Increasing Cycle Times

Discussion in 'Maintenance and Troubleshooting' started by euler7, Sep 17, 2011.

  1. euler7

    euler7 New Member

    I am a newbie here and have a question about things one can do to encourage longer cycle run times with a GSHP, particularly in cooling mode to wring out as much humidity as possible.

    Some background (warning: perhaps relevant, perhaps not, and most likely far too long):

    We had a 5-ton GSHP (2-speed FHP -- model AP061) closed-loop slinky design (5 x 750' @ 3/4") system (with desuperheater) installed in our new construction home in spring 2010. The location is eastern Iowa, where it often can be hot with high dew points in cooling season and quite cold in heating season. (Mid Sept through late Oct sure is nice, though!)

    The house is 2-story plus full finished walk-out basement, with 4000 sq. ft of conditioned space total. Our system is split into two zones, one for the upper level and one for main level and basement, each voting and controlled by a Honeywell VisionPro 8000 t-stats. R19 bat insulation in the exterior walls and R41 blown-in in the attic. No ERV/HRV.

    We are happy with the system so far and with how low our electric bills have been. We are also pleased with our installer, whose shop also did all our HVAC (duct work, bath fans, etc.) and electrical during construction. They are local and have been putting in GSHP for 30 years and have a good reputation.

    As to system sizing, I believe the initial heat loss/gain calculations were 58K loss, 36K gain. The installer said in our climate they prefer to size systems to cover 80 to 85 percent of heating needs.

    At any rate, in our experience so far (one winter, two summers), the system seems perfectly sized for heating but perhaps somewhat oversized for cooling. The performance data in the FHP spec sheets seem to bear this out. Hence my question about increasing cooling run times.

    Before we had our house built, I knew almost nothing about HVAC and general construction. Our general contractor was a good guy and I learned a lot along the way by asking questions and doing research. I still don't know much, but I know a lot more now than I did before -- enough to know I would do a few things differently with the house, but nothing major.

    As to the humidity issue, the house is never uncomfortable in summer. But there are times when RH does not want to drop below 55 to 60. This tends to be at night and in the morning before regular cooling calls are made. During warm days with long cooling runs, RH is right around 50 but never seems to drop lower..

    A great source of knowledge for me has been this forum. Cooling season is pretty much over for us, but I did pick up some tips here the past couple of weeks that I am eager to experiment with next summer.

    1. I set the t-stats' compressor CPH to 2 (first stage) and 1 (second stage). On an unseasonably warm day last week, this really seemed to help. Granted, the outdoor dew point was lower than the dog days of Jul or Aug, but indoor RH dropped below 50. Next spring, I may even drop first-stage CPH to 1 as well. I don't think I want it that low in heating mode.

    2. I also set the t-stats to run the fan for 90 seconds after the end of cooling call. But I keep the system in Auto, not Fan or Circ which I understand can be a bad idea because it reintroduces moisture off the coils back into the duct system before the coil has time to drain.

    Beyond these tips, I have questions about a couple of other options:

    1. Would setting our unit's DIP switch to low/dehumidify mode (which drops fan speed by 15%) be worth trying? Air flow seems excellent now. How could one tell if air flow were reduced to a worrisome level? Also, would lowering the speed run a risk of freezing the coil?

    2. Our loop system has a manifold. Is it possible to shut off one of the loops, in effecting turning it into a 4-loop system for cooling, then opening it again for heating? I suspect this one is a bad idea and I would not try it on my own without calling our installer. The 5-ton heat pump would be mismatched with a 4-ton loop in cooling season.

    3. Does the fact we have two zones play a factor in how well the system dehumidifies? The compressor lockout is set at 5 minutes (its max), but I was wondering if Zone 1 finishes a cooling call, then a couple of minutes later Zone 2 makes a call, would that pull moisture off the coil that did not have time to drain from the end of Zone 1's prior call?

    4. I do have a couple of portable dehumidifiers, which intermittently run in the basement to keep RH <= 55. Without them, basement RH would probably be 60 on a bad day. I have thought about a whole-house dehumidifier, but that seems to be overkill, especially since RH is controlled to where is is never uncomfortable. I just tend to obsess about these things and like the idea of keeping RH <= 50, even though it feels fine at 55.

