Hello, I am looking at installing geo as a retrofit. Have done significant improvements in the house envelope and per manual j it will reduce requirement from 9 tons to 6 tons (two systems). Based on current furnace runtimes before and after I tend to believe the improvement. This would leave me with over-sized ducting for the new system (reducing from 5->3 ton and 4->3 ton). Is there anything in particular to look out for in this situation?

What's the current furnace type?

Currently have 78 AFUE propane furnaces, 12 SEER AC.
House is in central Texas.
Electrical prices are reasonable here - for now

Oversized ductwork is rare.

Is the ductwork now in the conditioned space? Normally it takes the addition of sprayfoam to cause such a tonnage reduction.

I'd stay with the present ductwork and measure air flows to each room after installation. Compare them to Man J requirements and then take steps to balance - divert air from oversupplied rooms.

Sometimes envelope improvements right-size formerly undersized ductwork.

Some ductwork is conditioned, some not.

Improvement was complete replacement of outdated junk windows and doors. Plus some attic insulation. No spray foam (yet) in the attic.

Thanks!

What was the manual J load?
j

Hi Joe,

I have had 4 companies come in. Two have calculated the Manual J, but none have given me the results as I didn't pay for them as part of giving estimates (they would for a fee). Both came up with the same equipment sizing. All will fully review the Manual J with me if I select them as the installer. One company used the sizing results of the other two as an baseline for an initial quote for comparison purposes. Would complete and present complete manual j prior to install. All three of these would not do anything without a Manual J they felt comfortable with. Fourth company said "sizing of current equipment looks OK, we can start tomorrow". Guess which one is ruled out

Two are ground source, two are air source.

Reference checks on the two geothermal companies are positive.

Gotcha.
Company #3 is likely the reason the other 2 don't wanna give up the info.
There are lots of ways to calculate the load your self, but if there is a concensus.....
I'm not sure where you are located, ASHPs serve much better in climates south of me and are a good option in some cases.
To answer the duct question, I doubt you will find them oversized.
j

I am in Austin. Air source seams to be a reasonable choice, but I have overpriced propane and trying to get away from it totally for cost reasons. With air source I would need a backup heat source and my main electric panel would need to be upgraded for electric backup. I am told by everyone that no backup would be needed in these parts for geo. Also, I already need new linesets due to sizing and a leak. That appears to narrow the gap between air source and ground source costs, so I am leaning towards ground source. The one big benefit of air source is that I could replace the systems one at a time. Geo would at least need to drill all the wells to minimize mess and disruption - which comes with enough cost it makes sense to just do the whole job.

We kept our propane dryer and range (and gas logs used in the event of a power outage) when we installed our geothermal system. Our propane usage is now about 90 gallons/year for 2 adults and 1 child. I just wanted to mention this in case you want to consider keeping limited propane and saving the money on an electric dryer, range, and possibly electric panel upgrades.

Another solution for you may be a dual-fuel system if you already have the fuel source. High efficiency propane furnace kicks in when the outdoor ASHP can't keep up. If you have that flex duct like most home in TX, it's likely that they are not oversized due to the roughness on the interior of the pipe compared to steel.

Please keep in mind when considering LP versus geo equipement that a lp system with a 78 eff. needs about 130 cfm for every 10,000 btu's and a 6 ton geo would require about 350 cfm per ton (depending on geo system Mfg.) plus keepng in mind when it to comes to the sizing of a geo unit a person designs to the heating requirements which usally drives up the size of the unit. so this would lead me to believe that your ducts are probally are not large enough. being in texas i dont know what your heating and cooling requirements are. the ductwork is a critical part of the decision process when deciding what type of equipement to choose. The ductwork is the proverbial wheels on the car, A person would be upset if they bought a cadillac and the dealer had put on wagon wheels...you didin't get what you paid for. So in other words make sure of your delivery system (Duct-work) because it will directly affect the performance of any equipment. More times than not you can improve just the ductwork and see upwards of 25-40% reduction in operating costs!!!!

What is most likely is that your old load of ~100MBH was serviced by a ~150 to 200MBH and therefore duct work will be plenty big. It is unlikely that you are undersized.
You've had multiple estimates were any of those folks concered with your duct size?
j

Thanks for all the input.

The reason I asked was that one contractor mentioned that he would use a 3-ton unit rather then a 2-ton unit to get proper airflow. Said something like all the air would flow out the nearest vent opening and far rooms woudn't get any air.

Posted By aus on 21 Jan 2011 02:44 PM
Thanks for all the input.

The reason I asked was that one contractor mentioned that he would use a 3-ton unit rather then a 2-ton unit to get proper airflow. Said something like all the air would flow out the nearest vent opening and far rooms woudn't get any air.

Ok, to suggest it is cheaper to add a ton vs modify duct work is suspect.
What did the others say?
j

Over-sized and under-ducted...so common and so sad.

Posted By engineer on 24 Jan 2011 10:08 PM
Over-sized and under-ducted...so common and so sad.

What would be uncommon is to up the heat pump size to accomodate the ducts.

Lots of good input - thanks! I will ask more questions of the proposed installers. And will make sure to review the Manual-J before any work commences.

Ok - last guy says my ducts are undersized.

Where is best place to do my own manual-j? Is there a list of choices sites that I could look at?

HVAC calc 1 time license ~$50
Free sources as well if you google them.
J