Hypothetically
- in a heat centric environment
- water to air geothermal heat pump
- pond loop EWT 32F
- Tranquility 27, 2 stage with DSH
- Proper valid heat loss et al

-  if a building requirement was right on the line between a 4 & 5 ton what would the recommendation be?  Better off a little light (more aux), or a little heavy?  What are the major pros / cons of each?

In that specific set up I would look to see if 1st stage 4 ton would satisfy my cooling needs , if it would that would simplify my disicion
The pro vs con is simple install cost vs operating cost , usualy undersizing saves enough money on install that a whole lot of time to make the few days a year where the sightly oversized system outpreformed the under

Since with a pond loop you don't have high drilling or excavating costs, I would lean towards the 5 ton.

good call didnt think of that

I'd lean toward 5 ton, but I'm like an economist - always good for the "on the one hand...on the other hand" routine, so here goes:

1) Ductwork may not handle airflow required by 5 ton unit

2) Ductwork may be marginal for 5 tons, so operate noisily and inefficiently

3) 5 tons may be so oversized for cooling in a humid summer climate that occupants will be either hot and muggy or cold and clammy all summer - is that worth the increased winter efficiency?

Informed choice will be made only via Manuals J and D calculations specific to house.

FWIW SOP for cooling-centric humid climates is to slightly undersize - on hot days continuous operation results in such good dehumidification that missing setpoint by a degree or so still results in good comfort.

I believe this is a personal choice, normally the contractor makes for you. My personal choice would be the 4 ton. I feel being undersized means savings on the equipment up front, and over time a good chance of saving on operational cost.

Hey, you could always split the difference and get a 4.5 ton!  Just my 2 cents worth...

Regards,

You could go with the 4 ton but install more loop for the extra runtime and better performance.

Bob G.

Tranquility 27 packaged units put out the following amounts of heat per hour @ 32 F EWT:

4 ton = 37500 BTU's (cop 4.0) and 5 ton = 48000 BTU's (cop 3.9), @ 2nd stage.
4 ton = 31200 BTU's (cop 4.6) and 5 ton = 37500 BTU's (cop 4.3), @ 1st stage.

Though the 4 ton unit is overall more efficient, 37500 BTU's per hour output requires 7.5% less electricity from the 5 ton (1st stage) as compared to the 4 ton (2nd stage). Any additional load requirement, using the 4 ton will come @ cop = 1 aux heat. Using the 5 ton, you still get heat at cop= 3.9 up to capacity 48000 BTU's/ hour.

That said, for a given heating load the determining factors are: the duration of the load, the difference in the cost of the two units (installed) and the cost of KWH.

Considering humidity control aspects of small vs big dilemma, for people living in heating dominated ares I would like to add:

Among other Carrier geo systems, Tranquility packaged units have a factory installed whole house dehumidifier that is temperature neural, called ClimaDry. It can dehumidify without need for AC. It requires a tstat with humidity sensor. The extra cost for my 3 ton unit was $1350. I selected Tranquility 27 vs WaterFurnace Envision because of the HVAC installer and the dehumid option.

Regards, Masoud

Masoud,

Very interesting, I have heard of using a humidistat to control a system for humidity control.  Is the “ClimaDry” something different in operation?  What is the COP for this function and what is unique about it compared to just a humidistat on a normal system?  Bottom line is there something unique with the compressor, valving, fan, coils, etc…

Eric,

ClimaDry is a dehumidifier integrated to the heat pump as one unit. As you mentioned it uses a humidistat to control indoor humidity, regardless of a need for cooling. It cools the air to 55 degrees and with help from a low speed ECM fan humidity is extracted from the air. But, the extracted heat is not send to the loop outside, instead it is put back in the supply air. I think what is unique about it is a reheat coil. Thus for example, humid 70 F air goes in and dry 70 F air comes out.

I don't know the definition of COP for dehumidification, if there is one. I quote from CLM Geo Journal Vol. 1 Issue 17:

"...ClimaDry option is integrated into the ClimateMaster unit, eliminating additional components and a second compressor. The dehumidification mode operates at the high efficiencies of a geothermal heat pump."

