I am curious how many times your heat pump comes in an hour when heating and it is cold outside.

At around 35F outside, 10 min on, 15 min off or longer....
At around 20-25F outside, our Climatemaster systems are averaging 15-17 min on, about 10 min off.
Even down at 8F a few evenings, the run times were way less than expected (15-25 on), around 7-8 minutes off. No Aux.

It seems like all stage 1, but tstat doesn't show stage 2. (The WEL is going to be a Jan/Feb project). By looks of our electric utility daily charts, on the <20F days this thing is likely on Stage 2.... the electric usage on the low-temp days seems to spike a good bit higher, so I assume that stage 2 must come on.... (Note: I am set for 3 CPH stage 1, 3 CPH stage 2... the only setting I can mess with that may impact run times.)

The Question that comes out of this is: Why doesn't it run continously when it is down near the low design temp, shifting stage 1/ stage 2?  slightly oversized? or tstat? Cost is clearly much better, so not a big deal, but just not consistent with what people say about operation of these systems....

Dewayne,

Now that outside is 35 degrees my heat pump is on for 12 min and off for 6. This pattern continues until the outside temp drops below 15. Then it stays on all night to maintain 70 degrees inside. This is all 1st stage. At 0 degree out, inside temp drops to 69, but it is not enough of a differential to shift to 2nd stage. By the morning if there is sunshine, the heat pump takes a break and goes back to 12 on / 6 off cycle. I have a 3 ton Tranquility 27 with horizontal loop in SE Michigan. My thermostat settings are: temp = 70 degrees, differential = 1 degree for each stage, max cycle per hour = 4, and aniticipator = 4.

At my house there is a time lag between a noticeable change in outside temp and the heat pump's run time. For example, tonight outside is 20 degrees warmer than last night, but the heat pump is still doing its 12 min on 6 min off, as it did last night when it was 15. Appearance of sunshine almost immediately shortens the run time at my house.

We expect a record high of 61 degrees for tomorrow!

Regards, Masoud

Hi Dewayne and everyone else. Great question! I find cycle time to be an interesting topic. It is one of the final indicators of how accurate the sizing calculation were for the complete geothermal unit.

My system will go to 100% duty cycle when the outside temp is near 11°F. Out door wind speed and sun shine will affect this up or down some. My normal set point for my home is 70°F. Once the outside temperature falls below 5°F, the home will start to loose ground. What I mean by this, the system without auxiliary heat source will not keep up and the inside temperature of the house will start to drop below the set point. At a sustained temperature of -5°F for more then 24 hours, the house temperature will fall to about 67°F. I feel this is acceptable. Before installing the geothermal system we kept the house temperatures at 68°F during times we were home and as low as 61°F while at work and 65°F during the night. Recovery time for the geothermal is too slow for my liking, so, the temperature set point of 70°F all the time seems to work best.

Part of the calculation for sizing my system included logging data with the 90% plus propane furnace that the geothermal system would be replacing. I kept recorders of the outside temperatures and the duty cycle of the furnace to find a btuh loss number.

Where is the heat going? In the past year this question has influenced my choices to improve my overall geothermal system performance. Things I didn't think much about before, like the concrete basement walls. My basement walls have a one inch thick foam isolating board below grade on the outside, but what I found is a large loss comes from the above grade area that has no isolation. This can be easily seen with a low cost IR temperature probe. Other losses come from replacement of warm inside air with cold outdoor air. This happens anytime a cloths dryer is used, a bathroom vent fan is run, or if a gas water heater is being used. I plan on adding an air to air exchanger in the next year or so.

Great topic buddy,

I keep my home at 21C/70F.                   http://www.weatheroffice.gc.ca/city/pages/ab-50_metric_e.html

Today it's -10C/14F and my unit is not currently on. One of the great things when I  built my home is the back faces south so I do get the benefits of the sun (even though its quite low in the horizon) to help heat my home to some degree.

When it was -25C/-13F the unit ran in stage 2 pretty much all the time, if fact I don't remember it shutting down at all. Also, I built this home with my father so I did all the insulating myself so I know it is very well insulated.

