It would depend on how often you stay above worse case.
In my area I'll maybe see it three days out of the year..I could get by just find with the two ton.
I am about to have my HVAC system replaced and am seeking advice on sizing of the heat pump.
I carefully and conservatively calculated my heat loss/gain using HVAC-Calc. The results for my 1,400sf 2-story Florida townhouse are 18,162 BTUH heat gain (14,081 sensible) and 19,728 BTUH heat loss.
I am planning to have Trane equipment installed since I live 300' from the ocean and Trane warrants their condenser for 10 years. Amazingly, my Bryant condenser still functions after 9 years and 3 hurricanes, although it looks like hell.
I plan to use the 4TEE3F37A1000A variable speed air handler and I have been planing on using the 2-ton XL14i heat pump (4TWX4024A1000A). Per the 2001 spec sheets for the XL14i I found online, this combination is rated at 25,000 BTUH cooling (19,100 sensible) and 21,800 BTUH heating. By my math [(25,000-18,162)/18162], that translates to a cooling margin of 37%, a sensible colling margin of 35%, and a heating margin of 10% (the 10KW heat kit will bring that up to 35%).
The installer came out for a second look today as I'm having some returns added (this townhouse's cheesy 80's construction relied on undercut doors upstairs and the stairwell to get return back to the air handler in a downstairs closet with louvered bi-folds).
Anyhow, the installer expressed concern at only having 2 tons of cooling. He said that it was the second story, the West-facing end wall, and his suspicion that systems rarely perform to spec that led him to think I should use 2.5 tons.
My exisitng Bryant 3-ton condenser and big honking Bryant air handler (FK4CNB0006) still short cycles (~15 minutes)on all but the hottest days even as the condenser has turned into a salt lick.
So what do you guys think based on the info provided - stick with 2 tons or up it to 2.5 tons?
Thanks for your help.
CEL
It would depend on how often you stay above worse case.
In my area I'll maybe see it three days out of the year..I could get by just find with the two ton.
That cooling margin is only correct assuming your indoor design temperature is 80 degrees for cooling.Originally posted by cel
Per the 2001 spec sheets for the XL14i I found online, this combination is rated at 25,000 BTUH cooling (19,100 sensible) and 21,800 BTUH heating. By my math [(25,000-18,162)/18162], that translates to a cooling margin of 37%, a sensible colling margin of 35%, and a heating margin of 10% (the 10KW heat kit will bring that up to 35%).
HVAC-Calc tells you how much heating/cooling your home needs to reach the design temperatures you specify. Cooling equipment capacity is rated based on an 80 degree indoor temperature, though; if you want it cooler than 80 you have to de-rate the equipment capacity. The de-rating formula is 835 BTU * (number of degrees below 80) * system CFM / 1000. People rarely take this into account, and it's one of the main reasons why people mistrust manual J load calculations and/or think that equipment rarely performs to spec. Not that there aren't other reasons equipment doesn't perform to spec, like leaky ducts, lack of maintenance, improper charge...
For this two ton system rated at 25,000 BTU and a indoor cooling design temp of 75, that works out to a capacity derating of 3340 BTU, so this system is really 21,660 BTU cooling capacity at 75 degrees indoors. I'm just guessing you want 75; if you are using a different number, you'll have to adjust it to suit. Given 75, though, there's still some excess capacity there, so I wouldn't have any concern that this could be undersized.
Townhouses and condos are highly susceptible to oversizing because of all of the common walls. People who are used to sizing by square footage forget that this kind of structure just don't heat and cool the way single-family homes do. They tend to have VERY little exterior wall area compared to a single family home of the same square footage, even in end units.
I think the equipment you're looking at is a fine match for the load you cite, even if the design temp is 70. In your shoes, I'd stick with the two ton myself.
Remember, though, that if you specified in your load calc that you had window treatments on any west-facing windows, you really will need to keep them closed every afternoon for the load calc to stay accurate. And leaky ductwork is no good whether or not you have excess capacity!