Regards, Masoud

Masoud I have the Tranquility 27 but with regular Honneywell T-stats and a control board mounted to my unit.   Are you saying that my Climadry will not work without the humidy T-stats?  If so I need to research this before the cooling season.

Scott,

No reason to worry. If you have ClimaDry on your Tranquility 27, you should have proper thermostat - technically a combination thermostat / humidistat. It looks and works just like a regular tstat, except in addition to temperature settings, it has settings for humidity control. All major thermostat manufacturers market these gadgets. Read your "regular" Honeywell tstat manual, it should tell you about humidity control.

Regards, Masoud

When you say you are on the boarder, what percent of the heating requirement is satisfied by the 4 ton?
J

On the documents provided the 4 ton heat pump provides theoretically 97%.  This is what I have installed but its struggling and the distributor is now recommending swapping out for a 5 ton.  The pond loop 1250' is said to be big enough.  So giving the +/- accuracy of the manual j the benfit of the doubt it sounds like borderline.

Would you have to buy the second heat pump ? What happens to the first heat pump? What does struggling mean? Is it not keeping your house warm? If not, by how much?

Back to the EWT question? 97% isn't bad. Since this is existing, do you even have enough duct work for the 5 ton?
If the load and entry are correct 4 ton should do.
In mid MI, we would put twice the pipe down (if we're talking 3/4). So we're back to the entering water temperature.
Joe

I tried to keep this simple & general but to respect Joe & Dyawne's questions I have to get more detailed -

- I am in a very heat centric environment - Ottawa Canada
- When discussing my requirments with bidders for my system I made a point of my need to recover from a 2C nightime setback. A wife comfort non-negotiable requirement. While its certainly not why I chose him, the winning bidder said "ok, no problem" for the setback.
- After the heat loss et al, I was recommended a 4 ton Tranquility 27, 2 stage with DSH
- Using a 1200' (always flowing river) loop - while not very warm EWT 33F, practically infinite heat exchange
- The unit was installed in November
- A pre-exisitng 2 zone system was maintained, although the contoller was swapped out for 1 that could handle muti-stages.
- The distributor (Next Energy) supplied a document they call a BIN analysis. I don't know if this is an industry standard term or not, but it said the heatpump should run out of horsepower at 2F. I think you call this the 'design temp'.
- The unit seemed to maintain house temp (70F) while outside got down to low teens then ran out of horsepower.
- The installer, upon the recommendation of the distributor changed a filter of some kind in the refrigerant system & recharged it.
- After that the unit ran well but took 4+ hours to recover from the 3.6F setback (remember we call that 2 degrees (C).
- Upon consulting with the distributor the installer reports that Next Energy says the unit is too small, "we have to swap out for a 5 ton".
- How much is that going to cost whom? Will it cause air flow issues? Will it cause summer humidity issues? Don't know yet.

- So I'm wondering if the distributor has enough experience to know that a properly sized unit should recover 3.6F much faster, and therefore the heat loss was understated. Or, a bigger unit (too big) is the only way to meet the promise I was made about setback recovery, but the real answer is the promise was 'oversized' & the unit I have is 'right-sized'.

Thanks for your attention.

Now it makes more sense. Since the BIN analysis design temp doesn't match with real world data, then it appears that what should have been the right size unit is now too small. If you want faster recovery from the set back and since you have almost infinite heat exchange with the river, and if you are going to swap out the unit, go with a two stage 6 ton unit. Then you will have about 4 tons for regular use and 6 tons for fast recovery.

Just an idea.

I missed a clarifying point.

After the 'repair'..............
" -The installer, upon the recommendation of the distributor changed a filter of some kind in the refrigerant system & recharged it."
............... the unit ran fine and capacity seemed to match the BIN Analysis, no aux requirement until +/- 0F.  But took 4+ hours to recover from the 3.6F setback (remember we call that 2 degrees (C).

and

6 ton!!  I am worried about my duct capacity, and summer dehumidifation at 5 tons?  Or were you kidding?

OK then maybe a 5 ton with large heat strips then. If you want to have a fast recovery time, you need to add some serious heat capacity somewhere.