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  1. #1
    Join Date
    Aug 2013
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    61

    Confused are heat pumps more prone to mold problems

    I am a relatively knowledgeable layman, but I know when I am in over my head and this is it. I have gotten seriously conflicting advice from my local HVAC pros regarding the situation described below, so any guidance would be greatly appreciated.

    We had a two stage 16i Trane heat pump (with electric backup) in our home from 2007-2010 that had an intractable mold problem. The coil turned completely black after just one year, we replaced it, and it turned black again after another year. We had over a dozen HVAC contractors come out and try to address it, but other than giving a name to the problem (Dirty Sock Syndrome, as you probably all know), they were unable to do anything about it. We did all the standard stuff (UV lights, etc.) but nobody could fix it. For reasons unrelated to this problem we had to sell the house anyway, so finding the cause of this became a moot point. But it really was a nightmare. It stunk all the time and caused some health problems.

    Fast forward to today. We have just purchased another home and are putting a new system in (we are reducting the entire home as well). We would like to get a heat pump, but we want to make sure that we do not repeat the experience that we had before. In reading these forums, it seems to me that although any system can have mold in it, and although having a competent installer is the most important thing (I really do get this one!), heat pumps still tend to be more prone to mold problems than standard furnace/ac configurations. So our initial inclination was just to avoid heat pumps altogether. However, we do find the blasts of hot air coming from a standard furnace in the winter to be uncomfortable, and so as far as comfort is concerned we would prefer a heat pump.

    So assuming that I am correct that, in general, heat pumps are somewhat more prone to mold problems than standard furnace/ac configurations (I know that not everyone agrees with this), I am trying to determine WHY this is the case - because maybe our particular configuration in our new home will not have these issues (and then we can still get a heat pump).

    I have read (or been told by HVAC pros) three theories why heat pumps tend to be more prone to mold:

    1) Most heat pumps have the condensing coil before the blower motor, so the coil is under negative pressure. This clearly is going to make drainage more problematic than if the coil is after the blower motor, and under positive pressure. I realize that if the condensate line is trapped properly, that even a coil under negative pressure SHOULD drain properly. But things don’t always work in practice as they should in theory, and it still seems that having a coil under negative pressure is just asking for drainage (and therefore mold) problems.

    But if this is the main reason why heat pumps are more prone to mold, then this is good news for us because our backup heat is propane, and so our coil will be after the blower motor anyway.

    2) Another theory I have been told is that the condensing coil in heat pumps never gets hot enough in the winter to kill any mold that has begun to grow in the summer (unlike the situation in a gas furnace). In fact, I was told that the gentle warming of the condensing coil in the winter actually just fosters any mold that began to grow in the summer.

    But again, if this is the reason that heat pumps are more prone to mold, then this is good news for us, because our backup heat is propane, and so the coil will get heated (at least sometimes, when the backup heat comes on) to a high temperature, just like in a regular gas furnace (and unlike heat pumps with electric backup, where the heat strips are after the coil). True, it won’t happen every time the heat is on like in a regular gas furnace, but it will certainly happen a number of times throughout the winter (we live in Nebraska), and hopefully this will be enough to kill anything that started growing in the summer.

    3) The other theory I have been told is that heat pumps tend to be more prone to mold because the defrost cycle in the winter creates a problem. The condensing coil is obviously warm when it is in heat mode, but then it cools when it goes into defrost mode (creating moisture on it, of course), so when the coil heats up again you have a warm but moist environment for mold to grow on. If this theory is correct, then we might still be more prone to mold problems in a heat pump even with a propane backup.

    So this is the dilemma that I am trying to resolve. My wife is dealing with some health problems that make us want to avoid mold, yet those same health problems make her very sensitive to temperature swings, and so the gentle warming from a heat pump in the winter is very preferable. We don’t want to repeat the experience we had with our previous heat pump and so, as I said, our initial inclination was just to stay away from them. But if the issues that caused the mold problem previously are not concerns in our new house, we don’t want to give up comfort unnecessarily. Items #1 and #2 above will not be issues in our new house, but item #3 will still be. So I am not sure what to do. Any advice would be greatly appreciated!

  2. #2
    Join Date
    Jul 2010
    Location
    nebraska
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    1,629
    Dirty sock is from a type of bacteria not mold. It's very rare. Matter of fact the company I work for has been in business nearly 60 years and we have replaced two coils for being smelly in that time. Both were on heat pumps. Smelled like garlic to me.
    Coils turning black is corrosion from indoor air quality problems.

