could this work? solar storage tank as buffer for mod/con?

greenmountain

working on a very limited budget... got a great deal on a 120 gallon solar storage tank.
Any suggestions/criticisms for using it as a buffer?

trying to heat some small zones and want to keep in condensing range and avoid short cycling. I'm not sure how much stratification there would be... boiler circ is fixed at 4.5gpm. Tank is about 64" tall. Has two heat ex coils I'd pipe together. With so much volume and a large switching differential on the boiler I would think the on/off times would be relatively long, esp when only heating small zones. could this work?

ps. meant to put Delta P for the zone ecm circ (although I'm not sure which would be better here?)

hot_rod

What type of heat emitters? What temperature do they need.
Hot water option? What brand and model tank?

greenmountain

so far I have a variety of kickspace heaters... I believe they need 105F to kick fan on.
boiler is a Rinnai combi so would just use that for DHW.
solar tank is Velux but pretty sure it was manufactured by Bradford White.
It's older (8yr) so still looking for exact specs. will post when I find some.

I know the devil is often in the details and there's still a lot to figure out, but... just trying to get some sense of if this could, in theory, work as intended... Although one thing I'm really unsure about is if or how the boiler might modulate in this scenario, and to what degree that would affect efficiency??

greenmountain

the second one with the two coils:

hot_rod

Super simple, like this. The boiler sensor goes into the sensor well in the tank. So the boiler only “sees” the tank as it’s load. Outdoor reset enabled to run the boiler as low as possible.

Really all you are doing is building a high water content modcon.

kcopp

Thats an HTP tank.
Personally I think the Kickspace heaters and not a good match w/ the low temp.
You will be blowing cold air a lot.

greenmountain

High mass radiant (works better at low temps right?) is not currently an option for this project, maybe in the future.

I suppose I could work to find a balance between increasing tank temp as high as possible while still trying to keep majority of run time return temp<130? (Would probably work better with a tank designed to stratify but, like I said, working on a tight budget here and this tank was practically free)




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kcopp said:

Thats an HTP tank.
Personally I think the Kickspace heaters and not a good match w/ the low temp.
You will be blowing cold air a lot.

hot_rod

The tank will stratify when it gets up to set point temperature, to some degree. When flow from the boiler pump stops, the hottest water will rise up.
Next when a heat zone calls, the buffer will provide the heat energy until it reaches the turn on point. So determine the highest temperature required on design day, add that into the ODR settings. You may need to play around with the reset curve a bit and establish the lowest SWT that you can run without feeling cool air discharging from the kick heaters.
Unfortunately with fan coils you may not be able to run much below 115- 120F. And have a comfortable air flow.
Cast or panel rads would be sweet, run them down to 100- 105F, if they have enough surface area.

greenmountain

Maybe more modulation/stratification/efficiency by reversing the piping so the boiler is running through the coils and the ecm is circulating the tank water? The boiler has less volume, but not sure that would be a problem with such a big tank and the double hx...

Pretty sure I could come up with some old iron rads, then put them inline after the kickspace heaters...
maybe that way I could get hot enough water through the fan coils, then rads, and still have cool enough return...

psb75

You missed a great deal on panel rads at the Restore in Williston/Milton. 50 bucks each, regardless of size! They had hundreds of them. It will be important as to which coil(s) you hook up to. Take some time to consider stratification and how to make best use of it. That is the main feature of a vertical tank and esp. one with top coil and bottom coil. This is your "ace in the hole." Remember to keep your mod/con boiler condensing with low temp. return water. Take the highest temp water out of the tank for your loads and return it where it least disrupts the stratification.

hot_rod

If the connections are large enough build a diffuser tube to help build and maintain the thermocline. You want to stir the tank as little as possible when you move flow in and out. It will be easier to use the tank side as the boiler side, and pipe a means to stratify best. Without making yourself crazy in the process.

Excellent info in this issue regarding thermocline, exergy, stratification, sizing, etc.

hot_rod

One thing you will discover is a coil in a tank of still water will not transfer so well, you get a cold pocket around it until the pump that circulates thru the tank starts again.
If you have a dual coil with coil from top to bottom of the tank that will help some.

The Caleffi solar tanks were made by Bock and they ran some testing on using the upper coils as a hx take-off for radiant, etc. not what we expected

Nothing like two pumped flows to exchange heat, that one of the concepts behind flat plat heat exchangers, and their powerful heat exchange.

A 2 or 3 pipe method would really be a better piping IF in fact you want to squeeze as much energy in and out as possible.I

ScottSecor

I agree with the posts above, would not suggest the use of kickspace heaters when using a condensing boiler (especially for a home in the Green Mountain state). With regard to zone valve location we prefer to install them on the return instead of the supply if possible, as they seem to last longer due to the lower closing force needed. As an added benefit, installing the zone valves on the return reduces the noise and "slamming" with the fast closing types.

hot_rod

Pros and cons on zone valve location. On the supply you get 100% protection from ghost flow.

Also, in the return they see the exact same flow rate as supply, so shut off force required is the same.  The water hammer is more an indication of excessive flow, delta P circ or pressure bypass are a few options.

A bit cooler fluid on the return may extend their life.

ScottSecor

Bob (Hot Rod),
Just so I understand this,  if the ZV is on the return doesn't the friction from the loop piping reduce the head pressure on the ZV?  Let's say the pump produces eight feet of head at the outlet, then the ZV a foot from the outlet of the pump is also seeing almost eight feet of head.   However, if you put the ZV on the return and the loop has five feet of head for friction, doesn't the ZV only see three feet of head?   I could certainly be mistaken. 

hot_rod

correct, the zv would need to have a high enough shut off pressure 
In a hydronic system with a flat curve 1/20 hp circ, you have very little to close against

If a large circulator capable of developing say 50 psi, Your valve would need to be rated for 50 psi close off

Typically you will find valves with lower Cv have higher shutoff. A Caleffi 7.5 cv valve has a 20 psi shutoff
A 1 cv valve has a 75 psi shutoff

It is possible for the circ to force open a valve that does not have a high enough shutoff. The systems you work around, you may come across that.