Now that the infloor water to water unit is  back up and running, I believe this will maintain most the heating in the home. I am going to try and keep an eye on it further to let you know how things are running.

I would love to share some pics of my setup if I can figure out how to post them here if you guys don't mind.

Happy New Year!

Dean in Edmonton

Would like to see some pics.  You can post them to a free  site like PhotoBucket.  And then put a link to them here.

I've seen up to about an 80-90% on cycle time, low stage only, (high is locked out) on a design cold night, here about 30F (Jax, FL). That compares well with Manual J calculated heating load.

Engineer,

I have a few questions for you if you don't mind. When you state that “(high is locked out)”, are you referring to a two stage compressor with a stage locked out, or AUX (resistive heat strips)? Is 30°F the coldest temp you ever see in Jax, FL?

I understand that on new construction Manual J might be a good choice for most, but if doing a retro install, having data from an existing unit might give a better target in my opinion.

Regards,

2-stage system with Y2 wire from zone controller to unit lifted and capped. No aux strips on geo down here, typically.

32 degrees (not 30 - I was approximating earlier) is a design winter low for JAX, meaning that statistically only 2.5% of the time will it be colder than 32. Selecting a design temperature that way is standard practice. Thermal mass / inertia of a typical house lets it ride out short dips below design temperature. That and Manual J has some safety built in.

Bin data for Jacksonville, according to my Elitesoft SW, predicts 2 hours per year at 20-25 deg F and 26 hours at 25-30 deg F. The record low for Jax, set in 1921, is 7 deg. If anything like that is forecast, I'll reconnect Y2.

While run time data from an existing system is useful, I wouldn't rely on it for sizing. A unit can lose capacity over time as compressor valves wear or blower blades foul. Also, given that many, probably the vast majority of units in the past were not sized via load calculation, why perpetuate old bad habits? In the case of installed gas fired units, I'm less experienced, but do recall having read that actual AFUEs tend to be well below rated owing to various installation shortcomings, off cycle losses, etc. - that would seem to reduce accuracy of load calculations based upon run times.

Here's a representative sample of my heating run times at the moment, shown below.

The data is shown for a 3 day period.  The red line shows heating on/off.  When heating is on, the red line goes down - the width of the line is proporational to run time.

As you can see, any one run-duration for my system is not very long.

I have two units, a 5 ton WF Envision 2-stage variable speed unit for the family areas, and an identical 3 ton unit for the sleeping areas.

My 5 ton unit averaged running 3.6 hrs/day this month; my 3 ton unit averaged 4.6 hrs/day.  We have our family areas tstat set at 68° during the day / 65° at night, and our sleeping area tstats set at 65° except for a few hours in the morning at 68°.

The location is Dallas, where heating needs are not as significant as cooling.  Of the 112 hrs the 5 ton unit ran this month, all but 1 hour was in 1st stage.  And the 141 hrs this month the 3 ton unit ran was exclusively in 1st stage.  Noting such little 2nd stage heating, there's no need for aux heating strips, of which indeed I have none.

I don't have the Hot Water Generators hooked up for either unit.

Structure size is 3400 sf, single story.

Hope this contributes to this thread.  Holler if there are any questions.

Best regards,

Bill

Bill,

Thanks for posting your data. Are you at your peak heating needs this time of year? If yes, is the added capacity needed for cooling during peak cooling days?

Regards and Happy New Year,

Eric, looking at the below chart, showing my KWH usage by month for the past 12 months, I anticipate that my peak heating month will be next month.

The blue line, which is HVAC (only, including pumps) KWH consumption, shows that of the 12 months in a year, Jan. is my peak heating month (at just shy of 1000 KWH).  This is in comparison to this past month just completed (Dec.), where HVAC consumption was about 650 KWH.

It may not be a perfect apples-to-apples comparison, but I think it's reasonable.  Footnotes would include tstat settings, Hot Water Generator usage, and weather pattern.

I don't remember what my tstat settings were a year ago.  It's possible that we had them set at a higher temp than this year.  And this year, for Dec., we had the Hot Water Generator on for most of the month, and last year it was turned off.

