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Thermostatic Mixing Valve rec?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by kuribo, Feb 21, 2013.

  1. kuribo

    kuribo Feeling the Heat

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    Thanks for all the advice to date....

    Based on the advice received, I am going to go with a thermostatic mixing valve, variable speed circulator and zone valves from storage.

    I have been looking at mixing valves and am having trouble sourcing one of the right size for less than a fortune. I have 1-1/2" supply and return lines from storage (about 24 gpm total to 5 zones) and while I can find 1" TMV's all day for $100-$150, the cheapest 1-1/2" valve I can find is over $500.

    Maybe I should think about using 2 smaller circulators (taco bumblbees look nice) and two smaller, 1" TMVs, spliting my zones into 2 parallel loops...?

    Anyone have any recommendations for a reasonably priced 1-1/2" TMV?

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  2. PassionForFire&Water

    PassionForFire&Water Minister of Fire

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    Just to make sure:
    Do not use a standard thermostatic mixing valve for boiler return water protection
    They have to much pressure drop (Cv)
    Look for LK820 or similar.
  3. kuribo

    kuribo Feeling the Heat

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    This isn't for boiler protection. It is for setting zone temps with supply taken out of 1000 gallons of propane tank storage. Though I guess the same advice would apply...
  4. ewdudley

    ewdudley Minister of Fire

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    For 70000 btu per hour design load your net flow from storage will be about 2 gpm. Your flow through 2200 plus 2500 sq ft of concrete pex will be 15 gpm or so with all zones calling.

    Instead of a bigger pump and a bigger mixing valve to do both you may want to look into variable speed pump to circulate through the zones, and another variable speed injection pump with temperature control instead of a mixing valve.
  5. The size of the threads doesnt matter. You need to be concerned with the cv rating. And use that In your calculations. Often the same valve body is used with different size threads. So a 1" may have the same cv rating as a 1.5" if its even available from the same manufacturer.

    Nothin wrong with using a valve with 1" threads if the flow rating is sufficient.
    ewdudley likes this.
  6. kuribo

    kuribo Feeling the Heat

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    Not sure where you get the 2 gpm net flow figure????

    I was going to use one variable speed pump and one tcv with zone valves, but now I am leaning towards 2 smaller variable speed pumps and two smaller tcvs...

    I understand that Cv is what I need to be concerned with- it seems that the valves I have found so far have a direct correlation between size and Cv....What kind of Cv for a tcv with 20 gpm should I be looking for?

    Seems I could use one of the non-electric, manual setting TCVs or an electric actuator valve. What is most common in this case?
  7. ewdudley

    ewdudley Minister of Fire

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    2gpm * 70 degF deltaT * 500 = 70000 btu per hour from storage assuming at least 70 degF deltaT supply minus return to storage.

    So one small pump to pump round and round through the zones, and one small pump to inject hot water into zone loop. The injector pump adjusts its speed so that the mixed temperature is what you need.
  8. kuribo

    kuribo Feeling the Heat

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    I see....And when my storage temp drops and the delta T is only 10F then the injection pump has to output 14gpm, correct?

    And both the injection pump and zone circulator would be variable speed pumps, right?
  9. ewdudley

    ewdudley Minister of Fire

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    But with radiant slab your return temperature should always be as low as possible, and your storage stratification should be as close to ideal as possible, so your deltaT drawing from storage should always be 60 degF or higher. With good stratification, when top of storage temperature starts to fall you're pretty much out of heat.

    Yes the zone circulator would be constant pressure variable speed so you get more flow as more zone valves open and less flow as zone valves close. Grundfos Alpha constant pressure or Wilo Stratos constant pressure for instance.

    The injection pump would introduce hot water into the zone loop with closely spaced tees and would adjust flow according to temperature downstream of the injection point and before the zone valves, only need 1/2 inch PEX probably. It could be a Bumblebee variable speed constant temperature. The advantage would be that you wouldn't be pumping all flow through a high Cv mixing valve. Plus two small pumps, one with temperature control would be comparable in price to one big pump with one big mixing valve.
  10. kuribo

    kuribo Feeling the Heat

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    I am not following you regarding storage usable temps. The whole purpose of the storage, for me, is to be able to draw on it until the temp is down near my radiant supply temps. I am using two horizontal tanks, side by side (plumbed in parallel) so I won't get ideal stratification. I really don't want much stratification if I am trying to get as much energy into the tanks as I can- at least that is how I understand it...what do I have wrong?

