Hot Water Cylinder DIY Second Coil

I'd be inclined to make it the same length & tube diameter as the existing coil.

... alternative reply: Not Enough Information! You could go back to 1^st principles, but you'd need to know all about the temperature differential, flow rate &c. &c. (and know what sums to do).

Sums?!
Obiously any coil will work to some extent and the flow of the pump can be adjusted so that a sophisticated POP test can quantify the heat transfer coefficient (Paw On Pipe).
Assuming I go for the diy option, the basic dilema is do I go for 15mm pipe soldered into a rectangulat coil with umpteen elbows and an estimated length of about 8' or do I go for 10mm pipe which will have no joins but higher flow resistance and the potential to get squished during coil forming?
Sorry this is so technical
Del

Ah, but I can't see that until I've drained and opened the inspection panel. Just wondered if anyone had a rough idea. Also the prob is, I'd have to make a new coil sized so that it will fit on the inspection plate and slide in through the hole.
Blimey, you'll be wanting pictures next...it's so unfair.
Del

How much hassle would it be to A) measure the existing coil length by poking a bit of cable through it, and B) measure the volume by filling with water and draining into a measuring jug (and hence being able to calulate the diameter)?

Don't suppose you've got a spare NDT X-ray rig stuffed in a corner at the back of the garage? Are you sure? When did you last look? .

Ah, good plan, but...the existing coil is connectred to the central heating (CH) and I don't want to undo that.
That's one advantage of doing a new coil. I just blank off the old connections, add a new coil plus a header tank, fill it with inhibitor and antifreeze and I can run all year long with the solar completely separate from the CH.
The game plan is to do all thsi stuff and then next year fit a new CH boiler as the old one is pretty much on it's last legs.

So far it's one vote for the new homebrew 15 mm coil.
The next Q of course is how the heck do I connect it to/through the inspection plate.
That plate is another pro for keeping the old tank, it allows me to scoop the limescale out the tank. But, to balance that, the con is the heating coil on the old tank is prob full of crud (although I can flush it whan I fit the new boiler next year.
C'mon guys, do I have to make all these decisions on my own? Maybe we should have a meeting with tea and biscuits and an agenda and...nah...meetings never work, almost as bad as planning.
Del

You'll need the proper bulkhead fittings and enough slack in the coil to allow you to move the inspection plate out far enough to get to the coil fittings behind it.

Or, mount the coil so that the fittings protrude enough for service, but can be pushed back into the tank when the plate is attached.

Ha! Excellent, 'Bulkhead fittings' that led me to find just the right thing! I'd been looking at adaptors and tank connectors. Just found a nice bulkhead connector with compression fittings either side in 8 10 or 15 mm
Cheers, have a GA
Del

The coil is bulkheaded through the mounting plate, all connections are exterior, are they not? (at least that's how they are on all of the solar tanks we use)

A link to the original photos would help?

Obviously around here we generally approve of option two - the cost of screwing it up is usually still less than option one.

And since you built the first one, the odds you would buy the second seem slim, so clearly you are here just to be told to do what you want to do.

So this inefficient intermediate step is puzzling, I expected you would just post the plan.

Ah ,but yeah but no but. You forget how easy it is whilst sitting here to click on that seductive 'add to basket button'. No effort at all no pain, and in a few days a nice shiny new cylinder arrives. Ok it's not shiny 'cos it's covered in blue insulation.
Del

I know nothing at all about solar water heaters, thermal efficiencies of various sizes of copper tube or fabricating copper tubing heat exchangers, but I live in Arizona, so I consider myself eliminately qualified to give you advice.

It seems to me that the extra capacity of the larger tubing would compensate for all the 90° bends used to fabricate the "coil" and give you better heat transfer than the smaller diameter coil, and equal or better flow characteristics.

But, I can get all the hot water I need just by laying a 100 foot garden hose out in the sun. It was 114°F here yesterday, with little relief in sight.

I s'pose common cat sense would suggest I make the coil with either the same surface area or the same volume as the collector coils as a reasonable first approximation? whadda ya think eh? eh?
'Cos I can work out the approx area or vol of those coils.
My heat transfer should be better in the tank than in the collectors.
BTW here's the link to the original post about the solar panels.
Del

I wonder if you could use a 15mm tube bender to form soft copper into a helical coil. Say 200mm o.d. and 5 loops, which would give a bit less than 3m length. Just a guess without knowing what would fit through the tank opening. One advantage would be no internal fittings and no soldering/brazing.

Hello Dell, glad to hear all is well.

I have been pondering similar, but have alternate suggestion. (But I have plenty of space for it.)

I am looking at installing the solar unit (with its storage tank and all) in series before the standard domestic water heater so that pre-heated water is supplied into the domestic unit. I can set the thermostat on the solar at 85C (well above the domestic) and if ever the domestic gets too hot, the tempering valves on the outlet re-mix cold into the household lines anyway.

