View Full Version : system replacement advice
05-21-2012, 12:52 PM
The original 3.5 ton, 10 SEER York air conditioner in my circa 1999 home needs to be replaced. Wasn't generating the right amount of cooling, low refrigerant pressure and leaky evaporator confirmed by local HVAC company. We also have an 80k BTU, 80% single speed York Diamond furnace that works fine.
We're in the Maryland suburbs of DC in a 2400 sq ft, 3 story end unit townhouse (basement level is 1/2 underground at front of the house, walk out in back). The supply ducting is split with one set going to the first and second floors and another going to the 3rd, with manual dampers. The AC has always seemed adequte, keeping the house comfortable even on the hottest days. The heating if anything seems too strong, the house very quickly comes up to temperature and it does seem to cycle more than the AC does. Only comfort complaint is that the basement floor stays pretty cool in the winter (there's a gas fireplace there we occasionally use, it actually heats the whole house pretty well if left on for a while), and that the 3rd floor (3 bedrooms) is a few degrees warmer than the rest in the summer. I have the thermostat drop a few degrees around 1 am to get an extra shot of cooling in the summer. Review of bills shows that we spend ~$450-500 per year each for heating and cooling. We'll be in the house a minimum of 5 years, probably 10, possibly longer.
I called in 5 companies to provide estimates. All seemingly legit local businesses - 2 big ones, 2 moderate (one of whom I've used occasionally over the years for repairs/maintenance), 1 small. One of the big ones is listed on this site's contractor map. None of the 5 did a Manual J. All of them examined the existing equipment and measured the return ducts at the air handler. One of them (the small one, the owner was the guy who came out) did a walkthrough of the entire house and looked at the supply and return vents. Everyone has quoted a 3.5 ton AC for replacement and ~80-90k BTU furnaces. I think I'm OK with this as the existing stuff seems to be working fine, but I'm very curious as to whether a lower output furnace might be suitable.
I'm still going back and forth with them to get more details and clarifications, just looking for some general advice now. Based on the estimates so far, there seems to be some economies of scale for doing the entire system vs. just the AC. So to some extent not doing the furnance now is gambling that it'll last a good while longer. If we do decide to do the furnace, just replacing it with a new one that's functionally the same adds ~40-50% to the cost of the AC replacement. Going with a more sophisticated type (variable speed, 2 stage, etc) adds another grand. In some way it doesn't seem "worth it" to just put in a new one that's the same as the old one, but on the other, going with a higher spec system nearly doubles the cost of just doing the AC.
Realize I haven't actually asked much in the way of questions so here are a few specifics:
- is it a big deal that no one's doing the manual J?
- typical life of a furnance like we have?
- is a variable speed furnance really that much nicer? How about one of the mid-grade 2 stage furnaces?
- Quotes are for Bryant, Lennox, and Trane. From what I gather the hardware is all basically the same quality so any would be fine. Any particular models of each that should be avoided, or represent a poor value? I think I saw somewhere on here that the basic Trane XB300 AC unit wasn't well regarded.
05-21-2012, 01:59 PM
Yes a manual J is important, if you are replacing the system you could probably install a 3 ton if ductwork was setup properly and sealed, if you install a 3 ton you can often put in a 40-60K furnace(each manufacturer has different sizes unlike AC's where all make mostly the same sizes)
The furnace and the AC have to match up so a 3.5 Ton AC requires a furnace with a 3.5 ton or larger blower motor(generally only found in 80K and up sizes) So in your case a manual J could save you money by sizing your AC down half a size and your furnace down a full size.
Furnace "should" last 10-25 years depending on maintenance filter changes etc.
Variable speed units ARE nicer IMHO if ductwork is sized properly, all variable speed units that I know of are 2 stage.
Others that know your area better will probably chime in shortly.
05-21-2012, 02:05 PM
If maybe you have a report on a load calculation from when the house was built then no not one now is no big deal. There is no requirement for doing load calc. before you can be listed as a contractor. Yes I would only buy from a contractor who first did a legitimate load calculation.
