a different game. It's one thing to combine the heat from fuel burning sources able to produce water at 80 degrees. Incorporating solar requires some different engineering.

I looked at deriving heat for HW production from the AGA, but since the amount available is small and the fuel source is gas like the main boiler, there was no point. I don't know what you have, but that's something to consider.

Yes. I have the issue of a low flow rate as well and the pressure is not exciting either.

That's not a good way to do it. You run the risk of the cold running out before the hot and scalding in the shower.

A better way is to fit two tanks, join them with a length of 28mm tube just up from the bottom.

Put the float valve(s) onto one and the take offs (all of them) on the second; overflows on both. I used plural for valves because I was able to improve flow into the tank by having two valves fitted albeit run from the same supply to the loft.

They seem to have twin coil cylinders which is typically what is used when combining solar and fuel burning heat sources.

The important thing to remember is does the heat exchanger, be it a coil or external one match the heat transfer rate required. Coils on fast recovery cylinders are intended for transferring, without too much cycling, say 30kW or so from the boiler because that's what domestic boilers do. For transfering heat from the cylinder to the HW, especially if you are going to use a pump, you will need a lot more.

For example, a 31kW combi boiler will produce a flow rate of 12.7 litres/min with a temperature rise of 35 degrees. In the winter, the cold water can be 5 - 8 degrees, so you are just getting to shower temperature at that flow rate. Not very interesting.

Now add the pump and you will probably be delivering at least 20-25lpm at the hot tap, probably more. Ergo, you need to deliver at least 60kW to the heat exchanger heating the water. Since plate heat exchangers in sizes of 100kW or 200kW are not much more than the size of a couple of house bricks, it makes sense to go for a good sized one and not let the heat exchanger be the limiting factor.

I don't think that you will get that level of performance out of a coil

- the surface area isn't enough.

As to the point on complexity. if you are going to bring in heat from three sources to combine, you aren't really in the realms of simple plumbing any longer, are you.?

Personally, I think that an external plate exchanger is the obvious solution.

Reply to
Andy Hall

I have something similar, although it's a Stanley rather than an AGA. The back boiler in it does provide a measurable amount of heat to the current tank, so it's not heat I want to just throw away. The current back boiler is only 12 years old and while sevicable, after a winter of having a nice cozy log burner in the livingroom want to extend this to the dining room (where the back boiler is) Wood is free for the effort of collecting & chopping it (heats you 3 times, etc. ;-)

Solar is an added bonus (assuming I can get planning permission - as I have the tripple whammy of Grade II listing, Conservation area & Dartmoor national park, but I've had a nod that it won't be much of an issue in my case) I have a south facing roof and it'd be a shame to not get use of it - so solar in the summer (+ a bit from the cooker) to heat the tank, wood stove in the winter. (+ a bit more from the cooker & whatever the solar can give)

Well the shower is a proper thermostatic mixer which does cut off the hot when it can't regulate it, so that's less of an issue, however it's a good point.

Ah good. I plan to change the loft feed from a single 15mm pipe to 28mm too, so that will hopefully help.

They can fit what they call a thermal store coil - at the top of the tank which they use to heat water flowing through them. Comes with a mixing valve. I was planning on heating the tank directly from the cooker & boiler and using the bottom coil for solar as it will be a pressurised system. Everything else vented, and I want to optimise the number of separate F&E tanks in the loft, just for my own sanity if nothing else!

We used to have a coal-fired Rayburn and it would boil the water in the tank on-occasion...

Well yes. This is true... But if I can avoid a pump & flow switch then it'd be preferable...

I'll go & do some sums and see if I can get feedback about the thermal store coil in the tank.

Thanks,

Gordon

Reply to
Gordon Henderson

On 18 Mar 2007 09:00:27 GMT someone who may be snipped-for-privacy@drogon.net (Gordon Henderson) wrote this:-

Retro fitting is best avoided. Why not provide a coil for the solar panel and just leave it unused until you install the solar panel?

Given that the wood stove is more liable to overheating I would probably connect the cooker boiler to the shell of the store and the back boiler to a coil, in preparation for its replacement.

Heating can also be connected to the shell of the store and this can be used as a heat dump in the various circumstances where it is necessary to cool the store.

One thing to consider is a separate mains pressure coil for the shower, assuming the pitiful mains supply is good enough to supply a shower and still keep the storage topped up.

You might like to look at

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in particular the "Thermal Store" section. Like some other suppliers they will have one made up to your specifications in terms of location of connections and number/type of coil. Narrow and tall is best for stratification.

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some more sophisticated thermal stores.

Thermal stores with plate heat exchangers are generally only worthwhile in hard water areas. It is a lot easier to replace a furred up heat exchanger and this makes the added complexity worthwhile.