Please feel free to ignore this.

In your other proposal, I'd go for the cheapest of the two coils. Well lagged pipes will mean that any heat not transferred in first pass will go back to your collector and then exit at a higher temperature for the next cycle. Eventually the heat will get into your water heater.

Soft copper tubing also comes in 8mm reels. Very easy to form coils with.

I would be tempted to make a double (or triple) spiral, and solder the 4 (or 6) ends through a replacement plate, joining them to a manifold outside the tank. This would give enough flow, and the surface area for heat transfer.

Thanks for the ideas guys. I think I'll take close look at the ispaction panel and see what size aperture I've got. I'll post a pic.
The great advantage of the DIY coil is I can build it in my garage and then install it without disturbing all the other plumbing.
I just have a degree of trepidation about actually doing it. I remember wrestling with a coil of 10mm copper when doing my original panels, the supposedly good quality pipe bender didn't work very well and need some modification. I think modern pipe is getting thinner and thinner walled and harder and harder so bending isn't as easy as it was.
Del

You can see the tank is pretty old and chunky, the actual aperture behind the plate is about 4.5" diameter, it's funny 'cos I'd thought it was an oval, but it's circular, my old memory is playing tricks on me.
That's the cold water inlet, bottom right.

It's tempting isn't it? I mean it's nice and accessible, whereas the rest of the pipe work isn't, mind I'm not sure I'll have a lot of room to get the pipes out... (I can always cut a hole in the cupboard door)
Excuse the old eiderdown and duvet insulation.
Del

I am maybe a bit rusty with UK calorifiers, but to put a coil in needs you to cut a huge hole. Take the whole bottom off, damage any glued on insulation big time....

When I used to install heating in the UK (weekend hobby that paid REALLY well!), you could also buy a "coil" that could be installed in place of an electrical heating element, or using a new hole of the same size. It was the same shape as the heating element, long and slim, not wide and round.

They were placed so that:-

a) a present coil was not reached, check that point out carefully.

b) Short ones can be fitted in the side wall of the cylinder

c) Large ones from the top....

d) A cutter/kit can be bought to make the exact right sized hole...

A good plumbing supply should be able to help.....though my infos are probably years out of date.

I do believe it was possible to use a single coil from two sources of heat, though I have never ever seen one myself.....

I personally would buy a new tank.......because the "changeover" would take place in a few hours, instead of days weeks.....and you will still need to buy a new tank!!!

Shopping around should save a lot of money when planned in advance....

I'll check out a ready made coil. Woo hoo, just found one here, replaces an immersion element.
Did you see the pic of the cylinder #19 ?
I'm aiming to fit into that inspection plate.
Cheers Andy... a GA for you
Del

Ta !!

Try this:

Take your 10mm tube, fill with fine dry sand, and bend to fit inside of tank.The sand will come out if it is dry and fine,(Bootleggers used this trick to make moonshine coils, using a tree stump as a form.)Tapping and air pressure will assist in removing the sand.After cleaning the inside of tube, you are ready for step2:

Hint:When making sharp short bends, place the tube inside of a sturdy spring with a slightly larger ID than the tubing.This will prevent collapse.You can do long bends if you have the patience.Screen door spring works for some sizes.

Attach a cord to the top end of the tubing by any reliable means.

Step 3:

Thread the spiral copper coil into the tank thru the opening, while maintaining possession of the cord.The coil should end up more of less standing up inside of the tank.When it is all inside, pull the cord to bring the top end to within reach.Now you should have access to both ends.The pulling down of the top turn should not kink the tubing if you manipulate it as you pull.It should resemble a slinky that has fallen over at the end.

Good luck!

When faced with two choices, and you don't know which you want, flip a coin.

When the coin is in the air, you will have your answer.

Cheers, I use my own variation of the coin flip. When it lands, if you find your self saying 'best of three' then you have your answer.
I remember doing this to pluck up courage to ask out my first ever date... but it was worth all that teenage angst.

I like the dead slinky idea.
Del

Del,

Your existing coil is most likely able to handle the additional BTU input from the solar coolant. Your system, if it works now, will need an additional battery (ie more storage for the potable water if it is for DHW, or non potable if it is for a typical radiant system). The trickiest part of this question is to determine the "usefulness" of your battery, given your additional production capacity (I presume by upgrade, you mean to add more solar collectors). We find that the relationship between production, storage and heat consumption is the critical component in achieving capacity utilization, which will give you the most cost effective solution.

Think of it this way. If you need only domestic hot water, small volumes of high temp storage tend to work best, because storage is cheap, small volumes require fewer BTU's to raise to minimum DHW temp, and integral backup systems don't operate for long periods of time when solar is insufficient. We'd rather raise 30 gallons by 60 degrees than 60 gallons by 30 degrees.

