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You guys (and gals) with Water to Air HX experiance

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by ISeeDeadBTUs, Jan 7, 2010.

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  1. ISeeDeadBTUs

    ISeeDeadBTUs Guest

    Recently I heard someone say (not on the Net) that with hot air systems, you need a higher VOLUME of heated water. I shall not purport to understand that, but I am curious about your experiances.

    What made me think to ask is the post from a GreenWood operator who is having some trouble with his hot air system. These Refractory Mass Natural Draft unit have a very low water volume. The GW100 claims to have only 8 gals.

    I know nothing about pump size/speed nor W2A HX so I am hoping some will chime in with their findings and some ideas for the member in duress.

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  2. ihookem

    ihookem Minister of Fire

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    Don't know about volume but the temp has to be 140 or it just doesn't warm the house much . It just runs the fan. I would love to go hydronic and have 130.Some say 130 is plenty warm with hydronics .
  3. pybyr

    pybyr Minister of Fire

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    Volume seems irrelevant as long as you have the needed BTUs on tap in the water (temp, + ability to maintain the temp under flow/ load) and if you size your water-air HX to deliver the needed BTUs at the water temps you expect. Water-air HX's tend to be spec'd assuming 180 F or hotter input water, as that's the norm on fossil-fueled boilers; to extract the same BTUs at lower temps, you need a bigger HX.
  4. rwh442

    rwh442 Member

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    I will throw out some real world numbers from last night for you to ponder.

    I have a water to air HX, 16"x18", in the furnace plenum. I do not know the blow CFM. Blower is variable speed.

    At 180 deg supply, blower ran 100% for 6 min with 21 degree drop at 5.5 GPM flow so that was about 33 gallons.

    The blower then slows down to maybe 50%-60% for 2 minutes with a 13 degree drop so approximately 11 gallons used there.

    That's quite a bit more than 8 gallons used but I would assume the Greenwood could reheat 8 gallons pretty quick. Dunno.
  5. sdrobertson

    sdrobertson Minister of Fire

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    I know this is a ongoing debate with the Seton design and using storage but this seems to be where the numbers don't add up. With 8 gallons of water it seems that if you have a large heat load, you would be really working the boiler hard during calls for heat which is good, but after the heat load is satisfied, it wouldn't take long for the boiler to heat that 8 gallons up and then go into a idle. A heat exchanger in a furnace will take allot of BTU's while the furnace fan is on but it shuts off for extended periods of time after the thermostat is satisfied so no load.
  6. ohbie1

    ohbie1 Member

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    My original OB hybrid sys. came with a pretty standard single speed Grunfos pump (26-64) for the 1st floor zone and an even smaller 15-24 for upstairs. It's always been fine, and continues to be with the Tarm.
    I agree with ihookem . In my experience the water temp is key. The hx just can't extract enough BTUs from water under 140 °F to heat the house properly.
  7. Deere10

    Deere10 New Member

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    Thanks Jimbo found this one too.. As stated yes after the 130 to 140 degree mark the heat givin off is minimal. Dont see huge temp drops when fan is running. But does seem to run for 5 mins or so to come up 2 or 3 degrees in house at tstat(digital so some what accurate) Recovery of the 8 gals seem ok. pump on boiler is a Taco 007, Pump in house is a B&G PL236 maybe cant remember exact number but will look and post. Water to air HX is an 18x20 3 pass core.
  8. Deere10

    Deere10 New Member

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    PL 36 pump in house Dont know where i got 236 from..
  9. Frozen Canuck

    Frozen Canuck Minister of Fire

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    I think the problem is most coils or ready to install fan coils are sized to work with a fossil boiler that operates say 140 - 180. With a very small water capacity.

    On & off a lot, but thats ok, they were built for that & the fuel source suits this constant on/off cycle.

    Wood burners want a bigger usable temp range say 100 - 200. As well as a lot more water capacity (storage). So we can fire less & rest/enjoy more. :coolsmile:

    So when we go looking for coils we need to be thinking bigger, both in coil size & fan CFM.

    Bigger coils have more btus available to use. Bigger faster fans (more cfm) can extract more btus out of whatever coil is available.

    With my own set up, once I knew what the fossil burner chart said I needed for a fan coil size & cfm of fan to push through it, I looked at what I wanted to use for water temp minimum 100. That was nowhere to be seen on the fossil burner chart.