Reply to
David Hansen

If it's already there, then why not....

3?

- Chopping it?

- Burning it?

- Cleaning up after it?

It'll be interesting to see how that discussion goes at the council.

I used two Torbeck valves with floats set to different heights. Thus for small amounts of water, only one opens and is quiet. For larger amounts, both do and the rate is higher.

Only if it works.... You're already going to need a fair amount of paraphernalia to handle the solar stuff.

You could also ask what the construction is. If it's a coil of 22mm tube, unless it's very tightly packed, you won't have good surface area. Ordinary cylinder coils tend to be about 5-6 turns at the most. Given the tube diameter and the overall coil diameter, you can work out the surface area. Keep in mind that heat is transfered by the coil sitting in the tank of water. Heat arrives at it by convection.

With a plate heat exchanger, you have plates (no shit, Sherlock) arranged such that there is a chamber for primary and one for secondary alternately through the stack. The surface area is considerable. Added to this, both lots of water are being forced through it.

Here's a suggestion.

If you aren't sure, ask for two extra bosses to be fitted to the cylinder during manufacture. I did this for one that I bought and it added £10 to the total cost. That way, if the coil idea turns out not to work, you can implement the external heat exchanger.

Reply to
Andy Hall

Collecting, chopping, burning :) I'm not paying to have it delivered!

Well the current chief exec has pledged to make Devon the "greenest" county, so heres hoping ;-)

Good plan.

Thanks,

Gordon

Reply to
Gordon Henderson

Ah, I should have been clearer - I meant retro fitting the panel on the roof - it's not all going to happen at the same time. Solar might even have to wait until winter/next year. Tank will come with coil suitable for solar panel.

That's exactly where I'm looking at present ... Already been in-touch and he'd supply the extra connections I'm after.

500 litres starting size. Wonder how much they cost ;-)

Our water would appear to be very mildly acidic - probably something to do with Dartmoor...

Thanks,

Gordon (also from Edinburgh, way back!)

Reply to
Gordon Henderson

Another option to consider might be an accumulator. This buffers the incoming mains and can provide much higher flow rates (for a limited period).

Reply to
John Rumm

On 18 Mar 2007 15:53:53 GMT someone who may be snipped-for-privacy@drogon.net (Gordon Henderson) wrote this:-

Quite a lot I gather.

Reply to
David Hansen

Torrent RE Solar. See

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for more info. If you want I can get Paul Glehill to call you to discuss your exact needs. He is really switched on to renewables and individual needs.

Reply to
Stephen Dawson

I suspect it's probably me rather than David this is aimed at :)

Found what you're on about at:

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Gledhills own site is a bit thin on details & pricing that I could find!)

Prices are a shade on the expensive side, but if they're all pre-plumbed then they look OK. They also seem to have a bespoke cylinder build service, but I don't want to think how much they'd cost ...

As with everything, there is a bit of a budget, but we can be flexible :)

Cheers

Gordon

Reply to
Gordon Henderson

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>> outlines some more sophisticated thermal stores.

It seems like an overly complex/expensive solution to me. The problem with solar is that most/all of the year the flow temp will be quite low, and to get efficient heat transfer you need as high a temp difference as possible. The result of this is that to use a single cylinder you really need a tall, high volume cylinder to get a decent level of stratification, or some really complex baffle arrangement like that "Consolar" unit.

If I were to add solar heating (which is tempting) I think I'd add a second thermal store to my current Albion Mainsflow Direct. This would be a "low-grade" heat store, and pre-heat the mains cold water before it goes into the main thermal store. It seems a much simpler solution than trying to do it with one store. You could even dump other sources of low-grade heat into that store, e.g. from ground or air sourced heat pumps, or waste water heat recovery (depending whether it justifies the cost and effort).

An even simpler solution for those using a low pressure storage tank and a pump, instead of connecting direct to the mains, is to just put a coil into the bottom of the 50 gallon tank. The tank is lagged anyway, and I doubt the temp could ever get high enough to risk melting the plastic (an over temp safety device might be a good idea)? I think there might be a greater risk of Legionella or other bacterial growth though.

-Antony

Reply to
Antony Jones

On Sun, 18 Mar 2007 22:51:02 +0000 someone who may be Antony Jones wrote this:-

Pre-heating means another cylinder with heat losses, more space, more potential for high concentrations of legionella and so on.

A vacuum tube solar installation would be able to boil the water in the header tank. A flat panel solar installation would be able to bring the water to 60C.

They are a feature of systems which are likely to need it.

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is one of the available solar controllers and the "mounting instruction" link will take you to the manual which explains the various ways it will protect the store from overheating.

Reply to
David Hansen

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