    5. Since it is a brand new house, I really can't justify taking off the siding and reassessing the thermal envelope. One thing I would probably do differently next time is use polystyrene panels behind the Tyvek paper. Not sure if that would affect possible infiltration anyway, but it seems like good practice. I would also consider putting in ERV/HRV, though I understand that does not automatically reduce indoor RH either.

    Sorry for a post that turned out to be a (bad) novel! I appreciate any thoughts any of you can share.

    Dan Kitzmann
  2. geome

    geome Member Forum Leader

    If you are using the factory default fan settings, I would think a 15% speed reduction is fine, (or the manufacturer wouldn't have included this feature.) It made a noticeable difference in our run time, but so did reducing our thermostat set point by 1 degree. We prefer full speed in cooling mode, but you may like the 15% reduction.

    If your thermostat has a dehumidify in cooling mode setting, you may be able to "over-cool" (cool below your set point) by 1-3°F when the humidity is higher than you like.

    We are perfectly happy in heating and cooling mode with 1 CPH for first stage and 1 CPH for second stage. I, and a member of another forum, have observed that our Honeywell t-stats still run 2 CPH in first stage with this setting. The other member has a 2 zone system.

    If you're comfortable, I wouldn't be concerned about the humidity level reading on the thermostat.
  3. euler7

    euler7 New Member


    Thanks for the feedback.

    Out t-stats, unfortunately, are the VisioPro 8000 models without humidity sensor and control. My RH readings have been off a stand-alone hygrometer. If I did become more concerned about RH comfort levels, I'd consider upgrading out t-stats to the Honeywell IAQ and use the dehumidify overcooling mode you mentioned. It sounds like a nice feature.

    As to the 1-degree temp setback, are you referring to changing the deadband such that, for example, if the t-stat is set at 72, a cooling call is not made until the temp reaches 73 (or whatever increment is set for the deadband)? If so, I wish our t-stat had that feature. I think the Honeywell VP IAQ (one model up from ours) is the first Honeywell VP to feature it.

    Does anyone have experience with coercing an equivalent setback by (manually or by schedule programming) letting the temp rise two or three degrees, then setting it back to baseline so a longer cooling call is initiated? It seems that would reduce humidity more than having a constant temp with shorter cycles, but I'm only speculating. The downside, of course, is you have to babysit the t-stat. Sort of defeats the point of having a good t-stat in the first place. Plus, you experience the temp fluctuations. Sort of defeats the point of geothermal.

    Next spring I will try the slower fan speed on our unit to see how we like it. Our FHP is set at factory default normal speed now. Because the lower speed is detailed in the FHP install manual, like you I would think it should be safe to at least test for a while.

    I'd still be interested to know if turning off the valve to one of the ground loops (going from 5 to 4) is even theoretically a feasible idea. Again, I know precious little about GSHP science, so this might be a laughable and foolhardy notion. I wouldn't do it without asking an expert.

    Generally speaking, is sizing geothermal units tricky in climates that have higher heating than cooling needs, but which still experience uncomfortably humid summers? With a conventional system, it seems it would not be as hard since one could pick a lower ton A/C unit and a larger separate heating system. In our case, I suspect a 3.5 or 4 ton cooling load would be perfect, but that same tonnage would seriously undersize us for heating and really drive up costs with repeated calls to auxiliary strip heating on very cold days.
  4. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    You are unavoidably substantially oversized for cooling. Don't mess with the loop side.

    Feel free to try the dehu mode. The 15% airflow reduction will reduce the system's sensible heat ratio, improving its dehumidification performance. Supply air will be 2-3 degrees cooler and therefore more water will be extracted from it. Run times will increase and efficiency will drop just a bit. However, reduced humidity may allow slightly higher thermostat setpoints, compensating for the loss of efficiency.

    If not already done, consider a blower door test to identify and reduce sources of air infiltration. Air leaks raise humidity during cooling season.
  5. geome

    geome Member Forum Leader

    No - I meant just setting the thermostat 1°F colder to get longer run times (but if you are already comfortable, I wouldn't bother.)

    If your system in cooling mode goes into stage two, you can try the less aggressive cooling mode setting on the Honeywell. This may keep second stage from engaging as quickly, resulting in longer run times (and better dehumidification.) Another possibility is disabling stage two in cooling mode.
  6. euler7

    euler7 New Member

    Good thoughts. Our system ramps up to stage 2 occasionally, though I'm not sure if it does so excessively. Given our cooling needs and the outsized performance of the FHP AP061 even in stage 1 cooling, however, I can't imagine we'd ever truly need a stage 2 call anyway.

