Heat exchanger question

[nhjohn]

I know this topic has been discussed, especially by Sparke, but I don't there has been a concensus (at least not from my perspective)

I am getting my Tarm 60 next week. I'm in the process of finalizing my tank design and was hoping to ask you a few questions. I am thinking about going with pex or copper over the flat plate for several reasons. It is one less pump on both sides. Less chance of something going wrong (Hx failure or pump failure). I don't have to install water tight holes in the side of the tank. Also, after adding the price of the electric for the pump and some other components to run all the pumps, the flat plate HX adds up quickly.
Obviously, going copper is very expensive and I don't want to spend the money on soft copper or going to the hassle of making a hard copper HX.

So that leaves me with pex. I'm thinking of doing 2 separate HX's - one for the heat input (on the bottom of the tank) and one for the output (top of tank). The Tarm 60 is rated at 198K BTU's. I am thinking of using about 1000-1500 1/2 inch pex at 200 foot lengths each. Sound right?

For the output, I'm wondering if I can get away with less. I need to extract 80K BTU. Since I have radiant heat floor I only plan on having a 10 temp drop. My concern is when the return temp is 90 and I need to raise the water supply water to 100 and the water in my tank is only 105. I suppose the answer is always that I could add more pex. But am I better off with more shorter runs, or fewer longer runs. It appears to me that the longer the water runs through the PEX while in the tank, the greater the chance it will rise to the desired temperature.

Sparke - I know you have had success with Pex, but also indicated you'd go with the FP HX if you were to do it again. Is it only because of the labor in putting them together? What is your boiler rated at? Do you wish you used more pex?

Any thoughts or help is greatly appreciated.

 

[free75degrees]

I don't have to install water tight holes in the side of the tank.

I think you can get away with making the same types of holes with a flat plate as with coils. With flat plate you can get the input and output at various height in the tank by bringing the tubes into the top, above water line, and sticking them down to the appropriate height in the tank.

Do any other people do that, or do people make water tight holes?

 

[WoodNotOil]

Here are some very rough numbers on just the tank side cost of the three options you mentioned above:

Flate Plate
30 flat plate hx - $200
2 - Grundfos 15-58 - $160
2 - Zone Valves - $160
Rough Total $520

Copper Coils (2 coils@180'=360')
3- 3/4"x100' @ $355 = $1065
1- 3/4"x60 @ $215 = $215
Rough Total $1280

Pex Coils 3/4"(3x as much needed = 1080') 1/2"(maybe 1500')
1- 1/2"x1000'= $385
1- 1/2"x500' = $190
Rough Total $575

I like the flat plate option because no coils have to be built. I think sparke said it took him something like 8 hours per coil to build. Not sure how expensive it will be to run the extra pump for the life of the system.

I plan to bring all pipes in through the top avoiding the need for holes in the tank sides.

 

[nhjohn]

My understanding is that the flat plate HX must be below the high water mark in your tank (if nonpressurized). I think if you place the holes in the top of the tank the water will fall out of the supply and return pipe into the tank. You might be able to rig a pipe inside the tank to prevent this.

WoodnotOil - I think your numbers are a little low on the flate plate HX and high on the pex - at least for a Solo 60. I think I'm looking at spending at least 300 for the HX. As far as pex, you can get much better deals on supplypex.com. I talked to Watts and they said to go with a nobarrier because the oxygen barrier won't do any good sitting in the water. ALso, it will eventually wear off and fall into the tank as debris. The cost of running the two additional pumps could easily be 20-30 per year. That adds up.

I saw Sparke's coils and they are indeed very impressive. My plan is a little less sophisticated. I plan to build a base, put it in the tank and wrap the pex around the base. The idea being that all of the supply HX is at the bottom of tank and the demand is at the top. The two will be separate. It will cost a bit more, but make for a much simpler installation. It will also help the stratification of the tank quite a bit.

 

[WoodNotOil]

My understanding is that the flat plate HX must be below the high water mark in your tank (if nonpressurized). I think if you place the holes in the top of the tank the water will fall out of the supply and return pipe into the tank. You might be able to rig a pipe inside the tank to prevent this.

WoodnotOil - I think your numbers are a little low on the flate plate HX and high on the pex - at least for a Solo 60. I think I'm looking at spending at least 300 for the HX. As far as pex, you can get much better deals on supplypex.com. I talked to Watts and they said to go with a nobarrier because the oxygen barrier won't do any good sitting in the water. ALso, it will eventually wear off and fall into the tank as debris. The cost of running the two additional pumps could easily be 20-30 per year. That adds up.

