plumbing two water heaters

[juddspaintballs]

I'm in a bit of a hurry to get this done since I've got a guy getting parts for me for free...for now.

I've currently got a 50 gallon electric water heater hooked up. It's 12 years old and I'm on well water. I'm waiting for it to die.

I have a spare 40 gallon electric water heater sitting in the packaging in the corner. I'm thinking about hooking up both water heaters, but I'm not going to run 220v to power another water heater. I have a sidearm heat exchanger that I have not installed yet.

My thoughts are to wire the new 40 gallon water heater to the 220 electric so that it's installed 100% and working when I'm not running my boiler. I'm also thinking that I'll hook the old 50 gallon water heater up to the domestic water prior to feeding the new 40 gallon water heater, but not hooking up the electric to it. As such, it would practically be a holding tank for the incoming water. I'll add a valving in such a manner that I could bypass that water heater completely if it springs a leak. Then, I'll hook up the sidearm exchanger to the old 50 gallon water heater. When my boiler is running, that will make hot water in the "holding tank" prior to it entering the new 40 gallon water heater.

Let me see if I can draw this up...

 

[juddspaintballs]

yellow things are valves

 

[rkusek]

I'm far from an expert on DHW setups, but I think the diagram above will still use a significant amount of electricity heating the 40gal tank. The key from what I've gathered here is to superheat your tank using your sidearm boiler water and have a mixing valve on the output to the home getting it down to the safer temps. My new electric 80 gal heater looks like it can be set as high as 150 degrees so I would think one could use the sidearm setup to heat it to at least that temperature. The mixing valve has 3 connections the 150+ hot water, the cold water input, and the approx 125* water output to your DHW. NoFossil discusses these mixing valves on his site which you can read up on if you haven't already done that. He is actually using 2 of them in his design but that would not be necessary for most people. The beauty of the overheating is you should only use electricity when your temps drop below the 120-125* setting on your new heater. This would cover the cases where the boiler is cold or you have exhausted all the hot water and are re-heating, both would be acceptable times to use electricity in my opinion. Others here say they open the breaker during the winter avoiding electricity altogether. I would think that would lead to some unexpected cold showers every once in a while though.

I think you could basically hook both of your tanks in parallel by connecting the bottom taps of both heaters together. This would give you 90 gal of superheated water which I would think would amount to quite a bit of 125* water for use. This should make your sidearm fairly efficient as far as keeping up with the demand. I think I would only have the electricity hooked to the newer one. If you don't plan to heat with wood year round, then it might be better to eliminate the old tank completely for efficiency reasons. I never seen it done but always wondered why nobody ever puts a small circ on the potable water side of the sidearm to help recover the hot water faster. Most people that find sidearms using thermosiphoning can't meet their demands usually opt for a more elaborate flat plate exchanger. I'm going to wait another year before I try to heat my DHW with wood.

 

[bigburner]

I use a pump, 200 gal tank with a u tube HX boiler water then to the gas water heater which also a HX that does floor heat. The cold water goes in the 200 gal tank and sets there, when water is used it moves to the gas heater, but there is an Aqua sat on the wood boiler line that tells the pump if the line is hot and if so then the pump comes on and moves the hottest water form the 200 gal tank to gas heater. This isn't as simple as it sounds, keeping my self in wood heated DHW has been the hardest thing to get right and I thought it would be the easiest.

 

[juddspaintballs]

huskers, you made 2 very good points in your post. The first one is to run the heaters in parallel to get 90 gallons of really hot water (superheated describes water above 212 degrees but is not yet boiling) or to just eliminate the old heater all together. The second was to use a small circulator to move the water through the side arm exchanger for a faster recovery rate. The reason I chose side arm vs. flat plate is because I have hard well water although I knew I could have on-demand water with the flat plate. I may be able to get a small circulator to move the water. If I got the small circulator, then I would eliminate the old tank all together. That would leave me with only 40 gallons of hot water in the summer when I'm not running my boiler, but that should be fine for us for a few more years. If it's not, I'll upgrade to an 80 gallon water heater down the road.

So forget the fancy plumbing. I'll add a circulator between the bottom of the side arm and the bottom tap of the water heater. I was planning on using a mixing valve before the water made it to my house lines. I'll probably set mine at 135 or so.