Missouri System oversized....or something else?

Discussion in 'Maintenance and Troubleshooting' started by jk96, Jan 10, 2015.

  1. jk96

    jk96 Member

    Long story short I'm using way more electricity than expected for my heating and am curious if it could be a problem of an oversized system or if we need to be looking at other issues. Here's the details.

    Current kwh usage is an average of 90-100 kwh per day for geothermal heat only, this is heat only.
    Unit - Modine Geofinity water to air.
    Split 4 ton. 8 tons total.
    Two stage compressor with aux electric heat. Breakers are off for electric heat so it cannot kick on.
    6 zones total. Zone 6 is basement and is never used.
    Closed loop.
    Cycle times for 1 degree rise in thermostat temperature seem to be 12-18 minutes.
    Ground water in - about 44-45 degrees.
    Ground water out - 40-41 degrees
    Supply temperature - 97 degrees.
    (temperature reading from units built in orb controller readings)

    House - 3500 sq/ft. Lots of large windows.
    closed cell spray foam basement walls - 2" - R14
    closed cell spray foam above grade walls - 3-1/2"- R24.5
    closed cell spray foam roof - 5-1/2" R38.5
    House has 10' ceilings. Center of house is open to roofline. 27 foot peak.

    Here's a link to pics of the house to give you an idea of the openness. https://www.flickr.com/gp/jeremykovac/cC3C8U/ Zones 1, 2, and 5 are open to each other due to the vaulted ceiling and open floor plan.

    Floor plan with zoning details attached in pdf.

    Additional info. Last night it was on the extreme end of what we normally see for cold. I shut down one of the units completely and ran off a single unit to see how it performed. Here's the details.

    12am - outside temp 3 degrees, inside house temp 72 degrees.
    Single 4 ton unit running zones 2, 3, and 5, breaker shut off on other unit.

    8am - 11 degrees outside.
    Unit run time - 8 hours stage 1. 4 hours stage 2.
    Thermostat temps on active zones - zone 2 (72 degrees) zone 3 (72 degrees) zone 5 (72 degrees)
    Thermostat temps on inactive zones - zone 1 (68 degrees) zone 4 (67 degrees)

    Attached Files:

  2. jk96

    jk96 Member

    Here's some additional info from today. I continued to run only one 4 ton unit so I have not turned zones 1 or 4 on. As of sundown, about 5:30 pm zones 1 and 4 temperatures were 70 degrees. As of 8:30 pm as I am typing they have maintained at 69 and 70 degrees. The other heated zones have maintained set temps of 72 with unit run time on average of 20-40 minutes depending on if only 1 zone or multiple zones called for heat. Not exactly sure what unit down time was between runs but I would guess around 45 minutes. At sundown zone 5 temp was 74 degrees (two degrees abouve set temp) due to solar gain from large south facing windows. High temp today was 34 degrees.
  3. ohio

    ohio New Member

    beautiful home. HDR photography looks real nice.
  4. jk96

    jk96 Member

    Thank you Ohio. We finished the build in May. I guess I should say moved in in May. I've just about got everything wrapped up. I've been playing with HDR photos for about a year or so.

    No real useful data from last night. My run timers reset anytime the unit shuts down. Checked this morning and zones were satisfied. We continue to heat off one unit.

    Couple of questions if any of the pros are reading.
    1. From what I've read on here and other sites we should be running at or close to 100% of the time during peak loads, such as a couple of nights ago when temps were 0 degrees correct?
    2. What would you consider short cycling? When both units are on (T-state set to 2 stage 1, 3 stage 2) my normal run times seem to be as short as 12 minutes and as long as 30 minutes.
    3. How does heating capacity compare to cooling. More specifically, if a 4 ton unit is at the limit of heating our entire home on the extreme end of cold ( 0 degrees outside), how is the same size unit going to compare when switching over to cooling? (95 degrees outside)
    4. What would the minimum cycle times need to be in order for the desuperheater to be effective?

    Thanks in advance.
  5. birkie

    birkie Member

    I'm a homeowner, not a pro. It looks to me like you are at least 2x oversized compared to economic optimum. Some additional context is necessary to determine if there are other issues that are contributing to your high energy usage. For a given set of temperature measurements, what stage is the unit running in? How many units were running at that time? How many zones were being serviced? For example, your numbers look reasonable if they were taken when only one unit was running, and it was in stage 2 serving most of its zones. Your numbers look worse if it's running at stage 1 into a single zone (and would potentially indicate suboptimal ducting or zoning if that were the case). Also, it would be useful to understand where in the country you are located, and what your loop and pumping setup is.

    ...but as far as being oversized, it feels to me that you almost certainly are, and that so many zones is playing with fire. I don't know your climate, but if you lived in a cold climate with a design temperature of 0, my own opinion of a well designed, economical water to air system would be running 100% of the time well before then (somewhere between 20° and 40°F or so), and start using aux around 5-15°F. My intuition tells me that your house could probably do fine on 3 ton.

