HVAC Advice... Underfloor v. GeoTerm Forced Air

[SBoyd]

Greetings All,

In a nutshell, need to repair/replace heating system in a large volume (high-ceilinged), relatively small (<3000 sq. ft.) weekend house in "upstate" (not-too-far) New York. Currently system is all original (19 years old) oil-fired boiler with 3(!!!) Lennox airhandlers in the attics and a baseboard (Slantfin) system for a downstairs living space. There are four total zones currently. Everything is old, and I was hoping to move to radiant underfloor heat, as most of the main level (about 2,000 sq. ft.) is easily accessible from downstairs to do a stapleup retrofit. The living space downstairs needs to have to ceilings pulled down (about 800 sq. ft.) in order to do all of the mainfloor, but I don't mind and the sheetrocking was not that good and it will provide a chance to redo some bad wiring that was done by the original builder/owner. The baseboard heating works fine for that area, so we'll leave it alone.

The plan was to abandon the AHUs in the attics and do without AC, as the previous owners had never hooked up a condenser system to the AHUs - they were used solely to blow air through the unused cooling coils and then through an external heating coil that was fabricated onto the end of each unit. Well, after spending one very hot, sticky summer in the house, we want AC!!! This means I need to retain the ductwork and blowers in order to have chilled dry air, but it is so frighteningly inefficient for heating ($500 a month oil bills this winter), that I cannot continue this way, even if I did get a new high-efficiency boiler. BTW, gas is not available in this area, only propane. This is what has lead me to the underfloor heating. Other than the expense/hassle/effort of putting it in, I have never heard a bad thing about it, and mostly only superlatives. Also, it seems pretty easy to divide the heating areas into many zones, allowing us to not heat several rooms we do not use unless there are guests up for the weekend. This has implications for thermostats and controllers, but I'll post another question in the HA area...

I am getting quotes right now to have the tubing put in and possibly a new boiler (although the old one is still chugging along at 19...), with an option to add condensers to the 3 AHUs in the attic later, so as to provide cooling for next year. I am prepared to bite the bullet on this, but then I ran across some articles on Geothermal heat exchangers, essentially heat pumps that use consistent ground temperatures to provide both heat and cooling in one unit.

The main benefit to this (besides not having to have a boiler and an oil tank in my basement) is that these systems would keep the airhandlers in the attics and save me the expense/hassle/effort of having plastic tubing put in (and drilling many, many holes in my floor joists). While I am sold on the idea of gentle, non-drying, clean radiant heat, the idea of reusing the existing equipment, at a much greater efficiency, without having to put in three separate condenser units in the yard, is very appealing...

Has anyone put in (or know anyone who has) a geothermal unit? Would anyone please comment on keeping the forced air system for heating (just fix what is broken) versus putting the radiant tubing in?

Thank you all for your consideration...

 

[DavidL]

I have a high ceiling (37 ft) house that has a Hydroheat geothermal primary heat system. 68k BTU. Water is from a 1 1/2 hp well and is exhausted into my lake. I have a 100k btu high efficiency propane furnace as the secondary heat for cold days or when the power goes out (50 amps for the geo compressor).

Water comes in at around 45 degrees and is exhausted at 32.5 F (tuned for just above freezing). Works really well from an efficiency perspective. As the unit is running most of the time, it tends to distribute the heat more evenly than a traditional forced air system.

Big time diff in cost between running the Geo and running the propane alone.

There is more maintenance with the Geo as there are quite a few more parts to the system.

I also have radiant floor tubes in my basement floor slab, but never hooked them up. I bought the Geo unit to heat domestic hot water to use the hot water for the floor heat. But, never connected it yet. Not needed with the insulation everywhere - which is the other real key toward the energy efficiency of the house.

Doing it again, I would probably have opted for the reversable Geo unit for A/C in the summer. Currently I use a combo of a well water cooled evaporator and a more standard 5 ton A/C compressor.

 

[upstatemike]

So how much water do you pump in a day? Did you have to do anything special with respect to the well to ensure the system always works properly?

 

[DavidL]

not sure on how much gets pumped in gallons. The pump has to be upgraded to at least 3/4 hp which means the pressure tank is 80 gallons to keep the pump from short cycling. I went up to 1 1/2 for no pressure loss when washing cars, washing machine, dishwasher etc.

There is a freon controlled valve that regulates the water flow to get it to the right output temp. I have had to replace that valve twice since 96. I can tell when it is getting worn when the pipes start to hammer when the valve modulates the water flow.

 

[rocco]

Hi, SBoyd:

I'm in the process of doing two remodels, and have researched these same questions. I have not installed anything yet, but have settled on what I will install. I think my situation is similar.

Almost all of the top floor of one place is one big room, which acts as kitchen, dining and living room. It has a cathedral ceiling. I learned that forced air heat was inefficient for such an arrangement, because the heat rises to the ceiling and escapes through the roof. You can be expect the air at the ceiling to be 15 to 20 degrees warmer than the air at the floor. With radiant heat in the floor (which tends to travel into the furniture rather than into the air) the air at the floor is nearly equal to the air at the ceiling.

A disadvantage with radiant heat is the thermal inertia: you cannot heat-up a room quickly. A set-back thermostat will not be of much use, unless if you go away for a few days.

I investigated the geo-thermal units, but they were very expensive. With the cost of a reversible geo unit, and the cost of natural gas (the big question mark), I calculated that it would take close to twenty years to make back the difference in cost, which was longer than the unit was supposed to last. But propane makes for a different story, I presume.

 

[SBoyd]

Thanks folks, this is a helpful start. I had feared the GeoTherm stuff would be expensive to install, but I don't have any quotes on that yet. As the next few days unfold, and I find out how much the radiant system will cost, I suppose my mind will be made up for me!

I am looking to get a larger bladder tank as well, so I guess I am going to make some local plumber very happy...

 

[D365]

Geothermal is the way to go. There are a couple of good sites to check out with information on existing geothermal systems. They both have monitoring systems so you can see the systems in operation. One is www.duanesworld.net's geothermal HVAC page and the second is www.ourcoolhouse.com

Both sites offer details on the installations. I hope these sites provide good information for you. I found it helpful to get ideas and do research. There is another site which contains information on geothermal HVAC and sources for research, www.thegeoconnection.com

Good Luck.