    Air handlers and coils on top of furnaces with poor fitting filters, or lack of frequency changing them, are more prone to biological growth. The insulation in the cabinet gets dirty and it's moist so perfect for growth. Keep them clean and there will be no growth, nothing for it to feed on.

    Hope that helps clear up some confusion.

  3. #3
    Join Date
    Jun 2006
    Location
    Richmond, VA
    Posts
    2,914
    No to all 3.

  4. #4
    Join Date
    Aug 2013
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    61
    Quote Originally Posted by martyinlincoln View Post
    Dirty sock is from a type of bacteria not mold. It's very rare. Matter of fact the company I work for has been in business nearly 60 years and we have replaced two coils for being smelly in that time. Both were on heat pumps. Smelled like garlic to me.
    Coils turning black is corrosion from indoor air quality problems.

    Air handlers and coils on top of furnaces with poor fitting filters, or lack of frequency changing them, are more prone to biological growth. The insulation in the cabinet gets dirty and it's moist so perfect for growth. Keep them clean and there will be no growth, nothing for it to feed on.

    Hope that helps clear up some confusion.

    Thanks for your reply. Coincidentally our new house is in Lincoln, which from your username I assume is where you are too.

    In our previous place where we had problems, we changed the filters quite frequently. It may not have been Dirty Sock Syndrome, all I know is that it stunk terribly, the coil was black, and it was worse when we first started up the heat for the winter. We did everything we could to mitigate the problem but were not successful.

    Many professionals I have spoken with say that heat pumps are not more prone to this problem than standard furnaces. But others insist that they are.

    Of the three things I listed, it seems to me that the first one (the coil being under negative pressure) has to be a factor. Isn't this just physics? Drainage is bound to be more difficult if the blower is sucking condensate away from the drain as compared to pushing it down the drain, and poor drainage can lead to the kind of problem I described.

    The other two things I listed could just be "old wive's tales." But I don't see how being under negative pressure could not be an issue. In fact one manufacturer actually brags that they fixed the mold problems that are inherent in heat pumps by putting the coil after the blower. So I have actually been taking that as a given, and am trying to determine if the other two issues are valid concerns or not.

    Do you think I am missing something?

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,069
    In many areas people have gas or oil fired hydronic baseboard heat. And a central air conditioner, which has the coil before the blower. those A/C only units don't are not more prone to DSS then a gas or oil fired hot air furnace with an A/C coil on them under positive pressure.

    Your coil turning black in your other home may have been from Chinese made drywall. A common problem for a while.
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  6. #6
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    Jul 2006
    Location
    Virginia
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    4,665
    The trane hyperion air handlers have the blower before the coil
    We really need change now

  7. #7
    Join Date
    Aug 2013
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    61
    Quote Originally Posted by beenthere View Post
    In many areas people have gas or oil fired hydronic baseboard heat. And a central air conditioner, which has the coil before the blower. those A/C only units don't are not more prone to DSS then a gas or oil fired hot air furnace with an A/C coil on them under positive pressure.

    Your coil turning black in your other home may have been from Chinese made drywall. A common problem for a while.
    Interesting that you mention Chinese drywall. We had thought of that too. The person we bought the house from built it himself, and he did a terrible job in many ways (we found out later that his day job was as a nurse, not a contractor!). We had a lot of other problems in the house unrelated to the mold in the heating system that we put in. He did sub-standard work in some areas and did use some cheap materials (after we moved in neighbors told us that he started out with plenty of money to build his dream house but then went through a divorce and was very tight for money at the end). So Chinese drywall would have been right up there with the pattern we saw in other areas.

    However, this house was in Kansas, and from Googling Chinese drywall, my understanding is that Kansas is not one of the states where this was an issue. Am I wrong about this? Because if so, Chinese drywall would be a very good explanation for what we experienced.

  8. #8
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    Jan 2004
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    Lancaster PA
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    Can't tell you if any made its way to Kansas or not. Was all of that drywall bought local, or did he shop out of state also.
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  9. #9
    Join Date
    May 2009
    Location
    Savannah, Ga/H.H. Island, S.C.
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    1,486

    are heat pumps more prone to mold problems

    If you have a properly sized system with proper fresh air changes - you will not have humidity problems.

    No excessive humidity = No mold.

    I suspect your old system was oversized. Did anyone perform a manual J load calculation to determine if it was the correct size?

    Even if the equipment is already installed, be sure to get a load calc on your new home to make sure it is sized correctly!

    Maybe Teddy bear will come along and offer some more assistance. He is very knowledgable on mold, humidity, and fresh air infiltration.

