Expansion Tank Sizing for Storage

[Medman]

I know this has been covered before, but I would appreciate a quick answer...
I am planning to put in storage next month. I can get one or two 500 gal tanks.
What size expansion tank would I need for 500 gal/1000 gal?
If I use propane tanks for expansion tanks too, what size of this tank would I need?

Thanks for your help!

 

[WoodNotOil]

This thread https://www.hearth.com/talk/threads/32578/ should give you a good idea.

 

[boilermanjr]

Hello Woodnotoil. How is the flat plate working? Can you explain more about how you are using it?

 

[WoodNotOil]

Hello Woodnotoil. How is the flat plate working? Can you explain more about how you are using it?

Instead of using coils in my open epdm homemade tank, I use a flat plate to transfer heat from the closed system to the open one. The water is circulated out of the tank, through the plate hx, and then back. I have one pipe in the top of the tank and one in the bottom running the length of it with lots of small holes along both side to disperse the water without causing temp mixing. I draw from the bottom and return to the top for charging and reverse the flow when drawing heat.

It works extremely well and the stratification is usually equal to the delta T through the plate hx. It is between 15-20* which seems right on compared to what others are getting with coils. I do get some idling of the boiler when charging though which is an indicator that my plate hx is slightly undersized. I plan to replace it with one with larger ports and more plates soon which should solve the problem. When I built the tank the coils would have cost me $1000 each and I would have needed 3 for 1000 gallon tank with DHW. My plate hx only cost me $200 and I put a 100' 1/2" copper coil for DHW preheat for $100. That was a compelling enough reason to do the experiment.

Another interesting feature is that on the tank side of the hx I have two pumps in series facing each other that allows me to have flow in either direction. I used a DPDT relay to ensure they could never be on at the same time. This really simplified the piping and controls in that area. I contacted Taco and they said this would not cause any damage to the pumps as they are designed to be in series and the flow direction does not make any difference.

 

[Jeff S]

[quote author="WoodNotOil" date="1238251701" I do get some idling of the boiler when charging though which is an indicator that my plate hx is slightly undersized. I plan to replace it with one with larger ports and more plates soon which should solve the problem. When I built the tank the coils would have cost me $1000 each and I would have needed 3 for 1000 gallon tank with DHW.


What size plate do you have ,and what size do you figure you need to remedy any idling during charging ?

 

[WoodNotOil]

Medman is getting hijacked a bit here... Hopefully someone will come in with so more info for you.

What size plate do you have ,and what size do you figure you need to remedy any idling during charging ?

I have a 30 plate with 3/4" ports. I figure 1" ports will make a huge difference. I would also go up to a minimum of 40 plates for my 100kbtu boiler. 50 plates would probably be safer. A few guys on here got 70 plates just to be on the safe side and have had no idling at all, so I guess it works. Avoid 3/4" ports as the low flow is not effecient enough. Transfer increases both with larger ports and with more plates. So increasing either helps a lot with transfer. There are links to places to buy plate hxs on my site in my sig.

 

[Jeff S]

Sorry Medman

I have a program from John Siegenthaler book that helps calculate expansion size,there are variables such as.

- Temperature of stored water.

- How much water is in total system.

- what type of fluid(water,water/%glycol mix)

- height of system in relationship to your boiler

Not knowing your systems specifics,I went ahead and assumed with that large of volume you are probably using straight water.
also -storage at 180 degrees
-25 gallon for the rest of your system
-total system height of 10 ft.
This is what the program showed
525 - 40.5 gallon
1025 - 79.1 gallon

If you would like to give me more specific details I would be glad to put them in the program for you.

According to John's book there is a error of safety because not all of your fluid wouldn't be at this temp. due to stratification.

 

[2.beans]

i think 4% is the magic number. the last tank i set up was 500 gallons and i used 2 extrol 90s along with the original extrol 60 that was left. two exrtrol 90s give you 28.8 gallons of expansion i believe.

 

[Jeff S]

Its best if you know just what you are tailoring your system for.lets say your system involved 100 gallons inside a very large radiant floor system at 120* and your boiler held 20 gallon at 180* your total system of 120 gallons would need 4.4 gallons of expansion .

But if your total system of 120 gallons was at 180* then you would need 9.3 gallon of expansion.