    I have found a more reasonably priced 3 way mix valve- tekmar 712 with 741 actuator, about $350. It has a Cv of 20, so I think it should work well.

    If I had a constant, high temp supply to guarantee a large delta T, then injection mixing, as I understand it, would make sense. With a variable supply temp, I would need a large injection pump to handle the large flows necessary as my storage tank temp dropped. Also, the pump would use a lot more electricity than the mixing valve, if I understand correctly. If the pump motor fails, I have no heat. With the mixing valve, I could manually move the valve to at least get something....Does this make sense....

    Thanks again.
  11. ewdudley

    ewdudley Minister of Fire

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    Your return from the radiant loops will be nice and low, anywhere from 110 degF on down to 90 degF depending on heat load and how low of a mix temperature you can get away with. So the storage will be filling with water that is pretty much 'used up' and when the tanks are full of return water your supply temperature will fall rapidly to level that would be unusable if your supply temperature has been minimized properly. You get the same amount of heat whether you stratify well and draw the water once from storage dropping the temperature as much as possible for one lap, or whether you stratify poorly and draw the water repeatedly from storage dropping the temperature ten degrees on each lap.
    The 20 Cv valve should work fine. Or a small injection pump. In either case the minimum supply temperature you can use only affects the size of pipe from storage to the mixing point. If you can depend on good stratification and low return temperature to storage then you could get away with 3/4 or even 1/2 inch pipe from storage to mix since your storage is in the basement. If you want to play it safe then 1 inch or 1.25 inch will ensure that you can draw 10 or 15 gpm from storage if need be. Again it is the minimum deltaT from storage that determines the size of the pipe from storage to the mix point, not mixing valve vs. injection pump.
  12. 700renegade

    700renegade Member

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    You absoulutely do want stratification - it's the holy grail of efficiency in this type system.

    EWD covered this, but I'll try a different way. Lets pick some easy numbers - say your boiler delivery temp is 200* and lowest useable temp going to the floor is 100*. We want 100,000 btu to go to the floor.

    A system of 1000 gal ( 8340 gal ) of perfectly mixed water at 150* thus is storing 417,000 useable btu.
    Same tanks but perfectly stratified with the top 500 gal at 200* and bottom half at 100* also has the same 417k btu.

    The difference is this:
    In the mixed(ing) tank, you will initially need to draw 4 gpm off the tank ( 100k/500/50*dt). The second hour the mixed tank will average 138*, and you will need to bump up to 5.3 gpm to get 100k btu/hr ( 100k/500/38*dt). At hr 3 you are up to nearly 8 gpm. Eventually you hit the wall at about hour 3.5 and cannot pump fast enough to continue to extract btu's.

    In the stratified tank, you will initially need to draw 2 gpm off the tank ( 100k/500/100*dt ). Remember you are putting water at probably 80* back into the bottom of the tank. 2nd hour you are STILL drawing 2 gpm off the tank as its 200*. This goes on until you hit 4 hours and 10 minutes, the water at the top almost instantly goes from 200* to 100* and you are done. This is why EWD states you can get by with 1/2" pex.

    This same dynamic works identically when charging the tank from the boiler. Highly stratified water in the tank means the bottom of the tanks are maybe at 100* and as a result the required flow rate thru the boiler only needs to be a few gpm - ALWAYS. If you're mixing in the tanks, your return water temps go up, and your required flow rate away from the boiler goes up exponentially as you try to saturate the tank with max BTUs.

    Delta T, or GPM...... pick one. GPM requires big pipes, pumps and electricity, dt requires planning and control.
    I like dt.....
    Trex83 and ewdudley like this.
  13. Bob Rohr

    Bob Rohr Minister of Fire

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    You need 3 components for that tekmar setup. The 712 valve, 741 actuator, and the control that sends the signal to tell the actuator how to modulate. The actuator requires a 24 V floating action signal. Probably the least expensive option for control would be the 153 mixing setpoint control.

