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Your thoughts on the best way to run water-to-air HX for max efficiency?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by stee6043, Jan 19, 2009.

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  1. stee6043

    stee6043 Minister of Fire

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    I've been toying around with my water-to-air HX in the past three or four days and have made some interesting discoveries. I'm wondering what the boiler room thinks is the best way to run your setup.

    Currently I run a single water-to-air HX in a forced air NG furnace. I also have 1000 gallons of storage. I've been playing with the flow on my HX loop to see where I get the best heat to the house vs. how much temp drop I get. When I run the HX loop wide open I get about 20 degrees temp drop through the loop. Say 170 to 150. If I throttle the flow on the loop I can run the temperature drop all the way to up 60 degrees or so. The highest has been a change from 170 to just over 100. That's the extreme.

    I know 20 degrees is the typically the target. But what about the effects on tank stratification? When I run my HX loop at the 60 degree temp drop setting I get almost no mixing in my tanks. I can run the bottom of the tank all the way down 100 degrees and still have 160+ on the top. When the HX loop is wide open I get a lot of mixing and typically can't maintain anything more than a 20-30 degree difference between tank top and bottom.

    Anyone see any benefits to running either way? I'm running wide open right now because my pump does not like being throttled down. If I were to make it permanant I'd have to bust for an adjustable speed or smaller pump. Any thoughts are welcome!

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  2. pybyr

    pybyr Minister of Fire

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    Interesting- I was going to post a very similar question: I have a single water to air HX on top of my ThermoPride oil furnace's outlet plenum. The HX is BIG- 24x24, 4 rows of 5/8 tubes. The ThermoPride furnace's blower is rated at 1600CFM, so the air is moving at a lower velocity than many hot air furnaces- which I would expect would be good for picking up the heat from the HX (moving the air too fast would not allow it to pick up as much heat on the way through the HX.

    But- and even though it heats the house well- I am puzzled that I am not somehow getting more heat actually transferred from the water to the air. I've run the boiler at a variety of temps, between 168 and up into the 190s, and I'd have thought that once the blower was running for a while- the house temp would _soar_.

    the boiler and water-air HX are tied together by way of a primary-secondary system, with all of the piping being 1.25 copper, and the only smaller-diameter points are at the Taco Twin Tees, where the secondaries meet the primary (those are 1.25 main trunk, 3/4 inch secondaries) but the Secondaries then immediately step back up to 1.25. Circulators are Wilo Stars, which are pretty equivalent in operating specs to Grundfos 15-58 s - and I am running them all on the mid-speed.

    Many variables here I could start measuring and playing with (blower speed, pump speeds of the various pumps on the boiler feed to the primary, the primary's own circulator, and the circulator on the secondary feeding the water-air HX) but I'd greatly appreciate any hints from the pros and experienced DIY-ers on a "strategic plan of attack" of what to measure and what to then begin changing/ looking for.

    Thanks, as always, and hope I am not hijacking the thread- if I understand the post above, our two sets of questions are pretty closely related, and hopefully what might benefit one of us might also benefit the other
  3. Nofossil

    Nofossil Moderator Emeritus

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    This issue is a great example of the reason that forums like this are necessary. It's a case of 'everything you know is wrong'. Virtually all heating system designs are based on fossil boilers with an emphasis on simplicity and comfort - not necessarily a bad thing, but efficiency is notably absent from the list.

    Herewith a couple of things that may be generally accepted truths that just ain't so:

    1) Design for a 20 degree drop. Makes NO sense for a system where efficiency is important, and especially where storage is involved. You want the return temperature to storage to be as low as possible. Ideally, you'd draw off hot water from storage, use it for a pass through baseboards, use it through a DHW sidearm, run it through a radiant slab, then use it to preheat incoming cold water ahead of the DHW so that you can return it to storage at 60 degrees.

    2) Higher boiler outlet temps mean better performance. Again, not so. Best performance and highest heat output are obtained with the lowest possible inlet and outlet temperatures.

    3) Bigger is better. Boilers that are too large are less efficient and require a larger investment in storage. In a gasifier, wood that's too large doesn't provide enough wood gas generation to support effective secondary combustion.

    4) Long burn times are desirable. I don't know about everyone else, but I'm a lot happier only dealing with the fire for six or seven hours out of the day. Short hot burns are convenient and efficient. I typically touch the boiler three times per day - a start and two reloads. The start takes ten minutes and includes ash removal if needed. The reloads are a minute each, all in my slippers and bathrobe.