It doesn't hurt to error on the larger size it is just more expensive.

 

[Medman]

Don't worry about the hijack - its all relevant info anyway, as I have been toying with the idea of non-pressurized storage since I would like to add a drainback solar collector loop to my system as well.

Cost of non-pressurized vs. pressurized using propane tank is coming out about even if I have to buy a large expansion tank. My thought was to possibly use a couple of 100 lb propane cyls as expansion for now, charging them with nitrogen or CO2, then switching to an appropriate expansion tank as budget allows.

Non-pressurized system is attractive except for the cost of the HX - any leads on affordable options here?

 

[DaveBP]

My thought was to possibly use a couple of 100 lb propane cyls as expansion for now, charging them with nitrogen or CO2, then switching to an appropriate expansion tank as budget allows.

I am thinking about doing the same thing but I would stay away from CO2. When it dissolves in water it becomes carbonic acid. A lot of the CO2 gas will end up being dissolved into the water in the system and acid water is death to all that steel, tank and boiler. I have a CO2 tank and regulator so I really wanted to use that but I can't take the risk.

Remember if you're building your own expansion tank,the tank needs to be bigger than the actual amount of expansion. The larger the tank is than the amount of expansion the less the pressure will build up as the system water heats.

Simplified example: If your system volume is going to expand 50 gallons from minimum temp to maximum temp and the expansion tank total volume is100 gallons and it's empty at the minimum temp, when the water expands into the tank as it heats the tank will be half full at maximum temp. That will double the pressure the system was at when it was at minimum temp. Most of the commercial tanks I've been able to find details on had an "acceptance volume" of about half the tank size. That makes it safe to run the system at 12PSI to start and max out at 24PSI. There are a number of other factors that I am not familiar with or just don't understand but that is the basic principle.

 

[Medman]

Thanks for that - I had the 'no CO2' point in my notes here, just forgot about it. I have access to both N2 and CO2. I am really still up in the air on the pressurized vs. non-pressurized issue, since both are about the same from a cost standpoint for me. I like the idea of non-pressurized for adding solar collectors, though.

 

[WoodNotOil]

Non-pressurized system is attractive except for the cost of the HX - any leads on affordable options here?

One option is a flat plate hx. The prices here are pretty good. The 50 plate I looked at has 1 1/4" inputs. That is a lot of btu transfer. It can transfer twice the btus that my 30 plate with 3/4" inputs can. That's pretty good for $500. For an EKO 25 I would think the 50 plates would be plenty with the 1 1/4" inputs. You can check out my website for how I did my tank piping for a plate hx.

There is always the copper tube option, but that can be costly. Some guys have made some out of scrap copper that they get from heating contractors.

 

[Medman]

WoodNotOil, I noticed on your website that you have the system plumbed for concurrent flow through the HX during charging storage and counterflow when drawing from storage. Is this actually how you have the system set up?
I notice that you are running the return through the oil boiler before charging storage. Has this resulted in a lot of standby loss for you, and is that the reason for the redesign this year?

I am struggling with the plumbing of the tank, since no matter how I plumb it into my current setup I will end up losing heat when drawing from storage, as the water passes through cold components of the system.

 

[WoodNotOil]

WoodNotOil, I noticed on your website that you have the system plumbed for concurrent flow through the HX during charging storage and counterflow when drawing from storage. Is this actually how you have the system set up?
I notice that you are running the return through the oil boiler before charging storage. Has this resulted in a lot of standby loss for you, and is that the reason for the redesign this year?

I am struggling with the plumbing of the tank, since no matter how I plumb it into my current setup I will end up losing heat when drawing from storage, as the water passes through cold components of the system.

My system was setup in series originally because that is the method my installer chose. When I redo it this summer I will do it either in parallel, P/S, or with a hydraulic separator. I haven't quite decided. Either way, the tank side of my hx will not have to change. I do have it plumbed to change the water direction to preserve stratification and it works great. If you draw out a diagram of your system, we are always willing to give suggestions on how to pipe storage in.

 

[2.beans]

i dont know if this is helpfull but with my pressurized storage i used a series of expansion tanks to help with the cost. that way if one took on water you can change it without shutting down the system assuming you install a ball in front of the tank. sometimes you can pick up tanks on ebay pretty cheap. just a thought.