    All things considered the 3 way thermostatic is usually the least expensive option, and the easiest to install and service.

    The tekmar option would allow outdoor reset which is a nice feature on any hydronic system. It's like a cruise control that always modulates temperature supply based on outdoor conditions. It gives you near constant circulation and ultimate comfort.

    High flow thermostatic mixing valves work well as long as you watch the min. flow rate also. Too little flow and they loose some temperature stability.
  14. kuribo

    kuribo Feeling the Heat

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    Thanks for the explanation.

    I get the concept but wonder why my perfectly mixed tank wouldn't be at 190F? If I have the whole tank at 190F, don't I have more energy to distribute than if I had 190F at the top, 150 at the middle, and 110F at the bottom?
  15. 700renegade

    700renegade Member

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    Somehow when I wrote that post I imagined just that comment appearing! Lets try again:

    If your system is overpumping and constantly mixing the tanks, there is a 99.9% chance you will NEVER get the tanks 190* top to bottom. Think long and hard about how you would possibly move enough BTU into the tanks to move them from say 185* to 190*? If your boiler was putting out 140,000 btu you'd have to pump at 56 GPM for heavens sake! You'd need 4" piping from the boiler to the tanks.

    Now if your tanks were nicely stratified at say190* on top and 100* at the bottom, until that last 10 gallons of 100* water leaves the bottom of the tank, you can deliver the full 140,000 of the boiler with 3 gpm. You'll still end up with 990 gallons of 190* water, just like your 'perfectly mixed' example. 56 gpm or 3 gpm - which sounds better to you?

    Once again DELTA T is your friend ! It's equivalent to voltage in an electrical system
    PassionForFire&Water likes this.
  16. jebatty

    jebatty Minister of Fire

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    Maybe this example will help. I have a 1" thermostatic mixing valve to feed my in-floor radiant; then an in-floor sensor to measure floor temperature, which for my shop I set at 61F with 1F differential; 6 loops of 1/2" pex each about 270' long; on-off loop circulator controlled by a Ranco which reads the in-floor sensor; total flow through the 6 loops is about 2.25 gpm (about 1/2 gpm/loop).

    I heat 1000 gal storage up to as high as 190F top to bottom. The mixing valve is set at 100F. When the floor calls for heat, the mixing valve sends 100F water to the floor, return is about 70F, btuH = 30 x 2.25 x 500 = 33,750. There is a drop in mixing valve output as tank temperature falls to below about 110F, but even at tank temp of 100F mixing valve output is about 90F. Loop flow is constant, so btuH then also drops. This is not an issue because normal operation does not result in tank dropping below 110F.

    My in-floor is set in concrete, so slow response time to changing outside temperature, and floor always is set at constant 61F. This works well for my shop, which then maintains constant air temperature between a low of about 52F at outside -20F, normal air temp 58F-62F at outside +10F and above. For in-floor air temp might seem cold, but the warm floor and lack of drafts makes the air seem much warmer than it is. Hard to explain, you may know what I mean. The lack or drafts/air movement results in warmer "feel" temperature than results from moving air and cooling evaporation on the skin.

    BTW, my 1000 gal tank is horizontal and stratification on charging the tank is extreme.

    This is true for the example given but also is misleading. Typical burn might be 140,000 btu at high burn, tank charging and temp rising, but as wood load burns out to coals, btu's fall, and as tank reaches the 185F level, for example, btu's from boiler will be far less than 140,000 maximum. My 15-58 circ flows constant 12-14 gpm to the tank, the Tarm is 140,000 btu rated, and I easily can bring the tank up to 190F top to near bottom at this flow rate without idling the boiler in the process.
  17. Quincy

    Quincy New Member

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    A very interesting thread on stratification of storage .What is the best way to accomplish this ?My storage will only stratify when I am not heating storage for say 6 hours and the load is not calling for heat on a regular basis .I installed a dip tube for the supply to the tank from my primary loop was this incorrect .Should I have ported the supply to the top of the tank instead of through a dip tube.thanks for your input.
  18. jebatty

    jebatty Minister of Fire

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    To achieve possible stratification, hot water supply to top of tank and cold water return to bottom of tank. When system is calling for heat, depending on flow rates, warm water may be returned from system to bottom of tank and cause mixing, which may or may not be an issue. It is no issue with my system, because hot water serves only low temp radiant (100F). If warm water return and maximum stratification is needed or desired, then an improved design might consider mid-tank injection of warm return water.
  19. kuribo

    kuribo Feeling the Heat

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    Thanks again for taking the time to explain, much appreciated....