    This should start a few heated discussions ;-)
  4. sdrobertson

    sdrobertson Minister of Fire

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    I had the same problem with my tanks mixing. What I did was I purchased a Bell & Gossett controlled circ and I set it up so that my return to the storage is not above 125 degrees. This circ speeds up and slows down so that my set point (125) is constant. This has stopped my mixing and keeps my tanks stratified. Right now I'm charging as I have to work tonight and tomorrow night so I'll be running off mostly my storage with a total of two burns over the next 60 hours or so. My 4 tanks are 190 at top, 185 at middle and 125 at the bottom going to the boiler. I'll run the boiler till I get the bottoms about 150 or so and when I heat the house, my bottom will be at 125 and I'll still have 180's in the middle and 190 to the house. Its amazing to watch the temps change at the bottom and the middle and stay the same at the tops. In my case with my work schedule, storage is a life saver.
  5. mtfallsmikey

    mtfallsmikey New Member

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    Also keep in mind that a water to air HX is not the most efficient way to transfer heat.
  6. sdrobertson

    sdrobertson Minister of Fire

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    Summer project - retrofit to radiant floor heating.
  7. Duetech

    Duetech Minister of Fire

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    nofossil,
    Please let this be for a note of clarification in the abyss I try to use for mental storage (where stratification may not be desired but seems to occur any way and this may be one of those occurrences). First off you run radiant don't you? I am not yet at the point of storage with my system and I also have to contend with a less efficient heat transfer system. My unit labors to warm the (poorly insulated, old) house with 120*f water temps and anything less than 110*f will not work in still air and ambient temps of 25*f let alone wind factors. When my system is in the 170+ range I get the quickest response with my system and shortest furnace blower run times (reasonably) where a notable portion of my efficiency is prone to be. If I had storage allowing the top of the tank to drop to 140 would put me in a long call for heat and would deplete storage quickly and require a long hard burn just to get the house up to temp. I know having my unit in an unheated facility exacerbates the problem as far as response time but those are my circumstances. Outside of replicating your scenario what would be your suggestions be for my circumstances using your strategies? I want to put storage in but with temps like you are/have been talking I will have a cold house.?? (Maybe I should wait to build storage?) Thanks!
  8. Nofossil

    Nofossil Moderator Emeritus

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    I listed the ideal situation. I don't yet have a radiant zone or a sidearm, or even enough baseboard to do what I'd like. Stratification is always good as far as I can figure it, so in any system you'd like to arrange things to get the lowest return temp consistent with desired performance.

    Since I don't have enough baseboard, I have to obtain a relatively high boiler outlet temp if I'm heating the house and it's cold outside. Otherwise, I'm better off with a lower outlet temp. I'll juggle heat loads and circulator speed to accomplish that.

    In any specific situation, your options may be limited. I just wanted to clarify my view of the unimportance of the 20 degree rule.
  9. Duetech

    Duetech Minister of Fire

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    Thanks for your candid response. My concern was basically trying to determine where the next step is that I should take. I have been taking a slow approach towards insulation but may have to wrestle through that and radiant heat before I can go to storage. Thanks!
  10. stee6043

    stee6043 Minister of Fire

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    Nofossil, my thoughts were similar to yours on the 20 degree idea. My only concern with the "extreme" temperature drop in my case was the cold return to the boiler. When I get bottom tank temps at or near 100 degrees I get "less flow" through the tanks in general due to the Danfoss valve doing it's job to keep return at 140 and above. I wondered if this might be a potential "con" to that method of operation?

    I think most folks that have spent time on this site understand that radiant in-floor is best. But with a finished basement I am currently reluctant to completely destroy the place to install in-floor. Maybe someday. I rather like the simplicity of the in-furnace install. Quick and painless and it delivers heat very quickly. I also like using a standard off-the-shelf thermostat to control my system (with one cheap relay).
  11. stee6043

    stee6043 Minister of Fire

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    Cave, I have a similar situation. When the outside temps are 20 and below I can't effectively heat my house with anything less than 150 degree water. When I tried to do this my furnace and the HX pump ran nearly non-stop. That was part of my reason for playing around with my HX flow. In my simple mind the slower you draw hot water out of the tanks (when heating from storage) the longer you should be able to maintain the higher temps at the top. It turned out that this was in fact the truth. When I had my HX loop throttled to the max I could maintain 160+ on the top of my tank for a full 12 hours or more depending on the weather. When I would get home from work I'd have 160+/- on the top and 100-105 on the bottom. With the HX loop wide open I'd have a spread of 120's, 130's and 140's all over the place. Depending on how long the pump had been running I'd even get "temperature inversion" on occasion where my tank bottoms were higher than the top. It can get really strange (especially with the cold we had last week).
  12. Nofossil

    Nofossil Moderator Emeritus

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    I've heard (but not seen first-hand) some reports of Termovar and/or Danfoss valves that seem to restrict flow excessively with cold return water. If that's happening, then it's a problem that's independent of the basic premise that cold returns offer better performance.