    Then I went looking for a fan coil & fan cfm that would deliver the btus at that minimum 100 temp. I found that I almost needed to double the size & cfm that the fossil burner chart said I would need for a fan coil with water at 140-180.

    So long story short I bought the second largest residential model of fan coil that they made, according to the fossil burner chart this was way too big. I am glad I stuck it out & went with the bigger fan coil. Now I have a usable water temp range of 100-200. That equals less firing for me & more rest/enjoy time. Win win. Love those. :coolsmile:

    Only one downside so far, the boss likes her showers/baths hot enough to cook a lobster in, that is not easy to do with lower water temps, so I may have to move the bottom temp up to keep the peace, as well as keep my head attached to my body!!!!!! :bug: House temp is one thing her bath/shower temp is a totally different matter :red: . I kind of like my head attached to my body, so I best do all I can to keep it that way. ;-)

    One last thought, this GreenWood owner needs to give serious consideration to what Heaterman & others remind us of.....storage......no way is 8 gallons going to get it done with a wood burner. Think minimum 1000 gallons for starters, with plans to expand that in the future. :roll: Nobody wants their wood burner to cycle on/off like a fossil burner, right? :-S
  10. pybyr

    pybyr Minister of Fire

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    For those who are putting a water-air HX into an existing forced air system (rather than a pre-built fan-coil all in one enclosure), you can go very big on the coil:

    http://www.hearth.com/econtent/index.php/forums/viewthread/25090/

    My goal was to be able to transfer a lot of heat from water to air even at low water temps, and with minimum possible back-pressure for the blower, in order to be able to run lots of air CFMs at low velocity/ low noise-- and it seems to work.

    Still need to tie my storage to boiler (working on it- will be completed soon!) but at 154 (the min temp at which my boiler will circulate to the HX) the house starts warming up rapidly.
  11. mwk1000

    mwk1000 Member

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    I am having very good results with the HX in my existing furnace plenum. I have 20x20 HX 3 rows. I have the 90% efficiency propane furnace that I use the "low" speed line that was not used by the furnace rated at 1100 CFM. I also get usable heat down to 100 degrees from my storage tank. The furnace runs shorter cycles than it did when it was on propane. The Propane NEVER output higher than 85-90 at the ducts. That is what I get at 100 degrees, I have consistently measured about 15 degrees lower than the tank temp at the ducts.
  12. Gooserider

    Gooserider Mod Emeritus

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    Can't do personal experience, but my understanding of the theory would be "maybe..." Assume otherwise identical houses, one w/ baseboard, one w/ in-floor, and one w/ a water / air HX in a hot air system - since the houses are identical, they should need the same BTU/hr to keep them warm...

    What kind of water consumption are they going to use?

    The infloor only needs low temps, so presumably it would be doing a mixing valve setup, and just dribbling in the hot water, and returning very cold water to the boiler, so the house would be using a very low volume, but you are going to have some additional consumption because you'd be needing a lot more return water protection mixing supply water into the return... Water volume is going to be determined by the ΔT of supply and return, as in how many GPM it takes to carry the required BTU/hr, plus the extra for the return water protection. (Actually I think if one just figured the ΔT of supply and return to the boiler, the return protection would cancel out)

    The baseboard will need hotter water, and more of it going to the house, but will be returning hotter water to the boiler, so the return water protection should be less... However because you have a lower ΔT of supply and return, the volume should be higher in order to carry the BTU's...

    The HX also needs hotter water, and acts almost like the baseboard, except that it is concentrating the heat dump into one little area instead of spreading it around the house... So the question would be how does the ΔT work out - if it's about the same, then the volume would also be, etc... However, as a complicating factor it is at least claimed that forced air systems have higher rates of heat loss than hydronic systems, due to duct losses, and the greater infiltration caused by blowing the air in the house around - thus the W2A HX house would need addtional BTU/hr to compensate for the losses, beyond the amount needed for comfort - if you are having to flow more BTU/hr into the HX to get the same comfort level out, that is obviously going to increase the volume demand - but I have no idea by how much, and probably it would be very system dependent...

    Gooserider
  13. Tony H

    Tony H New Member

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    On my unit the water temp must be 140F or higher to make usable heat and the 3 speed pump is set on high giving me the most heat output. last night with a temp of -2f the cycle run was about 12 min off and 6min on
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