    Maybe we are talking about two separate approaches here: practical and theoretical.....I think from a theoretical standpoint, you want as many BTU's in the tank as possible. I agree that delta t is your friend but isn't the delta t that is important that between the tank and the floor return temp, not the delta t in the tank? So in theory, wouldn't having the entire tank at 190F be ideal?

    In practice, getting BTU's into the tank when the tank delta T shrinks requires more flow at a constant, rated boiler output, which, as said, can require very high flows. It would seem that one would have to use stratification if the boiler output is constant.

    Having read Jebatty's prior posts, I was under the impression that he is taking advantage of a wood boiler's natural decrease in output with burn time, as well as tailoring the wood charge (through weighing), to "modulate" his wood boiler's output to squeeze the btu's into his storage as the storage delta shrinks. Isn't this the optimum strategy?

    I am confused by your comment, Jebatty: "BTW, my 1000 gal tank is horizontal and stratification on charging the tank is extreme." If that is the case, does it cause difficulty in getting a uniform 190F as you say you get, top to bottom? Or is it the case that it helps because you are supplying water to the boiler from the bottom where the coldest water is located?

    Again, thanks to all that have replied.
  20. jebatty

    jebatty Minister of Fire

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    Yes, coldest water from the bottom of storage to the boiler.
  21. maple1

    maple1 Minister of Fire

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    My turn. :)

    Think of stratification as a sheet or blanket laying in the storage tank that separates the hot water on top from cooler water on bottom. With good stratification, that sheet rises & falls with draws from storage & charges to it. When charging, eventually that sheet will get to the very bottom & you will have a tank full of all hot water (say 190). As soon as you start drawing to loads, the sheet will start rising - all the while maintaining a good constant delta T (the higher the better) for constant flows throughout the draw cycle. With higher temp emitters, with a delta T of say 20, once the sheet gets to the top, then the overall temps will drop as storage gets used up. With lower temp emitters with higher deltas (say 70), once the sheet gets to the top, storage is about used up - but it will have taken much longer for the sheet to get to the top as the much higher delta would have required much less flow.
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  22. 700renegade

    700renegade Member

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    So in theory, wouldn't having the entire tank at 190F be ideal?

    Yes whole tank at 190* is great whether stratified or mixed.

    The difference is what you can do between that instant of time and your next boiler firing. Most importantly if you are using a high temp emitter like a coil in the furnace or baseboard heat. If you have any of those in your system you are functionally done when you hit 150* at the draw-off point in your tanks. There is where the stratified vs. mixed system will show it's colors.
  23. maple1

    maple1 Minister of Fire

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    Baseboard will put out heat down to lower temps than most people give it credit for.

    I just lit my boiler for the daily burn. A bit later than usual, and I didn't get it all that hot yesterday. My top temp probe, measuring the middle of my top tank, was at 130. House was warm - although it wasn't real cold out today. Right around the freezing point with some wind. I would for sure want to be at temps a lot higher than that for the frigid days, but I'm getting heat out of my storage quite a bit below 150, down to what would be about the lower limit for my DHW heating.
  24. kuribo

    kuribo Feeling the Heat

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    I have all in floor radiant. My supply temps at design point, with a 10F drop, are around 120F.

    While it may require more attention to my boiler firing and wood load, I can't help but think getting my whole tank to a uniform 190F is the right approach, at least, in my situation.

    Once the boiler is out and I am drawing and returning to storage, then it would stratify as the colder return water enters the tanks, wouldn't it?

    Thanks.
  25. ewdudley

    ewdudley Minister of Fire

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    No, with radiant you return water to the top of the tank so it will mix. 190 degree water is bad for the pex so you have to mix it down to 120 or whatever and if you return to the top the water on average gets cooler and this is easier on the mixing valve. And mixed water lasts longer.

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