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HVAC retrofit – how to understand comparative performance of different solutions

mountaineer1 | Posted in Mechanicals on

I would appreciate help understanding how to comparatively evaluate performance effectiveness trade-offs for different HVAC retrofit solutions (in addition to cost which is easy to understand).  House currently has 4ton supply only 2 cycle heat pump/fan coil.  

HVAC retrofit potential/proposed solutions:
1.  Keep current 4t 2 cycle heat pump, and ducting and integrate an HRV to supply per Bruce Manclark to create integrated balanced ventilation.  Replace existing spot exhaust fans with Panasonic Whisper Greens with humidistat controls.
2.  Install a higher efficiency variable speed heat pump system, integrate an HRV and replace spot fans as in option 1.
3.  Install a smaller higher efficiency variable heat pump system, downsize existing supply ducting and reduce the number of supply vents, integrate HRV and replace spot fans.
4.  Instead of integrating an HRV to either the existing 2 cycle or new variable forced air, install a fully ducted separate Zehnder HRV system. 

We are working to solve an exterior closed soffit moisture problem for our mountain house that was completed in 2014 in climate zone 5B that is also a wildfire and heavy snow area.  The house is 3 stories cut into a ridge with the top floor as the main living space (2100 ft2), the bottom floor is storage (650 ft2), and the middle floor is composed of an encapsulated crawlspace (1000 ft2) and occasional use space (380 ft2).  Additional house characteristics 1.8 ACH, all ducting is inside conditioned area, flat unvented cold roof with insulation on exterior, high performance walls and glazing.  Internal relative humidity measured by psychrometer is over 60% in summer with AC.

Remediation is focused on additional envelope sealing both inside the house and outside the house to stop communication with soffits and HVAC retrofit.  

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  1. Expert Member
    Dana Dorsett | | #1

    Downsizing the ducting from an existing oversized system isn't necessary when down-sizing the heat pump. Low duct velocity isn't a problem unless the ducts are outside of conditioned space, in which case moving the ducts inside would take priority. In your case that is not a problem.

    Integrating the HRV to the heating and cooling system isn't usually the most efficient or best way to go about it.

    A localized moisture problem is almost never an HVAC problem and only sometimes a ventilation problem in a 5B climate. Can you describe the problem in some detail? In most 5B locations summertime outdoor dew points average well below 55F, which would correlate to 50% RH @ 75F. At what indoor temp are you getting 60% RH? (The temperature to which the humidity is relative.)

    Even derated for altitude 4 tons seems like quite a bit of heat pump for a tight, better-than code ~3200' house, though if it's not a cold-climate type compressor it may need to be that big if the 99% design temp is well into negative digits F (is it?). Does it have resistance heat strip backup built in? If yes, does it ever kick in? Have you measured the duty cycle against outdoor temperature on nights when it's cold, but no auxiliary heat is being used?

    1. mountaineer1 | | #2

      Thank you. Our current daytime exterior humidity is 15% at 100F and indoor temp is 73. The cooling system does not show evidence it is condensing out internal moisture although it does cool (and heat) the house. The external moisture problem is drying out right now, but it is primarily active in our snowy winter. The auxiliary resistance heat kicks on at 20F and it comes on rarely.

      HVAC local experts have made a variety of proposed causative drivers of internal retained moisture: Heat pump short cycling, duct leaks, oversized heat pump, oversized ducting, too many supply vents, air intake not appropriately getting into house, no systematic exhaust, low effectiveness of force air return. In terms of the current system we are doing a variety of tests now (air filtration removed to test return effectiveness, setting mechanical air intake damper to locked open) to understand it.

      Understanding that a separate ducted balanced ventilation system is more effective than an integrated ventilation and forced air system, are there any "yardsticks" of comparative effectiveness for a lay person (me) to use?

      1. Expert Member
        Dana Dorsett | | #10

        >"...exterior humidity is 15% at 100F ..."

        That corresponds to a dew point temperature (one way to express the absolute, rather than relative humidity) of 44F.

        >"... indoor temp is 73."

        60% RH @ 73F corresponds to a dew point of 58F.

        The 44F dewpoint outdoor air is MUCH drier than the 58F dew point indoor air. Ventilation would have a NEGATIVE latent load, only sensible load. Barring excessive indoor sources of moisture the indoor humidity should be solvable simply by increasing the ventilation rate. An independently operated HRV is fine.

        In a tight house exhaust-only ventilation carries some risk if the ventilation path is unknown. Often the seam between the slab and foundation wall becomes a significant part of the path, bringing in soil gases including humidity & radon rather than clean-dry outdoor air.