    Unless you're in the warm south, Geo units typically have greater cooling capacity than heating. So a unit that is oversized for heating will be even more oversized for cooling.

    Desuperheater is around 10% of capacity or so. In your situation, your units could be putting out ~4k BTU/h from superheat, probably more. If you use 50gal a day, and cold water comes into your house at 50F, then you'd need about 7.5 hours total runtime from either of your units
    (@ 4k) to heat all that water to 120F.
  6. nc73

    nc73 Member Forum Leader

    Sounds to me like the installer didn't factor in how well your home is insulated. Someone didn't do a manual J. At least you have backup if one fails :) I'd be going back to the installer and demand some cash back.
  7. dgbair

    dgbair Just a hobby Forum Leader

    12 mins ... ouch ... that is really short.... even 30 mins is short assuming it's fairly cold outside. (not really sure what you mean by '2 stage 1, 3 stage 2')
  8. jk96

    jk96 Member

    I turned the second unit on today to compare readings and it looks like I have two issues that are working against me. First, I do believe we are way oversized on our setup. Temps outside today are 35 degrees. I have the thermostats set at a 2 (on at 71 off at 73) With only the single unit running my run times today are averaging about 30 minutes, always stage 1.

    In comparing the second unit my run times are the exact opposite. I seem to have an airflow issue as there is very little airflow from the vents. My runtime on the second unit for a 1 degree temp rise was 75 minutes. It took almost three hours to bring the thermostat from 70 to 72. With both units running I have one that is short cycling and another that runs almost non-stop with very little output.

    Birkie thank you for the info. I do know he ran a manual J on the house. He initial called for 6 tons and increase to 8. When I asked about the high tonnage and our insulating factor he said he needed the increased airflow. We were not charged for the increase. I will pick up a temp prob so that I can get some exact readings and will note the units stage and zoning. I'll also find out what I can on the loop field. My location is NW Missouri - Buchanan County.

    I do have a couple of other questions. Even if we get the airflow problems with the second unit fixed it seems that we are still going to be oversized for target run times. Using the assumption of 2x oversized for the sake of discussion, the easiest fix due to the ducting being split between two units would be to downsize units from 4 ton units to 2 ton units. Is there any drawback to running duel 2 ton units vs one 4 ton? What are the effects of the larger loop field? I would assume a larger than needed loop would be a positive and not a negative? What would normal protocol be between customer and contractor for fixing an oversize issue? Assuming we are oversized what should we expect of our contractor to rectify the situation? I'm not expecting a refund even though we may have paid for way more system than necessary, but what about the cost of downsizing units or fixing duct to connect all to one unit? What other questions should I be asking our contractor?

    Last edited: Jan 11, 2015
  9. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    The issue here is you have what you bought.

    I hear that there are ducting issues with the smaller unit if it is off and you made it through last week on just the larger unit you should be OK. I am going to re-read the posts above this one.

    I am not sure over-sizing is as much of an issue in heating as it could be in cooling. The main issue is the loop field sizing. If it supports what you bought then your solutions are not that tough to fix.

  10. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader


    Forget run time and see if you are comfortable.
  11. jk96

    jk96 Member

    Hi Mark. Both units are 4 ton each. Ducting issues seem to be with the unit that is turned off. The loop field was a very large field, I'll get exact size but I'm sure its plenty to support both units.

    No issues with cooling cycle through the summer. We moved in in May. My main concern is the waste in electrical usage by running two large units with short cycle times. How would electrical consumption compare in our situation compared to two smaller units with longer run times? Also - if we are running shorter cycles, what does this due to the benefits received or lost in hot water production?

    Mark I take some issue with this comment, especially from an industry professional. This was not a DIY install and a system that is 2x the system needed is not something minor in my opinion. (This may or may not turn out to be the case). Let's assume you designed and installed a system for a customer and were wrong in the sizing requirements by double. Would "You have what you bought" be your response to your customer if they brought the concern to you? I am not expecting any type of refund back but was asking how other professionals would rectify this situation? I guess I'm just hoping your statement is not the norm.
  12. moey

    moey Member

    Most the time you have to sort through the "experts" attitudes on this site. If they aren't being critical of you they are criticizing each other.
    Palace GeoThermal likes this.
  13. Tamar

    Tamar Member Forum Leader

    JK, maybe I missed it, but how do you know it is the geothermal that is using more energy than expected? How much more? do you have your geo on a separate meter?
    Mark Custis likes this.
  14. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    My comment about you owning what you consider to be over-sized equipment and the difficulties involved with removing and replacing thousands of dollars worth of equipment will be a very difficult task. Forcing that issue on a company could result in the company going out of business with out solving your issue.

    Sorry for the misunderstanding. Without boots on the ground, or seeing the results of extensive accurate testing, it is impossible to determine if the equipment is over sized.

