I am replacing a 3ton AC and gas furnace with a 2 ton GSHP. Some of the quotes I received say that only minor modifications would be needed to the existing duct system, another one says that the only way to make sure all rooms would be getting enough air is to redo the ducts completely. What do you think?

It depends on the blower. You may have had a 3 ton furnace and AC system with a 3 ton blower drive and now that you are putting in a 2 ton GSHP one contractor may be bidding the job with a 3 ton blower drive and another with a 2 ton blower drive so he is changing the ductwork for proper air delivery. Let us know what they tell you.

Also for refrence where are you located? It may help us answering future questions.

I am located in Raleigh, NC. I have quotes for WaterFurnace Envistion dual stage 2 ton and a ClimateMaster Tranquility 27 2 ton. Nothing in the quotes specifies that the units will be using a 3 ton blower. One of the contractor is adamant that new ducts are needed to get the system operating efficiently, while the other says that the same effect can be achieved with a some (small) updates to the plenums and installing dampers at the start of each run to rebalance the air flow. His actual words are "adjust the airflow of the equipment to match existing ductwork design", which I guess might also mean using a higher setting of the GSHP blower. Which approach makes more sense and is more efficient? Thanks, --aydan

That's a tough call to make via the internet.

Basically it comes down to performing a load calculation that reveals what CFM each room needs and then assessing whether existing ductwork can achieve required flow.

Be aware that in heating dominated climates (basically anywhere north of I-40), replacing gas forced air with geo will require more airflow to be accommodated since geo heats air less.

There is really no such thing as a 2 vs 3 ton blower. Units in the 2-3 ton range use the same blower, configured as needed. Units 4 tons and up use a bigger blower. Some 3-4 ton units can be specified with a larger blower to overcome duct issues, but 2 ton units don't have that option.

Be certain a proper load calculation has been done before selecting equipment. Anything less is a wild ass guess.

First of all make sure any ductwork that is updated or replaced is fully sealed, no point in leaking 20% of your efficient into the crawl space. And I have seen a two ton unit with a 800-900 CFM blower so not all two tons have a 1200CFm blower but I think those two units do.

Both units you mentioned come with an ECM blower. I do not see any compelling reason for duct modifications based on size change alone.
Did you also complain about disparite temps from room to room in your home when you were shopping?
joe

Thank you all for the information. Here is my attempt to try to summarize my understanding of the issue.

- When replacing one unit with another (regardless of tonnage), some duct modifications are necessary, just because of the facts that the new unit may have different dimensions and orientation of return/supply.

- An ECM blower tries to maintain constant CFM and does this by speeding or slowing down depending on the static pressure that it must overcome (up to its max ESP).

- If a duct system does not have dampers in place, dampers can be added to balance the flow from the supply plenum to each run (so that the air from a hot bathroom can be diverted to a cooler corner bedroom)

- Smaller units are configured with blowers with lower CFM settings, however, the blower has a range of settings that can be adjusted by the installer.

- Anything outside of the range of the factory default settings comes with a performance (kWh) penalty.

- In a heating dominated climate the ducts need to carry more CFM in winter than in the summer


So then if the ducts are relatively well sealed and in a sealed semi-conditioned crawl space, and they were sufficient for a 3 ton AC and gas furnace, they should work (with minor adjustments, e.g., adding dampers) when switching to a 2 ton dual stage geothermal unit with an ECM blower, but probably at the expense of more work from the ECM blower.

Does the fact that the new system is dual stage play a role? The first stage of a dual stage 2ton system is about 1.5 tons. Is this size now too small for the 3 ton ducts?

Thanks,
--aydan

You could slow down the blower via the DIP switches.....

Aydan,

You've managed to sum up the essential points fairly well for a newbie

In general, ducts really can't be too big - don't worry about supplying just 1.5 T into the ductwork, especially if you have installed balancing dampers - they'll work in low stage and high.