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Scorched Air vs Hydrocoil

NYplumberNYplumber Member Posts: 482
Good evening Wallies,

Always pondering, looking to learn & butting heads....this time was butting heads with a contractor. Not buying into his cheap workmanship and manipulation tactics.

In the world of forced error, there are two systems that one usually finds. Those two are furnaces and hydrocoils fed off of a boiler. It is my understanding that the hx on a furnace is no different then a hydrocoil heating the air. One has water flowing through it and the other products of combustion. Given the same surface temperature (coil or furnace hx) one would assume the air in the vent would have the same humidity level. I could be wrong though, and that is why I am writing on the wall. I have read Manual-P on psychometrics, but never polished the information.

Todays "heated" discussion was with a contractor who claimed that with any forced error system the air is dry and one would require a humidifier to add moister to the air.

His claim is that any hydrocoil that has been installed in the homes he built have customers that rave about the air not being dry. I think he and his client are full of hot air.

It is only logical to me, that with the correct airflow, and proper condensing furnace, one would replicate the hydrocoil.

If im wrong, quickly delete this post before he finds it.............only kidding.

Discuss so we can all learn.


  • same difference

    In both systems, there is no contact between the fluid doing the heating (combustion air or water) and the fluid that is distributed to the dwelling (air).

    Accordingly, the relative humidity of the discharged air is simply a function of the temperature. If the temperature of the air that is distributed is the same in both systems, the humidity must also be the same.

    People claim all kinds of things.
  • icesailoricesailor Member Posts: 7,265
    Air Temperatures:

    Its not my specialty.

    However, the HX in a warm air furnace will have exhaust temperatures as high as the combustion flame, regardless of the air temperature of the air exhaust. An Air handler HX will never be hotter than the hot water flowing through it. The hotter HX of the furnace will have a greater effect on humidity/moisture than will the cooler HX of the hydronic air handler.

    Customers may think that their house may be wonderful. Compared to what?

  • NYplumberNYplumber Member Posts: 482
    forced air

    More then conventional forced air.
  • nicholas bonham-carternicholas bonham-carter Member Posts: 6,535
    Warm skin

    If you are sitting in the sun, you will be warmed by heat waves, feeling more comfortable, without a lot of air moving across your skin.

    If you are sitting in front of a hot air supply duct, you will be warmed by the contact between your skin and the air. Such a large volume of air will be flowing across your skin, that more drying will take place (think hair dryer). I think this is the principal reason for adding more humidity to the air, especially when there is a lot of infiltration of dry outside air.--NBC
  • GordyGordy Member Posts: 7,111
    NY Plumber

    I think your on the right train of thought. If both HX's are the same temp the air flow should have same RH.

    Now the variables of the claims are. In the discussion you had is The FA guy taking this into account, and if so were the happy Hydro air people seeing the same coil temps? Same RH in the house itself? New construction with lotsa moisture to purge out of materials yet?

    Apples to apples I just cant see how hydro air vs FA could be different unless coil temp was lower, or it had a leak ;0
  • LimamikemikeLimamikemike Member Posts: 25
    scorched air vs hydro coil.

    We do a lot of hydronic fancoils around here, mostly in retrofits. There is a jump in humidity when a fancoil replaces a natural draft furnace because of the removal in our case of the combustion make up air in the return for the furnace, we replace with direct vent boilers and tankless units (open systems are very common here...) hence combustion make up is no longer required.

    less cold dry air infiltration makes it seem like a hydrocoil adds humidity.

    We also install modulating hi eff furnaces with the exact same effect.

    The required fresh air intake stays but the 6" combustion make up gets sealed up.
  • icesailoricesailor Member Posts: 7,265
    Surface Temperatures:

    The surface temperature of a furnace HX will always be higher than a AH HW coil because the exhaust gas on the inside will always be hotter than the temperature of a copper water HX. The higher temperature should drive the humidity out. The cooler water HX shouldn't remove as much humidity. The OAT in the heating season is always low in humidity, the colder it is outside. There's always infiltration.
  • Harvey RamerHarvey Ramer Member Posts: 1,911

    During heating, the RH of the conditioned space depends on three things. The out door RH, the tightness of the structure and the internal latent loads. As air passes through any type of heat exchanger and gets warmed up the RH drops. The hotter the air exiting the HX, the lower the RH. The Absolute Humidity or grains of moisture per pound of dry air will not change. When the hot air exits the duct, it will mix with the air in the surrounding space. As it does so, it will be cooling off and the RH will be increasing.

    IMO the largest enemy is infiltration and exfiltration. It does go both ways. Also, air is a gas and it will act like gasses do. When it is heated it expands and when it cools it contracts. Now consider what happens when you heat all the air in an entire house. It expands and pushes out through every nook and cranny that it can. As it cools it contracts and sucks in air through every nook and cranny. The outside air during the winter typically has a low RH. When this air is brought into the house and is warmed up to room temp, the RH drops even lower.

    Any kind of air heating system will act this way to some extent. That is why we call it scorched air.

    On the other hand, a system that uses some type of radiation to warm the space will ultimately establish and maintain a MRT. Once the MRT is established, the expansion and contraction of air will be negligible. Depending on the internal latent loads and the tightness of the house, no additional humidification may be needed.

    That is why hot water radiant heating systems are the best, period. They are more comfortable with less effort.

    Ramer Mechanical
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • John MillsJohn Mills Member Posts: 762

    Humidity is lowered by cold air coming in and being heated. Doesn't matter how, what temp it was heated to, etc. The house I grew up in was built in 1911 with a converted gravity system. In the winter, 2 console humidifiers couldn't keep it above 20% in there. My current house has scorched air but is 25 years old, well insulated, great windows. I have no humidifier nor need one.

    Doesn't matter what temp the air is heated to by hydro coil or heat exchanger, it's what the temp is where the humidity is measured that counts. The furnace or coil does not dehumidify. There is no condensate drain under the coil or the heat exchanger.

    Double mike has it right that getting rid of open combustion heating equipment is what makes for lots higher humidity. Those old low AFUE natural draft boilers  and furnaces used a LOT of house air for combustion AND dillution. Seems to me the figure was 30 cu ft of house air for each cubic foot of gas burned on those oldies.

    And the argument that hydronic heat dries less due to not having duct leakage is also valid - if the duct system is outside the conditioned space like so many around here.
  • radmixradmix Member Posts: 189
    Side Note

    Doesn't have to do with humidity but adding a boiler with all the associated material ie. expansion tank, water make up, air elimination, piping, wiring and glycol needed if the air handler is in the attic. You have another appliance to maintain and the cost of the installation doesn't make sense to me if you can just install a condensing furnace. You will also have a loss in efficiency through the heat exchanger.
  • 4Johnpipe4Johnpipe Member Posts: 475
    To hydro or not to hydro

    Pro's of a hydro coil are no combustion or exhaust pipe issues, less gas pipe, no condensation line issues for 90 plus furnaces. You will (in most cases) already have a water heater so why not let it make heat. The HTP Phoenix series has the capability to be your heat source as well. Installed a 199-80 with 5 zones of heat last winter. Average water temp in coil is 145 degrees. I do see the copper coil collecting moisture from the air on the heating cooling cycles. I think it may be a wash comfort wise however 145 degree copper verse 200 or more degree steel...I prefer the copper coil...Did I say modulating condensing water heater? Outdoor reset on an I series mixing valve...BAM!
    Considerate People, Considerate Service, Consider It Done!
    email: [email protected]
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