  15. jk96

    jk96 Member

    Tamar my method is not very scientific but I have established a baseline of overnight power use without the heat running. 6 ceiling fans, 5 floor fans, water heaters in standby, appliances in standby, etc. from midnight to 8 am we are using almost exactly 16 kwh ( 2 kwh per hour). I've done this multiple times to make sure its consistent. With the heat on and both units running my usage jumps to 6-6.5 kwh per hour. So about 4-4.5 kwh per hour for heat. If I burn wood in the fireplace my usage for heat drops to about 2.5 kwh per hour for heat. This has been consistent over and over.

    Jump forward to recently when I switched over to one unit - the night my single unit ran for 8 hours continuously on one of our coldest nights (3 degrees at midnight) my usage was 3.25 kwh for heat. Last night we ran the single unit along with the fireplace and my usage for heat dropped to 0.9 kwh.

    With that said, I do know I have a problem with the one unit we have turned off based on the cycle times of the unit that is running. Short cycle times for one unit running the whole house vs very long cycle times for the other unit when both are running. Also very little airflow from registers.

    Overall our whole house kwh usage has dropped by 30 kwh per day since switching to one unit.
  16. jk96

    jk96 Member

    Not trying to pick a fight Mark, just not expecting that reply. I do appreciate any input you may have.

    By the way, as far as being oversized, my cycle times and how easily one unit has maintained the house is the reason for my assumption. I worked from home today on some basement projects and logged my units exact cycle times. Here's the specifics.

    Outside air temp - 23 degrees and sunny.
    Thermostat set point - 72 degrees
    Unheated zones - tstats never dropped below 68 degrees and are currently at 69.

    Run cycles (clock is off on the units but started around 10am this morning.

    Heat on/off
    on - 22:50
    off - 23:14 (24 minute run time)
    on - 00:04
    off - 00:17 (13 minute run time)
    on - 00:50
    off - 00:58 (8 minute run time)
    on - 1:25
    off - 1:36 (11 minute run time)
    on - 1:59
    off - 2:36 (37 minute run time) two zones calling
    on - 2:50
    off - 3:05 (15 minute run time)
    on - 3:21
    off - 3:35 (14 minute run time)
    on - 3:48
    off - 4:37 (49 minute run time) two zones calling

    Again - this is with the single 4 ton unit running, 23 degree outside temp.
  17. Tamar

    Tamar Member Forum Leader

    Were you given any sort of proposed usage and/or did your installer include design documentation?
    Mark Custis likes this.
  18. Stickman

    Stickman Active Member Forum Leader

    Hmm...let's see... free advice from folks who have demonstrated their knowledge (through application of their suggestions to my own situation) and an obvious desire to help (just about everyone does get a response) versus trying and failing to get help from the screwball out-of-business contractor I chose (my bad). I'll take a little criticism. Not that I have seen any yet.
  19. Tamar

    Tamar Member Forum Leader

    Without knowing what the design temp is/was, with a somewhat unscientific measure of usage that doesn't take outside temps or windchill into account, and with your comment that you "were not charged for the increase in tonnage", it's hard to get too concerned (at least yet, anyway) about an additional 1 kw/hr to run the extra unit/extra zones. Though your readings don't change much with just running a single unit, it's possible your comfort level changes ever-so-slightly.

    There might be a control strategy that makes your system more efficient....I would not be looking to have well-functioning units switched out for larger or smaller ones. I'd see about control modifications that make better use of the second unit. Depending on how you use your zones, there might be one time of day (or day of the week) when the first unit is more efficient as your main heat source, and other times where the opposite is true.
    Mark Custis likes this.
  20. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    JK96: You can not hear the thoughts in my mind when I typed. I did not intend any offence. I was thinking about the financial ramifications of such a change in equipment. Not to say if there was an error you deserve recompense. It was my intent to try and help get the system doing what you think you want.

    Lets start over and see if I can lend a free internet hand to solve the issues.

    I am not bashing you or your contractor, but these things stand out to me:

    Why splits?

    Big Glass scares HVAC folks to death because window manufacturers print numbers with tongue used to replace the thumb when typing numbers.

    Big Glass scares HVAC folks to death because of the possibility of installation errors.

    In all the very professional pictures, I do not see duct work .

    What machine serves which zones?

    What are the ducting issues with the off machine?

    With the troubling machine/system turned off and I would guess 3 zones, what is your comfort level?

    It seems to me you have a wallet, (electric bill) issue.

    You are equating run time with electrical usage, by emotion, not measurement.

    The list above is a start. If I were you, as stated above I would try and make your home and the systems you own work. I will help, but I need more data.

    I think in air and water heat transfer. I am a tin banger, pipe fitter, plumber, electrician and master service tech. I know how each of these trades and common sense play together to make a home comfortable and economical to make comfortable.

    Warm regards,


    ps. Tamar and Stickman have gone through lots to get to where they and their comfort are today. They give good advice.


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