Indirect Dual Coil Water heater tank Woodboiler with 1000 gallon storage

[WoodWacker]

I'm currently building a new home. I have a boiler room off of the back of the garage where 2 500 gallon storage tanks and a Tasso boiler will be located. I have an abundance of wood so wood burning efficiency is not primary focus here. I'm pressed for money this year and the old cast iron boiler will have to do for now. Wood boiler and storage system will be pressurized using a head tank above the tanks. My current plan: I'll have a condensing gas boiler to heat the water in the indirect tank, 1 coil in the indirect tank will be DHW. The other will be water fed from the wood boiler/storage water. I want max surface area on the wood boiler coil to utilize as much of the storage during non-fire times. That way I can draw storage tank temps down and make use of the wood boiler heated water. I've done a little research online and there seems to be a many brands Heat-flo, Bock, boilermate, bradfordwhite, etc.

Has anyone used twin coil indirect tanks in this configuration? Any downfalls? Can enough heat be pulled from wood boiler system to keep my deltaT between systems minimal? Should I use just a single coil tank for DHW on it's own zone and use a heat exchanger on the gas boiler loop? I also plan on running this during the summer to provide DHW. I've attached a rough sketch of how I think the system should look like, I'm missing a few things, I'll have to add a valve of some sort to prevent pulling cold water from my offline boiler. The expansion tank connection may be at storage tank top too. Any input on indirect options/heat exchangers will be greatly appreciated.

 

[DaveBP]

I would locate your expansion tank connection to the boiler just as you have it in the sketch. Erase that squiggled-out line.

I have my non-bladder expansion tank that way but located on the second floor in a closet. The boiler is in the basement. That additional static head leaves the pressure in the expansion tank about 8PSI lower than the pressure at the loading unit. That gives you more expansion capacity(volume) for the same size tank without raising the system pressure at the boiler any higher. Pretty common installation in Europe from what I've seen online.

I run my tank sealed (by closing off the vent ball valve) and have run it that way for one complete burning season. I may have to add some air next year but last week with the system cold, after the idle summer, the temp was 68F throughout and there was about an inch of water in the bottom of the expansion tank. Not much of a margin but the system won't be that cold again until next summer.

The pipe up to the 2nd floor is translucent oxygen-barrier PEX so if I can see the water level in it should it get really low from air absorption into the system water. I can add some more air. It's pretty cheap.

I do use system water treatment chemicals, though. Same regimen as if it were an open tank. Same outfit that supplies Garn with their chemicals. Even if it only adds a year or two to the system life, I think it's well worth it.

And if I'm just kidding myself... well, ignorance is bliss.

P.S. just to add some detail, my system total is 1100 gallons and the expansion tank is 80 gallon 304 stainless steel.

 

[velvetfoot]

I'm no expert and I have a pellet boiler, but I can talk about my tank. It is a 119 gal buffer tank for my system that has a stainless steel coil in it for dhw, so it's like a reverse indirect tank, with the dhw in the coil and the coolant in the tank. It works real well for me for dhw since I let the tank temp go down pretty low to maximize the boiler run time. The water is always hot enough and I have a mixing valve on it so it doesn't get too hot since the tank water goes up to 180. I'm not sure why you would need two coils-the coolant water could be the same in both boilers.

Here's a link to the one I have. Maybe you could get a better price if you call. Certainly not for one on a strict budget.
(broken link removed to http://woodboilers.com/boilers-heating-supplies/thermal-storage-tanks/indirect-hot-water-tanks/froling-energy-tank-119-gallon-indirect-water-heater.html)

EDIT: Ooops. It's an open system. Sorry.

 

[WoodWacker]

DaveBP,

Is your 80 gallon tank full when you're topped up with heat? I was under the impression to size expansion 5% of system volume. I like the idea of closing the vent as long as there's a PRV in place and a sufficient air gap in exp tank. Obviously the bigger air gap the more consistent compress-ablity you'll have over the cool/hot volumes. I have a single story home and my head tank will only be at boiler room height, about 2-3 feet from tank top, 6-8 feet above wood boiler, and about 14 feet above indirect heater located in basement.

Anyone running dual coil indirect solar tanks using wood boiler as solar loop?

WW

 

[DaveBP]

Is your 80 gallon tank full when you're topped up with heat?

No. It gets about half full or so. The highest I've run the system temperature is just 190F top to bottom though. I typically see temperature excursions from about 90F to 180F but I'm not too fussed about being consistent with it. Pressure at the expansion tank runs 4 or 5PSI minimum to mid teens at max temperature so I'm guessing it might get up to 2/3s full when the whole thing is really at its highest temperature. The expansion rate (Tarm 40K) is slow enough that the water is about ambient by the time it gets to the expansion tank so it never feels even warm to the touch upstairs.

There's a common confusion in expansion tank terminology between volume of the tank and the volume of actual water that the tank will accept under design conditions. Water expands roughly 4% from densest at 39F to least dense at 212F so the 5% expansion is a good number to use because it's easy to remember and puts a little safety margin in there. But we need to remember that is the 'acceptance' volume you'll need to size your tank to and a lot of folks talk about total volume of the tank which is typically double ( or more ). Then you need to add whatever safety margin you or your state code is comfortable with.

The big expansion tanks we use with the large volume systems we have are anything but cheap, but I've never heard anyone around this forum say they wish they had a smaller tank.