Buildinga custom home in a hot humid climate. Customer wanted to price out geothermal since we steered them into it.

This is a very tight, very efficient home. Air sealed, foamed, cellulose in cavaties, super windows, continuous thermal break with foam sheathing, efficient floor plan.

Assuming the contractors entered accurate data in manual J calculations, I was told it would take a little under 2 tons of forced air for the home. I was also told it would take 6 tons of geothermal. For the same home.

Am I missing something?

This is not right. A ton of cooling is a ton of cooling. Can you get us the Manual J calcs?

Noting what appears to be the mindset here in the Dallas area, 6 tons of 'geothermal' seems about right for 2 tons of 'conventional.'

I know it doesn't make sense.  In fact, I don't understand it either.  But I'm increasingly understanding that there's a design mindset, right or wrong, that says, if using geothermal, that the cooling/heating load needs to be satisfied by 1st stage alone of 2-stage cooling.

For example, that's why I have, for a 3400 sf home, 8 tons of cooling that never runs in anything other than 1st stage, even in this current series of non-stop 100+° days in Dallas.  Many would say I have too much capacity - multiple well respected geothermal Dallas-area contractors would disagree though, and say I'm properly sized.

Thus it doesn't surprise me to see another example here in this example, of what sure looks to me like excessive geothermal based capacity, probably to keep the compressor in 1st stage always, chasing the higher of the COP/EER specs.

I always thought 2 tons is 2 tons, regardless of conventional air conditioning, air-to-air heat pumps, or water-to-air heat pumps.  But at least here in the Dallas area, it doesn't appear to be the case.

Also, with respect to the proposed 'tight' home:

My guess is the heating/cooling load is more like 2 tons.  But when a large home is heated/cooled with small capacity, because the home doesn't need more, due to the tight nature of the home, this means a deliberate design strategy is needed for fresh air and management of humidity.  With this tight of a home, you're going to need to think about how you're going to 'dump in' enough fresh air, and how you're going to manage the humidity of this fresh air.  Some kind of dehumid stragegy/equipment may be needed, depending on geographic location.

Best regards,

Bill

I'm well aware of the need for mechanical ventilation and dehumidification Bill. My question was regarding the two, completely different prescriptions for the two systems. The installer was aware of these specifications when doing the calculations which makes me even more curious. Running in the first stage makes some sense on the efficiency side but the sheer cost of 6 tons simply puts the system out of reach and, if it's more than we need, I question the ability of the system to assure comfort throughout the home.

Asked for the Manual J calculations, if they are using incorrect inputs (or if I don't see them) I'm moving on to yet another installer. We clearly are looking to do business with someone who is going to avoid 'garbage in garbage out' in regards to our mechanicals but seeing the quote, I think I need to keep looking.

I too thought a ton of one is a ton of the other. Why does this have to be so difficult?

I agree - a ton is a ton, particularly when paying attention to a dehumid strategy.

Best regards,

Bill

When we built one of the HVAC bids did the same sort of thing. When I asked them about it he said it was 3 ton in 1st stage and the the 7 ton was second stage. I asked why he had a 3 ton furnace with a two stage that was 1 and 3 tons, why didn't he spec the furnace that was 3 ton in 1st stage. He sort of stopped, like no one had ever asked him that... Neededless to say we didn't use them.

Brock, not following what you're saying. Was the total calculated load 3 tons? Or 10 tons?
Did the biddor submit a 10 ton (3 ton in 1st stage/ 7 ton in 2nd stage / 10 ton in full up operation)?
Or did he submit a 3 ton unit? Can you clarify a bit or did I miss something? Can you get a 1 ton unit?

I'm hoping someone can untangle Greenlife's dilemma. In our cooling dominant area, our Man J for cooling yielded 42,272 Btuh and for heating 42,289 Btuh. This is 3.5 tons in cooling. If we go with a 2-stage unit, I would expect 1.5 ton in 1st stage, 2 ton in 2nd stage and 3.5 ton in full up operation. For example, 1st stage would run when outside temps are 80's, 2nd stage would run when temps in 90's and both would run when temps are in the 100's or something like that. Is this reasonable to expect? Why would I need a 5 or 6 ton unit with a 3.5 ton 1st stage? Just when I thought I was getting a handle on the concept!!

The translation from Man J cooling/heating loads to actual equipment tons should be done via a Man S.  There are a number of factors, including latent cooling needs, that influence the number of 'tons' needed.

For at least WaterFurnace R410A w/ 2-stage scroll compressor GSHP units (maybe others too), 1st stage will be 0.67 of full capacity.  Thus, for a 3 ton unit, 1st stage will be 2 tons, and 2nd stage will be 3 tons.

While it may not be the case for other manufacturers/models, for the WaterFurnace Envision line, you can't get an 'in between' size as a 2 stage unit.  I.e., a 3.5 ton unit is only available as a single-stage model.  If you want a 2-stage unit, you'd have to get a 3 or a 4 ton unit.

Usually it's the tstat that determines how the staging is selected (non-zoned implementations).  For example, for my tstat, and for the way it's currently configured, it won't engage 2nd stage cooling until the setpoint temp is 1° or more below the actual temp for more than 40 minutes.

