HX needed?

[chuck172]

If I get a pressurized tank, like any of the tanks we have recently discussed, Would I even need a heat exchanger?
I'm piping my TarmSolo40 parallel with my boiler, I have a super-stor HW tank with it's own coil on its own heating zone.

 

[MrEd]

Assuming you are not using antifreeze in the system, I don't see why you would need a heat exchanger with a pressurized tank and a standard setup.

 

[Nofossil]

Short answer is that you don't, but......

You may want one of some sort to preheat water before it goes into your SuperStor.

If you add solar panels, that might drive you to add another.

I'm using an open tank with three coils in it. I think my ultimate system would be a pair of 500 gallon pressurized propane tanks with external sidearm-style convection heat exchangers for DHW preheat and solar system input.

 

[chuck172]

How big of an expansion tank will I need if I go with a pressurized storage tank. Is there a way to size them. Lets say 750 gal. or 1000 gallon.
If I were to go with a pressurized storage tank like lets say the one ecoheatonline offers as discussed in previous topics, get my larger expansion tank, I'm done. No HX. Simple piping too. I presume just in the bottom out the top, piped between the Tarm and the fossil fuel boiler.
What are the downfalls. Seems much more simple than the expensive, collapsable STSS indoor swimming pools or plate HX's with pumps or hundreds of feet of semi-precious copper coils.

 

[Nofossil]

How big of an expansion tank will I need if I go with a pressurized storage tank. Is there a way to size them. Lets say 750 gal. or 1000 gallon.
If I were to go with a pressurized storage tank like lets say the one ecoheatonline offers as discussed in previous topics, get my larger expansion tank, I'm done. No HX. Simple piping too. I presume just in the bottom out the top, piped between the Tarm and the fossil fuel boiler.
What are the downfalls. Seems much more simple than the expensive, collapsable STSS indoor swimming pools or plate HX's with pumps or hundreds of feet of semi-precious copper coils.

There are some discussion threads including a recent one that discuss expansion sizing in detail. Pressurized is simpler and allows you to eliminate a heat exchanger. However, it's not quite as simple as 'in to bottom, out the top'. The amount of usable heat that you can get out of a tank depends in part on how well you can maintain stratification. If you can keep the hot water at the top of the tank separate from the cooler water at the bottom, you can get much more heat out than if the tank is mixed to a uniform temperature. One of the advantages of multiple tanks is to prevent thermal mixing.

When charging the tank from the boiler, you want to draw cold water from the tank bottom and return hot water to the tank top.

When you're using the tank to heat the house, you want to be drawing hot water out of the top and returning cool water to the bottom. At the very least, you'll need an additional pump for running off of stored hot water. There are plenty of schematics floating around that show different approaches to this, again including some recent threads.

 

[chuck172]

So in a sense, I would be using two individually pumped systems. One going from the wood boiler pumped to the bottom of the storage tank returning to the wood boiler through the top. Controlled by a aquastate in the storage tank.
The second pumping is from the storage tank to the fossil fuel boiler, controlled by the oil boiler aquastat, where it heats my house through the heating zones, and my DHW through the super-stor tank. Then returning to the storage tank.
In other words, the storage tank is in the middle of the loop, feeds the oil boiler, being fed from the wood boiler, using two pumps and no HX's???
With this pressurized system I would of course have to add a much larger expansion tank to the system.
In this scenario the wood boilers only function would be to heat the water in the storage tank. Where it can hopefully run full throttle, with limited idle, to heat the 1000 or so gallons.
I hope I have this right, if I do It will be a mental breakthrough for me.

 

[chuck172]

Is my pressurized storage system concept correct?

 

[WoodNotOil]

Chuck - When charging the tank you want the wood output going into the top of the tank and the return water coming from the bottom. When drawing it is the opposite. Tarm has you put an aquastat in a well on the heat output pipe which senses when the tarm no longer has sufficient temperature to charge a boiler or tank. This aquastat would probably be the one you want to use to control the charging pump.

Likewise you could put one in the output pipe of the fossil boiler or use a surface mounted one if you don't want to put in a well. This would know when the fossil boiler was up to temp. You will need the fossil boilers own aquastat to control it as the backup.

