Using Spare Zone for Water Tank Heat Storage -- Will it work?

Hello. I'm new. This is a great forum and I've enjoyed following the discussions. Since I don't actually know anything about plumbing, I'd like to ask for comments/suggestions/advice on adding heat storage.

My situation is this: We live in a large, new house -- maybe 7000 feet -- radient in basement and first floor; baseboard in two floors above. We call this place "Plywood Mountain."

We have an oil burner rated 186000 btu/hour. 25 gallon capacity.

We also have a Memco M100 wood boiler (50 gallon) rated a nominal 120000 btu/hour. The hot side of the wood boiler goes into the return line to the oil burner then out the top of the oil burner back to the the wood boiler. The circulator in this loop runs continuously as long as the temperature in the wood boiler is at least 130 degrees. This time of year we set the oil burner aquastat so it doesn't come on till the temerature falls to 125 degrees. It cuts out at 145 degrees.

As long as we keep shoving wood in the boiler, we can "surpress" the oil burner. But it takes a lot of tending; hard to match wood boiler output to demand. Heat storage would help.

So I plan to build a 900+ gallon tank as per pbvermont's method (fiber reinforced plastic panels, foam board insulation, and pond liner.) My plan for getting heat in (and out of) the tank is to 'appropriate' an un-used zone in the basement. 3/4 inch supply and return lines for this zone run right next to the proposed tank location. It wouldn't take much to "T" into these lines, and divert the flow in this zone to a heat exchanger in the storage tank. Each line even already has a valve in it to cut off flow to radient loops. The pump for this zone would then move heat into (and out of) the storage tank instead of this half of the basement floor.

Normally a zone is for moving heat from the boiler to some end-use point. Is there a reason why one couldn't use a zone as a bi-directional way of moving surplus wood heat to a storage tank, and then under different conditions, running the same circulator to move heat out of the tank and back into the return line of the oil burner? This would save alot of plumbing and an additonal circulator.

Originally I planed to use a flat plate heat exchanger. For planning and cost-comparison purposes, can anyone suggest an appropriate size? Lately (like this morning) it occured to me that if I used a conventional coil in the tank, I wouldn't need an extra circulator on the tank side -- just the existing zone circulator. But the problem is the cost of copper. Does anyone know how much 3/4 inch tubing I would need to have an appropriate, effective heat exchanger? Would it be better to use 1/2 inch (cheaper; longer -- more surface area)? Could I use aluminum tubing, and just add some extra length?

The reason for doing things this way (stealing a zone etc) is to disrupt the existing system as little as possible, and save money on what could be a pretty costly project.

One final thing: how to control the flow of heat in and out of the tank -- basically when to turn the zone circulator on and when to turn it off. I hope to do this by PLC -- a controller called an Arduino (like a Pallalax Stamp, only it cost $36 including postage!). You program an Arduino with a (free) C type language via an USB cable. The idea would be to develop rules for various situations -- e.g. if the oil burner comes on, turn the tank cirulator off. Or, 'if the wood boiler circulator is running and the boiler temperature is greater than 145 and the tank temperature is less than the boiler temperature, then run the tank ciruclator.' The goal would be to manipulate the tank circulator so as to maintain, say, 140 degrees in the boiler.

On this subject, does anyone have experience using National Semiconductor LM34 fahrenheit temperature sensors or DS1620 digital thermometer/thermostat? Any other suggestions about how to measure temperature from outside the pipe?

As you can see, this controller thing is a regular science project for me. But the payoff would be two fold: first, we wouldn't have to get up in the middle of the night to feed the beast. And second, we could burn short, hot fires of Spruce-Fir-Ceder junk wood and save on expensive firewood. If the tank held heat well, we might be able to keep it hot all summer, and thus surpress the oil burner completely. (We use a BoilerMate for hot water.)

So to summerize:
Is the 'Zone-Appropriation' idea ok?
What would be a 'right-size' flat plate heat exchanger?
Or, alternatively, about how much copper tubing would I need?
Could I use aluminum tubing?
Any other 'alternative' heat exchanger ideas -- like maybe submerging a few sections of baseboard heating element?
PLC enthusiasts: check out the Arduino controller. Lots of fun!

Thanks
Chris

Welcome to the forum and to the boiler room.

It sounds like you're aiming to put together a system quite similar to what I've done. I've written mine up on my site - link in my signature below.

I use the bidirectional zone for heat storage - works great.

I'd suggest putting the boilers in parallel. No sense heating the oil boiler if you don't need to. If you do that, each boiler will need its own circ with check valve. Again, I've got plumbing and electrical schematics on my site.

Disclaimer - I've documented my approach. Works for me, though there are things I'd do differently if I started from scratch. There are many other approaches, and there's no single approach that will meet all needs (and codes).

Hi Chris. Sounds like you're up to a project like this. Should be fun. I'd recommend nofossil's website and system design as a great place to start, then tailor your design to your particular needs.

The heat exchanger thing is a real dilemma for most of us, mostly due to the high cost of copper, as you suggest. Everybody seems to have a different idea, and most seem to work pretty well. We have many threads discussing both the theory and practical application of different hx strategies. My current favorite (though as far as I know, untested) approach would be to use a couple of cast iron radiators as a water-to-water heat exchanger. Getting a ci rad into a tank and keeping it from corroding would be a challenge, but it's a pretty cheap ($100 per rad, more or less) solution to an otherwise-expensive obstacle in your system design.

Just a thought. I built a monster two-part hx out of rigid copper pipe, but haven't hooked it up yet or filled my tank. As soon as the weather cooled off and I learned a bit more about operating my boiler, I really don't have much need for a tank, although I think it will be really useful as spring returns and for domestic hot water in the summer.

Anyway, welcome to the Boiler Room. I think you came to the right place.