I saw Sparke's coils and they are indeed very impressive. My plan is a little less sophisticated. I plan to build a base, put it in the tank and wrap the pex around the base. The idea being that all of the supply HX is at the bottom of tank and the demand is at the top. The two will be separate. It will cost a bit more, but make for a much simpler installation. It will also help the stratification of the tank quite a bit.

I think the plate can be above the water line, but the pumps should be below. I think that can still be accomplished with holes in the top and the pipes coming back down after exiting the tank. I am no expert, so someone else can chime in on that one.

My estimate on the flat plate was for my tarm 40, so yes you would spend more for the 60. Additionally you may need longer coils as well. If you are planing on running two separate coils in the tank, I would think in 1/2" pex they would each need to total 1500' to have enough surface area. It would be more practical to have two pumps and be able to pump one coil in either direction.

 

[nhjohn]

Also, don't forget the $40 flange with shut off valves that your have to buy with each pump.

 

[WoodNotOil]

Yes there are the costs of flanges, fittings, etc. The encouraging thing is that people have made these systems themselves and made all of these options and others work for much less money than buying a STSS like system. Whichever option you choose, be certain to size it large enough that you can transfer all of the heat the tarm 60 produces so that it does not idle during burns. Otherwise a lot of efficiency is lost. If you do pex, leave room in the header to easily add more later in case you discover it was not enough. I would be interested to know what you mean by "building a base" for the pex. I might consider going that option too if it was not time consuming. I don't have 8 hours to spend on each coil. The plate hx would be faster in my case to get going. I want to stop burning oil as soon as possible. I am starting tank construction within the week.

 

[MrEd]

Are there actually people who have successfully built a non-pressurized tank and used a flat plate hx? I've seen lots of talk about trying it, but can't remember anyone actually chiming in declaring that they did it, and that it works as planned.

Certainly seems *a lot* cheaper. I was quoted close to $8000 for a STSS solution that would handle a Solo 60.

 

[chuck172]

MrEd, If you read my most recent posts, I have asked the same question. I hope you get a better answer than I did. Seems like many feel Plate Hx's are a better and more workable idea but I don't know if anyone is actually using this application for basic residential heat storage.
Personally I have given up for a while. I'm getting my Solo40 this week. I'll finish piping and wiring it in parallel with my boiler and SuperStor. I have a good woodburner upstairs and will attempt to throttle the Tarm by adjusting the amount of wood in the firebox this winter.
Hopefully Tarm will come out with their storage solution soon like we talked about.
Everything is subject to change for me but I know one thing for sure, keep cutting wood.

 

[WoodNotOil]

I think jebatty has one working with plate hx, but his is open and used as a radiator so it is not the usual residential application. However, he successfully transfers heat from his tarm to it. Perhaps I will be the guinea pig in this instance. I hope to have mine up and running in July. I spoke with a tech at www.radiantheatproducts.com describing the project and he helped me size the plate and pumps. He thought it would work as intended. I may need to add a filter on the tank side and clean the plate once and a while, but I am confident it will work as intended.

 

[free75degrees]

Here are some very rough numbers on just the tank side cost of the three options you mentioned above:

Flate Plate
30 flat plate hx - $200
2 - Grundfos 15-58 - $160
2 - Zone Valves - $160
Rough Total $520

Copper Coils (2 coils@180'=360')
3- 3/4"x100' @ $355 = $1065
1- 3/4"x60 @ $215 = $215
Rough Total $1280

Pex Coils 3/4"(3x as much needed = 1080') 1/2"(maybe 1500')
1- 1/2"x1000'= $385
1- 1/2"x500' = $190
Rough Total $575

I like the flat plate option because no coils have to be built. I think sparke said it took him something like 8 hours per coil to build. Not sure how expensive it will be to run the extra pump for the life of the system.

I plan to bring all pipes in through the top avoiding the need for holes in the tank sides.

Another option is the hard copper homemade coils. I used 10 foot sticks of 1/2" L copper. I have a total of about 350 feet and it cost me about $600 by the time I bought all the pipe and fittings. There are 4 5 foot tall vertical coils in parallel. There was a lot of labor making them, but I consider that part of the fun.