I'd suggest going over your load calc in detail with your contractor to make sure he agrees with your measurements, inputs, etc., on a room by room basis. Also you may want to point out that this system is rated for 25,000 BTU, which is unusually high for a "two ton" system. Many 2.5 ton setups are only rated at 26-27,000 BTU, so this is close to the 2.5 tons that his intuition tells him to use anyway. And you already know that three tons is way too much. You will have plenty of heat even if the heat pump is on the fritz; 10kW will more than cover your entire heat loss in Emergency Heat mode.
Your outdoor unit will thank you for hosing it off with a garden hose regularly to wash all that salt out!
If you load calculation is correct ,and indoor design is 78°,stay with two tons.
If you can increase the attic insulation,to reduce the actual load even more,makes the two ton choice even safer,plus cuts your electric bill.
Thanks for the replies. I'll go with the 2-ton condenser per your advice.
I have one follow up question related to the refrigerant connections. The condenser connections are 5/8" and 5/16" and the coil connections are 3/4" and 3/8". Where will the installer make the transition or another way of asking the question is what size should the lines be?
CEL
If I was your installer I would have my opion and let you know that if you do not do as I advice then all problems with system would be on you ask the installer thier opion. and hear them we are not the ones holding the warr.
keep Jesus in control
so, what if your temp drifts to 82F for the 5 days when it is 100+ outdoors? even 85F --
I could live with that --
remember that rotating equipment is at its highest efficiency when fully loaded.
harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!
What temp was your Manual J based on? HVAC-Calc lets the user pick that. Standard tends to be 75. Since many like it cooler, I often bump that down a bit. If you had your desired indoor temp picked, I sure wouldn't go over 2 ton. Why the big air handler? Won't the 031 match and get better latent?
wyounder;HVAC-Calc tells you how much heating/cooling your home needs to reach the design temperatures you specify. Cooling equipment capacity is rated based on an 80 degree indoor temperature, though; if you want it cooler than 80 you have to de-rate the equipment capacity. The de-rating formula is 835 BTU * (number of degrees below 80) * system CFM / 1000.
I have seen this deration formula specified in Carrier/Bryant documentation. Does this formula work for other equipment manufacturers?
re BaldLoonie's questions...
My Manual J temps were 75 inside and 95 outside with 130 grains of moisture. Since I'm across the street from the ocean it rarely goes above 95.
I'm leaning towards the 037 rather than the 031 as the XL14i heat pump spec sheet shows the 2-ton condenser/037 air handler combo is about 2% more power efficient than the 2-ton condenser/031 air handler combo (1.98KW vs. 2.02KW cooling and 1.86KW vs. 1.90KW heating). However, doing the math it doesn't look so good for payback. Assuming FPL rates remain at $0.10 KWH (1st 1,000 KWH) and 20% system duty, that translates to just $7 savings per year.
Per the XL14i heat pump spec sheet, latent is actually better with the 037 (5,900 BTUH) vs. the 031 (5,500 BTUH).
Two other factors that have me leaning towards the 037 are 1) the normal CFM of the 037 is slightly lower - which I assume may make it quieter, and 2) the liquid inlet of the 037 is 3/8" vs. 5/16" - given that they both use a 3/8" coil it sems odd to me that the 031 restricts at the inlet. Trivial concerns probably.
CEL
PS: Thanks again for all the inputs. As I saw another person note, the posts on this site have helped me be a more informed consumer. In the end that should help both me and the company that does my install since I'm more likely to be satisfied if I understand what I'm getting, even if I do annoy them along the way by requesting specific equipment and asking lots of questions.
cel,
Most ratings you will find for any brand or ARI,are at 80°F indoors.
You deuduct 835 sensible btus ,per 1000 cfms (air flow),for every degree below 80°F .
So with two tons,800 cfms,it's 835 times,.8 (cfms) ,times 5(degrees)= 3340 sensible btus losss,when maintaining 75°F inside.