As I mentioned earlier, there are examples here in the Dallas area, where GSHP 2-stage units are used and sized to intentionally run almost always in 1st stage, designs done by well respected HVAC contractors.  I don't understand what the design considerations are that cause this, including designing the borehole field to the full (2nd stage) capacity of the GSHP unit(s).  (I.e., yesterday was a 106° high, and my units, 8 tons total for a 3400 sf home, never got out of 1st stage.)

Best regards,

Bill

Ahhh! Yes, I'll expect the geo contractor to size the eqpt per a Man S.

I had read at HydroTemp's site about a 3 stage using dual compressors:

"... A dual compressor system normally consists of both a small and a large Bristol Compressor. This unique design allows the unit to provide three stages of heating and cooling: stage one uses only the small compressor, stage two uses only the large compressor, and stage three employs both compressors at the same time. Dual compressors run on the same refrigeration circuit, allowing the small and large compressor to run on coils sized for both compressors -- resulting in increased efficiency."

I see where I incorrectly applied 3-stage thinking (2 compressors) to a 2-stage application (single scroll compressors). In my case, don't think I need a 3-stage system for a tight ICF home. Thx for responding. Dave

I mistakenly omitted that I was refering to WaterFurnace R410A 2-stage units.  I corrected my comments above.

Bill

Another issue which arises in extremely tight, low load homes is stratification / stagnation. I'm not referring to the need for fresh air but rather the fact that as design load drops from the old school 1 ton per 400-600 SF to 1 ton per 1200-1500 SF in extreme but increasingly common cases, total system airflow gets so low as to make register location and grill selection much more important. Operating at reduced CFM / ton for increased dehumidification exacerbates the concern. That new tight houses have ever higher latent vs sensible load adds yet further to the issue.

The issue can be overcome by careful selection, sizing and placement of registers and grills, (ACCA Manual T) but the care required is an anathema to the many hacks in the residential HVAC field.

Anyone confusing 2 tons of "conventional" with 6 tons of geo should seriously consider a career change

Farmboy their load calc's for our house was 3 tons. The two options he gave us were a 1 ton low / 3 ton high natural gas forced air or a 3 ton low / 7 ton high geo system, I didn't get what brands he was selling. After I asked him why he had a 7 ton spec’d for geo with 3 ton for gas his reply was, That’s the way you size geo units, to run in low all the time. I asked, isn't 3 tons, 3 tons? He said no, it's not the same with geo, they are rated differently. That’s when we moved on.

What precisely is meant by a "1 ton low / 3 ton high natural gas forced air"?

It may be that a Wisconsin house needs 3 tons of cooling but 7 tons of heating. This is quite plausible, even highly likely. So if one aims to use geo year round vs just air conditioning in summer than the geo is sized to meet most of the winter load. That'll make for some massive ducts and poor summer dehumidification unless the oversizing is judicious and met with a properly sized 2 stage unit.

What I don't buy is the plan to exclusively size geo to stay in low stage. Why pay the substantial cost adder for a two stage unit and then only run it in low stage? Granted it'll be a bit quieter and more efficient, but again, dehumidification will suffer and the additional cost is considerable.

Wish my command of the English language was so good that I could write like the above!  I think this is an exceptionally well worded summary of the HVAC challenges associated with very tight homes in cooling-dominated climates.

Best regards,

Bill

Sorry again it was 3 tons heating and cooling, with the natural gas furnace he said it was two stage, with one ton of heating or three tons of heating, I didn't ask for more details.  For cooling without geo it was a 3 ton conventional unit.  Which is why I thought, why the heck is there a 7 ton geo unit?  Honestly I think it is to cover their behinds on the system, so they just oversize it and even if they screw up the install, it still should work.

Either he's having you on or there's another misunderstanding. I doubt any gas furnace goes anywhere near as low as 12,000 btuh, and I'm skeptical a 2 stage furnace is capable of such a radical difference in capacities between stages - factor of 3. My experience with / understanding of gas furnaces is limited, though.

In Mid-Michigan; a home could easily call for 58kBTU heating and <30K cooling. Given a loosely 20% capacity reduction for closed loop a 6 ton unit would be about 58K out. So while a ton is a ton; 12KBTU cooling is actually 9.6KBTU heating and in MI we load based on heat not cool but... generally stop at cooling load +2tons. So if you happen to be in the Gr. Rapids area, these numbers jive but....
depending on sq. footage you would use a 4 or 5 ton and aux. heater. Be glad to load it for you.
Where are you?
How many squares?
Lot size?
Good luck,
Joe

The home is in the gulf of mexico, hot and humid. Not too interested in heating.

3000 square foot.

Whoever told you 2tons air / 6 tons geo definitely needs to elaborate.

A 3000 SF gulf coast home requiring only 2 tons is at the very high end of current construction methods and materials. Not impossible, but it would take the utmost care and consideration of all aspects of load, infiltration, solar gain, number of occupants and their living habits to be certain this willl work.

I have 2.5 tons cooling 3000 SF in North Florida

Thanks for assuring me that I am not a complete lunatic. Now that I have proper sizing done by a third party I need to find an installer who will work off my specs and still warranty their work.