We’re replacing our single unit HVAC as part of doing an addition. Since the house is 2 story, we’re going with 2 zone system, one for each floor. The HVAC guy I talked to today suggested going with a lower efficiency furnace (below 90 AFUE) in the attic because of the need to vent the water from combustion on the 90+ unit. If the drain line backed up or froze on the attic unit, that certainly sounds like a recipe for disaster. To avoid this, he said to go with a lower efficiency unit up there that doesn’t need drainage.
I don’t really like the idea of using a lower efficiency unit, but his logic does make sense. Is this a common practice? Surely every two zone system confronts this problem (at least in areas of the country where it gets below freezing;-)
Any thoughts?
Replies
This has nothing to do with zones, it has to do with condensing equipment. Are you maybe referring to a two-stage unit?
The best option would be to install the furnace somewhere other than the attic.
Another option is to heat trace the drain, and/or run the drain in some way so it will not freeze. The lower efficiency unit is an option, though not the one at the top of the list.
try posting your question at HVAC-talk
http://hvac-talk.com/vbb/forumdisplay.php?forumid=1I suspect that the furnace for the second floor is not going to run very much and going with a 90% might not give you much return on your money.
In reference to Timbo's comment:
This has nothing to do with zones, it has to do with condensing equipment. Are you maybe referring to a two-stage unit?
The best option would be to install the furnace somewhere other than the attic.
I'm referring to two furnace units, both two stage. One will go in the basement, primarily heating the first floor, and by convection, providing some of the second floor heating. The unit that goes in the attic will provide supplemental heating for the second floor.
I am calling the first floor a zone and the second floor another zone, hence 2 zone. I guess it's really 2 different systems, rather than a 2 zone system as I called it. I was referring to all the heating and cooling for the house as one system, regardless of how many heating and cooling units are involved. I guess my terminology is wrong...?
By "heat trace", do you mean apply some heat source to the drain line to keep it from freezing?
By "heat trace" I mean a low powered resistence heat "wire" that is thermostatically controlled to add just enough heat to keep the line from freezing. Typically used when water lines have to be run in potentially freezing conditions. Versions are self-adhering, but most are simply wrapped around the pipe (like 2 wraps/ft) and insulated over.
As far as the terminology is concerned, you are refering to two (2) separate systems, not a two-zone system.
Given the greater detail on the situation, an 80% furnace for the upstairs portion is not really that bad an option, as it will on need to run to pickup a lttle extra heating load from time to time when the lower level furnace does not heat the whole house, which, if sized properly, could do so about 75-85% of the time.
What Timbo said. He's a HVAC guru.
Try looking at http://www.buildingscience.com for ten common mistakes in home design and building. The HVAC system outside the conditioned space is on that list.
Another thought is to upsize you single replacement system and zone the distribution system. A good system designer can do the load calcs and distribution design for you.
Dave
First, if you put a 90 plus furnace in an attic subject to freezing temps, you must build an insulated room for the equipment. There must also be a drain, so the room is best located near the upstairs bath or laundry.
In a two furnace and a/c system, the first floor system will actually heat most of the home because heat rises. During cooling season, the upstairs system will cool most of the home because cool air sinks. The larger the open space between the first and second floors, the bigger the problem. This is good to keep in mind when sizing equipment because the real installation is not a static Manual J calculation. Thermostat set points between first and second floor may need to be a few degrees apart to force the other system to operate.
I would strongly recommend an insulated room for the equipment even with a non condensing furnace. Even though there have been several links posted concerning attic system horror stories, I have never run across an installation that has caused those problems, so the quality of the installation has some merit. Good or excellent attic ventilation is probably a factor.
You will be ahead of the game with a non condensing furnace in the attic due to the short run times. I would consider a variable speed drive and a high efficiency ac for the attic system to maximize energy savings.
A master float safety switch or moisture detector wired in series with the thermostat wiring, placed in the equipment drain pan is also a good idea. That way the equipment is shut down if there is a water problem, forcing you to go up and take a look before the pan is full. Cheap plastic pans can crack.
"I would strongly recommend an insulated room for the equipment even with a non condensing furnace"
Our current heating/cooling system uses a package unit that sits outside, on the north side of the house. So it is often exposed to freezing temps during the winter. I'm not clear why we'd need an insulated room for the furnace. Is this just a question of efficiency?
I like the idea of the float safety switch in the pan, however. Great idea.
If you have equipment designed to sit outside, no problem. I made the assumption that standard equipment was to be installed in an attic.
If this is new work, consider bringing the entire attic space 'inside' the building envelope. Avoids the whole issue you are facing, and pays dividends down the road in energy savings.
This is replacement work in a house built in 1935. I don't think that bringing the attic and basement into the conditioned space is going to be too easy to do.
Generally a basement can at least be made safe from freezing. It doesn't have to be fully integrated into the HVAC. An attic is a little more difficult, of course, especially when you consider the hazards associated with a power outage in cold weather -- you can keep most of the house above freezing, but likely not an attic area that isn't designed as part of the conditioned space.