05-21-2012, 03:57 PM
It's really been interesting talking with the estimators having spent some time reading this site. I'm a mechanical engineer and pretty detail oriented so the consensus here to have the manual J calcs done appeals to me but it in no way seems to be a common practice.
I did just spend some time on the phone with a company who will perform the calcs however. I believe they're the company who did the original installation, as there are some markings on the ductwork with their name on it. I explained to the guy that I've gotten several estimates but that no one did anything beyond measuring the return ducts. He said they'd measure the rooms & windows, check what types of windows we have, check out the foundation & what parts are underground, examine the walls, orientation of the house, etc. The stuff I understand is included. Then it would take a day to turn around the calcs and provide recommendations for the equipment.
He also said that heaters in our area frequently seem oversized due to the desing criteria being maintaining 72 indoors at a 0 degree outdoor temp (it's almost never that cold here and we tend to keep it at 68), and that sometimes the blower size required for the ducting (which is presumably driven by AC requirements? - my assumption) requires the use of a minimum size furnace which also might be higher than the typical load would need. He also said this is one of the reasons the two-stage furnaces are becoming popular, which makes sense. He's coming out on Wednesday.
05-21-2012, 04:01 PM
Sounds like this guys knows his stuff, even if his company originally put the equipment in doesn't mean they did it right 15 years ago. I look back at what my company did 15 years ago and its nothing like today, 15 years ago a load calculation was MUCH harder to do and not as accurate. Plus many things change over time and you may have upgraded parts of the house so a new calculation is needed.
05-21-2012, 05:01 PM
80,000BTU input 80% efficient furnace is 64,000BTUs output. A 80,000BTU input 95% furnace is 76,000BTUs output. Doesn't sound like you really need to go up in size on your furnace. Does your current furnace have to run continuous when its 0 outside?
05-21-2012, 05:27 PM
your climate is similar to mine, so I'll chime in that it seems oversized, IF the DUCTWORK is sealed properly. if existing ductwork is untouched, then likely staying with what's installed currently would be wise.
ductwork sealing can half again increase the cost of the install, but it'll pay for itself in energy savings, and installation costs, IF it reduces the equipment costs.
one thing to note about ductwork sealing, is it the ductwork is all inside the building envelope heated space, then sealing is not going to make any difference.
in other words, if the ducts are in the attic or a vented crawlspace, sealing them is critical. if it's in the walls or between floors of the house, then it's not going to make much difference.
05-21-2012, 10:47 PM
beenthere - not anticipating upgrading the furnace beyond 80% due to costs associated with new exhaust, so you're correct that I don't see any reason at all to increase the capacity. It seems the 80% Bryant furnaces are either 88k input (quoted systems) or 70k or so. So the Bryant folks are quoting the 88k system which is 10% more than my (maybe?) somewhat oversized existing system. Lennox and Trane both have 80k units.
vstech - the 1st and 2nd floor ducts are within the envelope, the last 10-20' of the ducts for the 3rd floor are in the attic, which is uninsulated. The ducts themselves are mostly some sort of insulated flexible circular tubes. They seem to be continuous from a central distribution box (which is also clad with insulation) to the registers in the ceiling.
Thanks for the thoughts everybody. I'll report back on the results of the manual J calcs.
05-24-2012, 12:26 PM
Had the gent out to take the measurements for the load calculations yesterday. He measured the dimensions and arrangement of the rooms, windows and doors, jotted notes about various aspects of the house. He also did the usual measurements of the ducts around the furnace and inspected the existing equipment.
Called me back this morning with the results. Said he came up with 3.7 tons of AC required and "a little over 50k btu" for the heating. Given my experience in the house that would seem to confirm (assuming some error in the calcs) that my existing 3.5 AC is OK and that my 80k/80% furnace putting out 64k btu may in fact be oversized. He sells a product line that has 60k and 80k input units, which would result in a slightly undersized unit, or another of what I have now. His recommendation was to stick with the 80k in a two stage model which would therefore use the lower stage most of the time. Thoughts?