  10. #10
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    Aug 2013
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    61
    I assume it was local

  11. #11
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    May 2004
    Location
    south louisiana
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    3,186
    I like heatpumps and have a hp unit in my home.
    yes we have these same problems along the gulf coast.

    You have the answers for a heatpump-
    place coil under positive presssure,
    trap is sized for the static pressure of blower.
    measure static pressure and oversize trap to assure this problem is eliminated.

    If you want a sure fix, consider a variable gas furnace and
    lower the bonnet sensor for the fan to come on at a lower temp.
    most are set at 165, set at 145 for the softer heat.
    may need to add humidifer for your climate, long run times.

    have this new to you home tested for air infiltration and then size
    fresh air requirements.
    fresh air should be ducted to the return side of new a/c system.
    this fresh air can be filtered thru an air exchange unit to recover lost heat -
    ask for performance data as some manufactors do show a significent savings.

    chinese drywall attacked ALL metals. disimilar metals seems to show the black covering before pure metals.
    did you notice any black on decorative metal objects or utencils?

    best of luck.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  12. #12
    Join Date
    Aug 2013
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    61
    Quote Originally Posted by Brent Ridley View Post
    If you have a properly sized system with proper fresh air changes - you will not have humidity problems.

    No excessive humidity = No mold.

    I suspect your old system was oversized. Did anyone perform a manual J load calculation to determine if it was the correct size?

    Even if the equipment is already installed, be sure to get a load calc on your new home to make sure it is sized correctly!

    Maybe Teddy bear will come along and offer some more assistance. He is very knowledgable on mold, humidity, and fresh air infiltration.

    We did not have an excessive humidity problem in our old system. We purchased a very accurate humidity meter (not the cheap ones you get from Radio Shack; it was a meter used in scientific labs), and the relative humidity was routinely in the lower 40s in the summer. It was a two stage system and we always just turned the temperature up or down a degree at a time to keep it near set point and therefore in the lower stage. So excessive humidity in the home was not the issue.

    This is why this was all so perplexing. There were just no obvious reasons why the coil should get so moldy as it did. All I can think of is that it was not draining correctly because it was under negative pressure. I realize that there are tons of other systems out there under negative pressure that drain just fine. But maybe being under negative pressure is just more problematic (the condensate line must be trapped properly, for example - this is a perfect example of a negative pressure configuration being more problematic), and our installer just missed something.

    Note that when the unit was on and I stood outside and looked at the condensate line, no water would appear to come out for 20 minutes at a time - even though it this was in the middle of summer and presumably the condensate trap was already full. And even then barely any water came out.

    Also we kept the fan running 24/7 (as I was told this would help prevent mold growth); maybe that kept it from draining correctly.

  13. #13
    Join Date
    Aug 2013
    Posts
    61
    Quote Originally Posted by energy_rater_La View Post
    I like heatpumps and have a hp unit in my home.
    yes we have these same problems along the gulf coast.

    You have the answers for a heatpump-
    place coil under positive presssure,
    trap is sized for the static pressure of blower.
    measure static pressure and oversize trap to assure this problem is eliminated.

    If you want a sure fix, consider a variable gas furnace and
    lower the bonnet sensor for the fan to come on at a lower temp.
    most are set at 165, set at 145 for the softer heat.
    may need to add humidifer for your climate, long run times.

    have this new to you home tested for air infiltration and then size
    fresh air requirements.
    fresh air should be ducted to the return side of new a/c system.
    this fresh air can be filtered thru an air exchange unit to recover lost heat -
    ask for performance data as some manufactors do show a significent savings.

    chinese drywall attacked ALL metals. disimilar metals seems to show the black covering before pure metals.
    did you notice any black on decorative metal objects or utencils?

    best of luck.

    Do you need a trap if the coil is under positive pressure? I am under the impression that this is optional when under positive pressure, especially if the condensate line drains into a utility room (which our new system will). In this case the lack of a trap is really just like a small leak in the system, but there will be a number of turns in the condensate line between the unit and the drain so this will be minimal. And also, it will be heating/cooling conditioned space anyway so a small "leak" is not a big deal. And then not having a trap will insure there are no drainage problems caused by the trap.

    I do understand that if the coil is under negative pressure that a trap is mandatory. But we will be under positive pressure.

    We were not planning to install a fresh air exchanger. Unlike most people though, we do open our windows and air the entire house out every week or so, even in the winter. As long as we do this, do you think a fresh air exchanger is still something that you think is important to have?

    As far as Chinese drywall is concerned, we did not notice any problem on any metals anywhere else in the house. That's why we didn't think this was the culprit.

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