I have seen a number of threads where duct size seems to be an issue, especially for retrofitting geothermal.  I understand that duct size is a funtion of air volume (and velocity) and that the needed air volume is a largely a function of supply air temperature.
 
Question:  Do geothermal units provide heating/cooling at different supply air temperatures than conventional heating/cooling, thus requiring higher airflows and larger ducts (or better ducts with lower static pressure drop)?

Most duct work I have seen in existing houses is pretty bad, with long runs of flex, poor insulation, etc., but are goethermal units more susceptable to these problems?

ICF Curious

Absolutely, Geo heat is a gradual, consent heat, where as heating with oil or gas is a blast of hot air when the systems kicked on. The same is true for cooling.

I don't understand.....how and why would supply air temperatures for a geothermal heat pump be different from a normal air cooled heat pump?  I can understand how a gas furnace could create higher supply air temps than a heat pump, but aren't the evap coils for cooling basically the same as an air cooled heat pump or a regular air conditioner?

ICF Curious
(who is becoming Geothermal Curious, too)

A properly designed and installed geo heat pump shouldl provide warm air bit warmer than air source.

Problems arise in heating dominated climates where cooling loads are low and relatively brief, and heating loads are met by an oil or gas furnace capable of a much greater rise in air temperature. Higher temp means less airflow is needed. Retrofitting geo into such systems may run up against ducts insufficient to move enough warm air in winter. If the house is now heated and cooled solely by an air-source HP with electric backup, then geo should be a good fit.

In any event, cooling should work as well or better - evap coil is basically the same with both systems

You've hit a nail on the head with your observation that most duct systems are designed poorly. Indeed many of the retro jobs we do require significant duct improvement as well.
In theory an air to air heat pump would require the same duct work. The code actually calls for 3x's the minimum return to a non-specific heat pump.
Locale has an impact on required BTU's and wether the focus is heating or cooling focused.

I live in a cooling dominant area, south Kansas. Coincidently, my Man J shows cooling and heating loads to be almost the same, about 42K Btuh each.

1. Sizing for Cooling and Heating. So can I expect a properly designed and installed duct system to work well in either mode so long as the CFM for rooms doesn't vary too much? Some examples:

Guest Room Cooling 159 CFM Heating 156 CFM Diff = 3 CFM or 2%
Office Cooling 382 CFM Heating 337 CFM Diff =45 CFM or 12%

We plan to use radiant floor heating so I wouldn't expect to use force air heating mode often and would consider having electrical backup strips in the air handler if necessary for those extra cold days.

2. Sizing Duct for a Geothermal System. If my Man J designer shows the external, supply and return static pressures, pressure losses and CFMs, would these numbers be specific for a geothermal system given that he's recommending a GSHP? Which in turn allows the Man D designer to size the duct system without having to adjust for GSHP?

By the way I'll be calling him next week to verify, but wondered what our forum pros have to say.

3. Code and RA. Joe, are you saying code calls for the min RA CFM to be three times the supply CFM? or something else?

Dave

Thanks!  That answers my question.

ICF (and Geothermal) Curious

RE Code,
Forgive me if the numbers aren't exact, but in MI we deal with the Michigan Mechanical code which is an amended International Code. There's no reference to cfm just duct size.
Chapter 5 "specific appliances" 918.2 calls for 2 sq inches of supply duct for every 1,000 btu/h output limited only by manufacturers' installation instructions. It calls for like size return duct.
918.3 calls for 6 square inches of return per 1,000 btu/h, for non-specific "heat-pumps" with no such limit based on manufacturers' instruction, but concession to the listing conditions of the appliance. It makes no reference to supply duct. This is one of the few areas of the code that has specific duct sizing instruction vs reference to Manual D or ASHRAE.
Because the code is a minimum standard that routinely avoids specific design, it would be my guess that this clause would be to address the frequent return "choking" we've all seen from un-educated installers. It is difficult for an inspector to reject a project based on Manual D on a residential application. While this gives inspectors a tool to help consumers avoid the pit falls of poorly designed ducts, many inspectors are, unfortunately, under-educated themselves.
Joe

Boy, don't you wish inspectors knew what to look for to get rid of crappy installations!

Emphatically- Yes I wish inspectors new what to look for (might be a thread in there). People (and yes that includes inspectors) are limited to their experiences, and most HVAC guys aren't heat pump installers.

In my county all inspections are handled as construction progresses by a single individual. In my case the guy was a former framer, so framing issues got the lion's share of attention. That was fine - I'm glad those issues were thoroughly addressed. Happily that's an area of weakness of mine. I don't think electrical, plumbing or HVAC got more than a passing glance.

It is hard to expect that a single inspector have great depth of expertise in all these fields.

Yep.

It's just pathetic, though, that inspections aren't complete. A lot of shady stuff gets passed because an inspector doesn't have a clue what he's looking at. Why are building permits so expensive anyway? They are supposed to pay for that inspection. If it takes a mechanical, electrical, and structural inspector, instead of just one guy, then that's what it needs to be.

I'm an electrician, HVAC contractor, driller, and designer. I'm sick of people thinking my higher price is the same job as "Bubba" down the street. Bubba doesn't do a heat loss. He skimps on the wiring, cuts corners, barely flies by, etc. He absolutely doesn't size duct correctly, not to mention the crappy fittings he uses. He chokes the R/A. Each room gets just a 6" supply run with a 4x10 register. It may have 35' of flex, taken around 5 corners to get there, but he thinks that's enough. He has no idea about static pressure in his duct, or static pressure loss, etc.

Duct sizing, design, and installation is KEY to a comfortable, efficient, reliable system--no matter if it's geo, air source, or fossil fool system. I just wish inspectors knew enough to require a duct design, and then knew more than just a couple of things to check for on seeing if it's good.

Requiring a duct design on every job is not required by the code so in my area can not be required by the inspector. What is required is something like a Manual D design (of which there are several "suggestions") and a loose requirement for a minimum sustainable temperature and the availability of some sort of balancing dampers. Nothing in the code insists that it can't be 90 in one room to achieve the minimum of 68 degrees in another. Building department politics often lean towards the path of least resistance as well while homeowner/builders and hack installers defend poor work with the "you can't make me"/ "show me the code number" arguments. The notion that code ensures quality is way off. Inspectors must compromise on most projects as none are 100% by the book- rule of thumb for me is 3 minor or 1 safety violation before I get my pen out.
So- buyer beware; ICF, your questions are the right ones, you'll make sure that you- the end consumer, will get it done right. The code official can't.
Tuffluck, you might want to check out my suggestions on training your inspectors.
Joe