Edit: Chuck - I am assuming you are using a series hookup from your description above. You are right that you need no hx and two pumps. This is true for both a parallel or series hookup. The only thing that changes is the piping configuration between the two.

On page 20 of the tarm manual you can see piping concepts. They call the series a "parallel" layout which causes some confusion. There you can see the position of the Honeywell L4006 B aquastat I am referring to above. What people on the forum refer to as parallel seems closer to the primary/secondary layout in the manual. Those layouts are also available at: http://www.woodboilers.com/piping-layouts.asp

I hope that helps

 

[chuck172]

nofossil and woodnotoil, your explanations help me understand storage much better, thank you. It seems to me that pressurized is the way to go. Much simpler piping, no HX's. As MrEd and I have come to know , Tarm may very well be coming out with their own pressurized storage tank system. Reading from previous posts, other wood boiler dealers are starting to offer the tanks also.

 

[jebatty]

The L4006B aquastat is not for controlling output to tank, it is an overheat aquastat, normally open, and closes on temp rise to 210. It's purpose is to turn on the pump(s) to a dump zone to get rid of overheat.

I installed a pipe surface mount aquastat on the return line to my Tarm which I set at 150 (fully adjustable). I also have the Termovar to insure return temp stays up for optimum boiler operation. When the Tarm is fired, the boiler circ pump is not running until boiler temp rises and the Termovar controlled return hits 150. Then the aquastat closes and the pump turns on, circulating hot water to storage. In full firing mode the return temp, Termovar controlled, will not again drop below 150 until the fire is mostly burned out. The differential is about 10, so at 140 or so the aquastat opens and the pump shuts off.

Since it is possible (likely) storage will be above 140, the return aquastat may not shut off the pump even though the fire is out. To prevent this I also put a timer on the boiler/pump electric supply which I normally set at 6 hours, which is about the maximum load burn time, to shut the system down when the fire is out.

 

[WoodNotOil]

Jebatty- The overheat aquastat is the L4008B which is located in the top left corner of the right side of the tarm boiler. I have one of those as well, that is not the one I am referring to. In the "parallel" (which is really series) concept layout on page 20 of tarm manual, you will see the the aquastat I am refering to. It is not used in a tarm only installation, so you may not have one in your system. This controls pump C-3 in the layout on the return line of the tarm. In the "parallel" hook up it controls the flow between the two boilers and in the case of the piping for my tank it will control charging the tank instead. The nice thing about it's position is that it knows when the fire has gone out and turns the circ off, no timer is needed. I am going to use a surface mounted aquastat on my oil boiler on it's output pipe to control drawing from the tank. Further details on my current setup and planned tank setup can be seen at my site in my signature.

 

[jebatty]

You're correct on the L4006B and L4008B, my error. Thanks for noticing. But I think my comment on the timer still applies, let me know if you think it doesn't. The L4006 is close on rise, so it turns the pump on when boiler output hits the set point. If storage temp is above the set point, then L4006B will stay closed and C3 will continue to run, even if the fire is out. Am I missing something?

 

[WoodNotOil]

I just looked at mine and it is the L6006C not a L4006B which acounts for our confusion. The L6006C is a R-B opens, R-W closes and it operates like a regular thermostat with a swing that you can set. In ther words it turns itself on and off without the need for a timer. It works really well. I always assumed it was the 4006 because that is in the diagram, but the contractor that worked with me must have chosen this one instead because of the series install. He has been doing setups like this a long time and has some tricks of his own. I will try it with the storage and hope it works alright.

 

[jebatty]

I don't think that the L6006C will eliminate the need for a timer in my situation. If the boiler is feeding storage, and return from storage is within or higher than the set point differential, and fire has burned out, the R to W contact will remain closed (circ "on") until water temp drops, which could be a very long time. This is where the timer comes in to shut everything down after a sufficient period to insure the wood has burned out. My install may be different than most, as I use an LP tank for storage, and use the tank itself as a large radiator to heat my shop. I have no traditional zones.

From Honeywell:

"Upon a drop in boiler water temperature (to dial setting, less differential), makes R to B burner contact; breaks R to W contact, preventing circulator operation. Upon a rise in boiler water temperature (to dial setting), breaks R to B burner contact, makes R to W circulator contact."