And here is Heating and Cooling Degree Days we had here in Dallas this past month, Dec.:  531 HDD, 15 CDD (I don't have the info for Dec. a year ago.

The GSHP capacity is such that I don't have to use 2nd stage cooling even in the peek cooling months.  Basically, I almost never use 2nd stage at all, ever.

I used to think that I'm way over sized on capacity.  And I still may think so.  But, I'm not privileged to the actual costs of putting in a vertical closed loop water-to-air GSHP system, so I'm not sure.

If you look back a ways here on this forum, you'll see that there was a posting by someone in the HVAC business, detailing actual costs, including overhead, salary, and profit needs.

I looked at that info recently.  And one observation I had was that the incremental cost, at the contractor level, to add in a second unit does not appear to be sizable when compared to the total cost of the job.  Especially when there's just one water loop (such is the case for me) and it being the bulk of the cost of the job.

I have 8 tons of capacity for a 3400 sf single story home, of moderate insulation and air-tightness.  I think everyone's reaction, including mine, is that this is way too much.

But if I knew the economics of a system with detailed price breakdowns, maybe it's possible that I only marginally paid more for the second unit.  And in return, I have an 'insurance policy' of being able to heat/cool half of the house if one breaks, I didn't have to pay for heating strips, I'm always running in the most efficient mode (1st stage), and I have further performance benefit due to having enough 'rest' between cycles where the 'down deep' earth temp has a lot of time to keep recovering from heat extraction.

Certainly reliability due to short cycling is a negative of my installation.

And many would be concerned with lack of dehumidifaction capability in the cooling months.  But, as it turns out, I didn't have any difficulty mainting near 40% RH during the summer time.  I think this is due to the huge surface area of my WaterFurnace evaporator coils.

So I'm currently wondering if the only downside of my capacity is reliability due to short cycling.  What I don't know, is if I paid a lot, or incrementally so, for the 3 ton unit in addition to the 5 ton unit.

Best regards,

Bill

Bill
a couple of things I noticed on your last comments,
the one that peaks my interest is that you stated you felt that since the cost of adding an extra was not that much, you agreed to add that 3 ton unit to zone the home, althought it may have appeared to be a good deal however, what you will see is higher operating costs due to the starting and stoping of the equipment and the energy used to start such equipment.
Your humidity level is designed to be handled by the larger coils so you have more surface area to create the drops on the coil. if the unit has short cycles then you are correct this will not handle the humidity levels
due to the coil temperature not getting cold enough to allow the drops to collect and eventually go down the drain, you may want to check if the diip switch for the dehumidify option has been selected this may help. and next I would make sure you do not get second stage untill it needed by temperature change not by amount of time it has been on first stage this may help in creating longer run times.
longer run times are the answer to geothermal equipment. Many installing contractors and suppliers have differ opinions on the what is a proper size for equipment, obviously the Manual J figures are the guide for how much, however hitting the mark on geothermal to hit the load in heating costs during design temps may not always be the most econonical choice as far as operating cost. the design temp is not reached but maybe 5 to 10 percent of the season.
I imagine in dallas the whole scenario is different than here in wisconsin, its not like you have a backup cooling unit like we would need for heating, but the principles still apply on oversizing. long run times equal better dehumidification, and if your design temp difference in cooling is 25 or thirty degrees then at say 110 degrees outside the unit should not shut off ever and maintain the 75 degree setpoint inside
so all that being said sizing your equipment for the cooling load not heating load will make the heating run times cycle shorter.
sorry for the run on sentences, grammar was not always my strong suit.

Dallas summer design temp is 100, so 25 degrees cooling TD is about right. I also like the comment about avoiding starting stage two based solely on running time of stage one - very good point that stage two should only come in upon failure to make thermostat setpoint by 2+ degrees. Long stage one run times are good during both heating and cooling seasons.

Short cycling owing to oversizing might be countered with a bit of care in running each unit - depends on geometry of house and ductwork. 40% RH is a remarkable achievement in a city whose summer design wetbulb is a muggy 75.