Does the fact that our basement living room has a gas fireplace, which we use from time to time to heat up that room (it's always colder down there) and has been able to keep the house safely warm during multi-day mid-winter electrical outages, suggest that maybe the smaller 60k furnace would be safe? And if so, the last question I guess is if that 60k furnace's blower is suitable for a 3.5 ton AC? Thanks everybody,
05-24-2012, 03:41 PM
Some details on a proposals I've recieved:
- For just replacing the AC, I have quotes for the Bryant 113 and 126 models and Trane XB13 and XR13. They are all within a few hundred dollars of each other, the 126 is the priciest by a bit but the installer is throwing in an extended 10 year labor warranty (of their own, not Bryant).
I think I've decided it'll either be just the AC, or the AC with a 2-stage, variable speed furnace. As mentioned earlier our heating and cooling bills aren't too high so there's not much incentive to spend on higher than basic SEER, and upgrading over 80% on the furnace is impractical.
- Bryant Evolution w/186BNA042 AC, CNPVP4217AT coil, 315AAV048090 furnace with the Evolution thermostat. A couple guys quoted the same equipment with similar pricing
- Trane XR15 model 4TTR5042 AC, 4TXCB048BC3 coil, XV80 TUD2C080AV4 furnace with the Trane 803 thermostat
- Trane XL15i model 4TTX5042A1 AC, 4TXCC043BC3 coil, XV80 TUD2C080A9V4 furnace with Trane 802 thermostat
- Trane XR13 model 4TTR3042D1 AC, 4TXCC043BC3 coil, XV80 TUD2C080A9V4 furnace with Trane 802 thermostat
This last option is being offered at an extremely competitive price. Vendor was identified through Trane's website and seems to be a legitimate local business. I pulled the AHRI certificates on these combos and found that the XR13 only gives up 0.5 SEER to the XL15i, and 1.25 to the XR15. Not a positive ROI for pricier systems. Sound is all within a db or two per Trane's materials. All of them are probably quieter than the existing York machine. Is there any drawback of matching the XR13 with the variable speed furnace? Would I be missing out on anything tangible? Thanks,
05-24-2012, 03:48 PM
Here in Portland OR we rarely go above 13 SEER for AC's, only go to high SEER for heat pumps. The XR13 has been our best selling AC and is a great product when matched with the XV80 furnace. The XL15i is significantly quieter in my opinion and thats what i have at my house(heat pump though)
05-25-2012, 01:01 PM
Just did a verbal agreement with the small company mentioned in the original post for an XR13, XV80, and I believe 4TXCB048BC3HCB coil. I believe he specified the 4 ton coil in the narrower ~17" cabinet due to the physical dimensions of the space and existing ductwork. Combo is on the AHRI list, and we actually have the same relative combo now so don't see any problem there. While he didn't perform the manual J calcs, I got the best overall vibe from him and by what came back in my research. Thanks everybody,
05-25-2012, 03:25 PM
Joe the same thought has crossed my mind, and it would probably be a safe way to go. It's really just a hunch on my part that's at least somewhat reinforced by the load calc (the rigor of which I have no way to evaluate), so I dunno. I may run the thought by him next time we talk and get his opinion. From what I gather the 60k Trane furnace puts out something like 1225 cfm maximum, which is somewhat marginal for a 3.5 ton cooling load I think? Anyway perhaps some aspect like that or the duct sizing puts higher minimum limit on the furnace size?
All that said the temps are turning up around here and I don't want to get too deep into the heat season. If there's one thing I've found is that it's extremely hard to find someone who will delve into the details as some of the folks around here would seem to. Given that, and knowing our existing system basically works fine (if somewhat oversized on the heat), I'm OK going forward.
05-25-2012, 04:51 PM
It would be a good idea to first evaluate your homes "envelope" before making any decisions. If the house has excessive air leakage & has insulation issues, they should be addressed first for several reasons. Improving the envelope can reduce your utility bills, increase the overall comfort of your home by eliminating cold/hot spots & drafts. Just because the ducts are in the "envelope" it doesn't mean leaky ducts will not increase infiltration/exfiltration, and also decrease your systems efficiency. Do you have returns that simply use the wall cavity as the return? Have you checked how much that wall is connected to the attic through air leaks? Your home is as much a part of the comfort equation as is the HVAC system.
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