    Do you know whether simply changing the t-stat system setup to 2 Heat/1 Cool (heat pump) would disable Stage 2 cooling? Or might that disable Stage 1 instead and default all calls to Stage 2? Would disconnecting the appropriate t-stat relay wiring be a better method?

    Thanks for the input. Next summer I'll try dehu mode.

    I've thought about a blower door test. Though it's a new house, I suspect some attic infiltration. We have a lot of recessed lights on the upper level, which I understand can be a source of leaking if they are not well sealed. Our builder did a nice job overall, but thorough sealing of every conceivable attic leak might be one of those things typically left undone unless explicitly instructed otherwise. Add this to my list of things to pay more attention to if we ever build again!
  7. geome

    geome Member Forum Leader

    Interesting thought. Our Honeywell setup asks for the number of compressor stages, so the setting chosen (1 or 2) would be for both heating and cooling modes.

    That said, when our setting is set to 1 compressor stage, it is first stage that runs. Not sure what your (different thermostat) setting would do. If your system doesn't run much in second stage, it's probably not worth experimenting with locking out stage two. Our system rarely runs in second stage cooling too, except for when it is extremely hot outside. It runs in second stage more often when cooling our house to a comfortable sleeping temperature (we don't maintain a constant set point.)

    I ultimately decided to take Joe's previous (good) recommendation and leave the compressor set on 2 stages and just let the system run. :)
  8. euler7

    euler7 New Member

    That is good advice. I should just let comfort be my guide and not monkey around gratuitously. Tinkering can sort of be a fun pastime -- until you screw something up!

    Our t-stats are setup now for 3 Heat/2 Cool (the extra heat stage is for emergency strip heat). I'll leave them there,
  9. geome

    geome Member Forum Leader

    Changing to 2 Heat may disable the second stage compressor and leave the strip heat enabled as auxiliary heat (as a second stage.) Not a good idea in heating mode (if this is indeed how it works.) But... calender reminders to change settings has worked for me in the past. :cool:
  10. euler7

    euler7 New Member

    I wondered about that as well. Our model specifies 2 Heat/1 Cool (w/ aux heat) for the mode in question, whereas there are other modes for 2 Heat/2 Cool (w/o aux heat) and 3 Heat/2 Cool (w/ aux heat). From this I deduce the 2 Heat/1 Cool mode would jump from Stage 1 heat to emergency heat. Bad idea in winter. But as you said, old-fashioned calender reminders would avoid the problem! :)

    I forgot to ask before about your note on the t-stat's less aggressive cooling setting. Ours calls this something along the lines of "Choose this if room temp is cooler than set point."

    To your knowledge, does this affect staging directly (i.e., keeping it from jumping to stage 2 as aggressively)? Or does it instead alter the set point sensitivity -- i.e., when the set point is at, say, 71, the cooling call ends at 71.2 degrees instead of going all the way down to 71.0?
  11. geome

    geome Member Forum Leader

    We use the less aggressive heat setting to delay aux heat engagement for the purpose of recovering from a nighttime setback. This setting seems to increase the temperature differential at which higher stages engage.

    I forgot to mention that decreasing the CPH will increase the temperature swing. We are comfortable at 1 & 1 CPH, and like the system not cycling as much, so we left the setting there. System efficiency should be better with fewer, longer, cycles. Others, and myself, have noticed better DSH performance as well with fewer cycles. Unit longevity may be better with fewer cycles, but go with the settings you like best.

    Our settings maintain +/- 1°F around the set point (although Honeywell will only show you the setpoint as the "actual" indoor temperature within this range.) We are comfortable with this (+/- 1°F swing), but everyone may not be.
    Last edited: Sep 18, 2011
  12. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    You guys are on this

    .....but I would slow the airflow.

  13. surviverguy

    surviverguy Member

    The humidity can be reduced somewhat doing simple things to minimize air infiltration like putting gaskets on all wall outlets and switches on envelope walls. Few builders do that and it's an easy and cheap diy project which can make a difference to the RH. Dehumidifiers in bathrooms are also helpful during/after bathing. Check the window seals and look for cracks to fill with caulking.
  14. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    slow down the air flow in cooling

    let the thermostat do its job.


Share This Page