If you are using the additional collectors to provide space heat, being able to store more of your gain may require that you add storage capacity. Again, finding the point at which you consume the greatest percentage of your production is more cost effective than finding the greatest percentage of your production that you can store. Usefulness trumps efficiency of the system, in this case.

BTW, an indirect copper heat x coil will usually (even the smallest) move 40KBTU per hour, This would be 5 good evacuated tube units or 8-10 flat plates.

I already have the system running with CH and solar running through the same coil (see the link to my original thread about the system #14), the fundamental problem is the sheer volume of water in the CH and solar combined makes it prohibitive cost to put antifreeze in and thus I can't run the solar over winter. Also the two systems can fight eachother unless carefully set up. The Separate solar coil solves both these problems.
In the Uk I probably have lower solar temeratures than you get over there.
It's just a DHW system.
Del

I see, then a coil it is! There are Vaughn coils that sound like they might bolt right in. I take it you already have controls, but you will need some reconfig, I'm guessing.

You might check out Solvelox, bu SolarHot. I've used a lot of them, and it's an external heat x with two counter flow pumps/ one solar (glycol) one potable, on a flat plat heat x. You would strap it on your tank, you would then have the ability to add production, too. You can build one yourself, but these are very nice, well engineered units. I'm in the business, and I buy the from Dan at Solar Hot, although we use immersed coils like you are suggesting most of the time. I like one pump systems. He's very well informed and knows as much as anyone should about solar thermal systems.

why can't you remove insulation from tank, coil 10mm around exterior of tank.(solder to tank for heat transfer)and re-insulate over new coil wrap? I've done this around a woodstove pipe to heat water by convection and it worked.(KISS method)

Yup, sounds like a good idea.
Not sure I'd get such good heat transfer and accessibility for soldering would be a prob. (I think my soldering wouldn't be up to it, I never seem to have enough heat for those big jobs).
If I had the tank out and no inspection plate I'd go for it, but that plate is just grining at me asking to have holes drilled and a coil fitted.
Del
(BTW isn't it about time someone gave this thread some stars?...)
... _{oooh what a tart I am}

Try a heat gun instead of a soldering iron for the big jobs. Only took 10 minutes to silver solder a radiator top hose tube on in freezing conditions. (Still needs space, though)

_{Your wish is my command - but can I borrow a cross-bow for an hour or two sometime in exchange? There's this little job I have to do} ___

Del, have a look for Solaplug on Google if you have an immersion heater boss on the tank. Retro fit coils using the heater boss.

Cheers, interesting, but they look a bit more expensive than the one I've found because they are for unvented tanks.
Del

The immersion heater element in many hot water cylinders is usually redundant, on the basis of the bought-in value of the 'juice' needed to run it being higher than using other methods of heating the water. A DIY way of using it would be to connect it to a wind generator or a set of solar panels, and to hell with the plumbing to a liquid-based collector. Just make sure that the output impedance of whatever is driving it is the same as the resistance of the element.

Might be another option chez les châts, peut-être?

and to hell with the plumbing to a liquid-based collector.

Only one problem - PV panels capture way less energy than a wet system.

OK I'll bite. Minimal requirement for electric KW to BTU would require watts to raise 40 gallons of water by 60F = 40 x 8.2 lbs = 340 lbs.

340 x 60 = 20,400 BTU's

20400/3412 = 5.97KWH

Average output for pv Virginia =4.5 (Full sun hours per day) x panel rating

5970 / 4.5 = 1326 watts of panels, or 6 225 watt panels.

assumes 100 % efficiency of all conductors (you should count on 20% at lower voltages)

Quite.

225W @ 14V gives an output impedance of <tap, tap, tap> 0.9Ω, while the element has a resistance of 3kW @ 230V... <tap, tap, tap> ....er, 18Ω.

Wire them in series!

I've pretty much committed to the DIY 10mm copper coil.
I'll post pics if it works!
Del

Please post 'em even if it doesn't work - then we'll know how not to do it !

Nice one! Shiney, too!

Excellent!!!! gold stars all around!

Del, that is very nice. Congrats! I would be happy if one of my guys put that out.

You could solder a straight rigid piece of 22mm copper to the inside of the coil, leave the ends open, for more surface area. You could also neatly wind some solid copper grounding wire (8 or 10 GA) between the coils. A touch of solder keeps them neat.

Smooth copper is a little quick. A variable speed pump controller could help too. What kind of collectors?

The Kitty provided a link on post #14

Thanks G. It's all coming back to me. Anti scald provisions are code required in US for solar, and because we use Evacuated tube collectors, even in winter, the water can high limit at 180F. High temps mean small storage. Of course, high temps mean lower collector efficiencies. Catch 22. BTW, all of the immersed heat x we use are corrugated or ribbed copper.(for her pleasure) Your system is very cool, Del.