        Sometimes in a retrofit it's less expensive and easier to install ductless HRVs such as the Lunos e2 (which works in pairs that switch the direction of flow every minute or so, one pulling air in while the other exhausts). Having multiple units is similar to having multiple registers, placing the HRVs where ventilation is needed the most. With the low-impedance paths of that type of HRV it can be combined with a modest amount of exhaust-only ventilation (say, a continuous low cfm bath fan), since the major path of the air drawn by the exhaust fan is guaranteed to be via the HRV.

        ASHRAE 62.2 calls for 0.03 cfm per square foot plus 7.5 cfm x (number of bedrooms +1). Building Science Corp calls out that as excessive, and prescribes 0.01 cfm per square foot + 7.5x (#B.R. + 1.) For a 3 bedroom 3200' house the BSC prescriptive would be 32 cfm + (7.5 cfm x 4)= 62 cfm,. which is about what you get out of 3 pairs of Lunos e2 at max speed. That's almost surely going to be enough to solve the humidity issue unless you have a tropical jungle garden indoors.

  2. MattJF | | #3

    I still am not picturing the issue you are having. You say “solve an exterior closed soffit moisture problem for”

    Tell us more about where this soffit is (roof?) and how it is constructed. You said flat roof, so this soffit is really confusing. Pictures?

    Exterior moisture problems are not usually solved with interior HVAC. While maybe not optimal, I don’t think hvac modifications will have any effect. What is your winter indoor RH?

    If interior air might be getting into the soffit, is there a spot you could hook up a blower door fan to pressure and pump the soffit full of fog from a fog machine. The holes will quickly become obvious.

    1. mountaineer1 | | #5

      Thank you. We have evaluated the envelope thermographically with blower and have a clear remediation plan for the envelope. We are addressing both the envelope and the HVAC.

      My struggle is how to comparatively differentiate the HVAC retrofit performance trade-off choices in addition to price and ideal systems as we reduce indoor humidity. The indoor RH remains relatively constant winter and summer. Lack of other sources suggests current source of the indoor humidity is human. There was a humidity source in the crawlspace, but it was spray foam encapsulated early 2017.

      I apologize if I have provided a red herring by referencing the soffits.

  3. user-1110677 | | #4

    I am sorry you provided so much data, but so little usable information.

    Your system is sized for heat load and not cooling. For cooling it is significantly oversized.
    Your moisture levels RH: 60% technically passible, but you want it lowered. Your house is already pretty tight at 1.8ACH.

    I guess you need a ManJ to determine the correct size, next for someone to draw your ductwork and check the distribution system.. I do not care if your Heatpump two stage or even one or VS ... it should be lowering you RH.

    Ventilation is nice but you need functional heatpump. And my understanding it is not. HVAC dealer is telling you that it is short cycling.

  4. mountaineer1 | | #6

    Thank you. What is a ManJ?

  5. Expert Member
    Akos | | #7

    Oversize heat pump or not, if there is enough cooling load, it should control humidity.

    From what the OP is saying (no condensation collected), my guess the AC is either not set up right, that is too much blower flow rate, or the system is programmed to run the fan all the time (condensate re-evaporation from the coil each time the compressor cycles off).

    Both are reasonably easy to to check before digging much further. The good thing is the air there is pretty dry and hot, the air leaks in the house should actually help with drying the place.

    Removing the filter is the wrong thing to do in this case, it only increases the flow across the AC coils lowering moisture removal.

    1. Jon_R | | #9

      +1 on this solution (plus source control). And if it's not enough, add a dehumidifier. If Winter humidity is the problem, then consider a HRV.

  6. MattJF | | #8

    Thanks for clearing up that you have other actions to address the soffits.

    It's not super clear what the HVAC problem is. It sounds like you would like the indoor RH to be lower.

    1) Do you find the summer RH uncomfortable?
    2) Or are you worried about building moisture issues. 60% RH in summer usually not really a problem for moisture. In winter, 60% RH at 68F might present condensation issues. Are you seeing water on you windows in the winter?

    For the summer issues you see right now, what is the temperature you get when you insert a digital meat thermometer in the supply register closest to the air handler? What is the speed setting for the fan? You may need to turn the fan speed down.

    Given your local very dry climate, you should be able to ventilate your way out of any moisture issues at the expense of additional energy for sensible cooling or heating. I would have someone verify the flow rate though your fresh air duct. An HRV won't dry your house out any better than a fresh air duct. It will just do so with a reduced energy penalty.

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