    If you can configure your loads to provide colder return temps, then you're extracting more heat from every gallon that's circulated. That means you need a lower flow rate to deliver the same amount of heat. That's a good thing. Here are three scenarios, all of which deliver the same amount of heat per hour. Flow rates are through the zones, taking into account the effect of the Danfoss mixing valve.

    Scenario 1: 12gpm, 180 degree boiler outlet, 160 degree return temp

    Scenario 2: 6gpm, 180 degree boiler outlet, 140 degree return temp

    Scenario 3: 4gpm, 160 degree boiler outlet, 100 degree return temp

    Scenario 2 is more efficient than scenario 1, and scenario 3 is the most efficient.
  13. sfriedri

    sfriedri New Member

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    Have you checked to see how the air temperature coming out of the exchanger varies with the water flow rate using the same water temp?

    I also use a water-air exchanger in my system and have a three speed pump supplying flow, but with all the other tweaking I have been doing I have not tested the different speeds on the pump. I know that the water-air exchanger BTU ratings are based on a combination of water temp, water flow rate, and air flow rate, so theoretically using a slower water flow rate will reduce your BTU output.

    With my pump on high speed I usually get about 30-35*F stratification in my tank. The tank is about 6' tall and has baffles on the inlets.
  14. Duetech

    Duetech Minister of Fire

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    SWF,
    "I know that the water-air exchanger BTU ratings are based on a combination of water temp, water flow rate, and air flow rate, so theoretically using a slower water flow rate will reduce your BTU output."

    That is exactly the way the auto industry has been doing heaters in cars. High flow rate high thermal output unfortunately heat is a by-product for them while it is an objective for home comfort. I have a one speed circulator so I can't do flow rate/output testing. These systems are two different critters in some regards as radiant is wide area slow dissipation and forced air is concentrated high volume transfer. If the water temperature and flow rate are not high enough with forced air you will lose btu output integrity as some of the air will not get heated and will discount the quality of the air that is heated before it gets out of the register. Certainly a case where the radiant proves it's superiority. Nofossils statements about the low return temperatures has drawn very valid points about storage and it's wide applications that has helped clarify what I should expect with my system when I get storage up and running. It has also shown me the limitations that my system will probably experience. Stee6043 on the other hand has shown it is not only possible but viable and reinforces my thoughts on what I need to focus on for my current system. (Thanks guys)
  15. sdrobertson

    sdrobertson Minister of Fire

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    With the variable speed circ I'm able to stay between Scenario 2 and Scenario 3. My problem was that I need the zone to be running all the time as I have two air handlers on the same zone and the return temps was changing when one, two or neither handler is running. Now I'm aways at 125 return which leaves me with enough BTU's in the lines to satisfy both air handlers. With a constant return temp, my tanks stay stratified where before I would end up with 4 tanks at 170, 160, 150 etc. as the bottom temps would change with the non-steady temps. With no load on either air handler I would return 175 degree water to the bottom which would stir the tanks and totally destroy the 190 degree water at the top.
  16. stee6043

    stee6043 Minister of Fire

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    I like this information. I installed a dimmer on my HX loop last night as a temp fix until I can buy a three speed pump. I'm back to my low flow/cold return setup. In case anyone is interested I found that my Taco 007 would not run with a fan speed controller on anything other than high speed. But with a light dimmer it runs anywhere from 50-100%. Just an FYI...
  17. DaveBP

    DaveBP Minister of Fire

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    How hot does the dimmer get after the circulator has been running a while?
  18. stee6043

    stee6043 Minister of Fire

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    Not hot at all. The dimmer is rated for 600W (cheapo switch style unit) and I do not believe the fact that it's a motor on the other end has any impact on the switch. Whether the dimmer will affect the life of the pump remains to be seen...
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