 

[DenaliChuck]

Here are some very rough numbers on just the tank side cost of the three options you mentioned above:

Flate Plate
30 flat plate hx - $200
2 - Grundfos 15-58 - $160
2 - Zone Valves - $160
Rough Total $520

Copper Coils (2 coils@180'=360')
3- 3/4"x100' @ $355 = $1065
1- 3/4"x60 @ $215 = $215
Rough Total $1280

Pex Coils 3/4"(3x as much needed = 1080') 1/2"(maybe 1500')
1- 1/2"x1000'= $385
1- 1/2"x500' = $190
Rough Total $575

I understood the plate hx system to need one pump more than the other systems since all systems needs one pump to move fluid through the boiler. If this is the case, then the rough total for the plate hx is $80 too high and should be $440.

I think I just answered my own question...are you going to use the same plate hx to load and draw heat from the tank?

DC

 

[WoodNotOil]

To maintain stratification in the tank you need two pumps on the tank side of the hx plate. Pumps are needed as usual on the boiler side, but you would probably utilize the existing ones depending on how your system is piped.

 

[free75degrees]

If you can get zone valves for cheaper than circulators then you could go with 1 circulator and 4 zone valves. Do they make valves that choose between two output instead of open/close (like a three way light switch). If so you caould use 1 circulator and two valves.

Both purple valves are open while both green are closed for charging the tank. Reverse for drawing heat from the tank

 

[WoodNotOil]

Zone valves are about the same cost as the Grundfos pumps I listed. So two pumps and two zone valves seems to be the least expensive option on the tank side of plate hx.

 

[DenaliChuck]

Ahhhhhh. I didn't envision the hx being used for loading and drawing heat from the tank! Makes sense; thanks for the diagram!

 

[nhjohn]

free75degrees - what type of wood boiler do you use? What is the max output? Is 350 feet of copper enough?

 

[free75degrees]

free75degrees - what type of wood boiler do you use? What is the max output? Is 350 feet of copper enough?

I have Tarm 40, not yet installed. If 350 is not enough I'll have to add another coil or two, but I am hoping it won't come to that.

 

[sparke]

Hello, Overall I am more then happy with the pex coils. I do not remember for sure but it seems to me, the first coil took a long time but the remaining 3 went much quicker. They do PERFORM. My boiler is rated 90K output. On the coldest days it keeps up with all usage with hot water to spare. When my tank is down to 120* with not other usage but charging the tank-the boiler CAN NOT keep up with the pex Hx. I have my aquastats set up so they stop the pump when the return water hits 140*. I have 700-750 gallons of storage. I have 4 coils in parallel at 200' each for a total of 800'. Perhaps too much coil charging but I think this is a good thing when I am drawing from the tank. I have plenty of pump. A Grundfoss UP 26-99. I am installing a Danfoss tempering valve so this will take care of the 4-6 hours of cycling. When I said I would do it differently I was referring to a few things. One I would not want to make them again but now I have the hang of it-not so bad. Since I only have 750 gallons, I would go with the Plate because the coils take up alot of space. Although probably not a s much as I think when it comes to liquid volume. Obviously this is not an issue with a bigger tank. The more I study Hydronics - I am not convinced a Plate Hx can perform as well because of the stratification issue. I understand you can go with 2 pumps or CV's for directional flow. However, I still think the mixing of the water I.E. turbulance in the tank must affect stratification. The jury is still out on that...

 

[jebatty]

I did use a flat plate, open storage, plate hx mounted above the top of the tank, pump was mounted just off the bottom of the tank, so water always was in the pump. The pump flow was vertical, so any air in the line would move straight through the pump. Do not plumb horizontal through the pump, as air locks can develop. Keep in mind that with an open system, it will drain down to the lowest water level. When the pump kicks in and pushes water through the system, there will be air, and venting is needed at all high points. I had a vent on the top side of the plate hx to release any air that got trapped at this point. If you get air locks, the system will not work.

 

[nhjohn]

Sparke - thanks so much for your input. I'm am leaning more and more toward the pex. I think it will be the easiest to install, offer the best stratification, and be the least expensive. The other thing is that if I don't have enough pex I can just put in more. I very much appreciate your input.

Jebatty, you almost had me with your lobbying efforts for the plate HX, but there are too many variables to worry about, the cost is more than just another pump (most notably the power consumption of the pump), and they seem (never actually done it) difficult to install given that you either need to go through the tank below the high water mark or make concessions for air bubbles.

Now the question I am trying to answer is this: If Sparke is having success with 4 coils of 200' each with a 90K boiler, should I go with 8 coils of 200' for my 198K boiler? Or 9 coils? Is it better to go with more shorter runs? I would think not as the hot water might not have time to exchange before its on its way back to the boiler. Should I go with longer runs? 3/4" runs? Or 1/2" runs?

Sparke - do you only charge your tank up to 140? Why not higher?