Any home HVAC sizing experts here?

[skyactiv]

I'm renting the previous house I lived in to a Jewish refugee family for $500 a month as a favor to my synagogue. A comp in the neighborhood sold for $176K and the rent covers the property tax, insurance and not much else. The renter called saying the AC runs non stop. It did when I lived in the house. It does struggle to keep the house at 75 degrees when it's above 80. Above 90 there is a thermal expansion valve that starts making a racket unless you set the thermostat at 78.


The outdoor unit is a Rheem, 2 ton 10 SEER dating back to 1992, installed years before I bought the house.
My brother who's a refrigeration tech for a major grocery chain installed a new 90+ 2 stage furnace along with a FIVE ton evaporator and a thermal expansion valve.
He put that in when he was new to the field.
The house is a 1521 square foot ranch.
He now admits he misjudged the evap coil size but he also said he does not size units in his field, that is left up to someone else.
I think the house needs a 2.5 ton unit and a 3 ton evaporator coil. Does that sound right?

 

[379KITTY]

Certainly sounds like a mismatch in size between condenser and evaporator coil, not sure how that would affect system performance. A system that old and only 10 SEER is likely way less efficient then the 20+ SEER units avail. today. Undersize unit will run more/longer than it needs to and probably won't drop temp as much as desired. With that in mind, an oversized unit won't run long enough (cycle too often) and won't dehumidify properly either. So sized correctly is best. I think it would be well worth replacing a unit that old, unless it was cooling completely to your satisfaction. Which doesn't sound like the case.

 

[spasm3]

I have always heard 600sq ft per ton. I'd go 3 ton compressor unit and evaporator coil.

 

[JustinH]

We have a 2.5 ton American Standard 14 SEER unit in Central Texas on 1030 sq foot house that is pretty well insulated.

I believe it is sized correctly.

The correct way is to get a contractor to do a manual J calculation and they will see what you really need.

 

[walterjay]

There are many factors to enter into the equation here. Insulation values, window type, exposure, air infiltration, expected outside temperatures and location. Have a professional do a heat gain calculation and determine the size needed. Guessing can be expensive.

 

[danez_yoda]

When is the last time you checked the charge on it? It mght be low on freon.

If you upgrade, on an older house (leaky) 3 ton top and bottom might be the ticket.

 

[NormanBuntz]

Originally Posted By: walterjay
There are many factors to enter into the equation here. Insulation values, window type, exposure, air infiltration, expected outside temperatures and location. Have a professional do a heat gain calculation and determine the size needed. Guessing can be expensive.


This plus number of windows (total glass area) and height of ceilings. Having a proper load study will pay future dividends. And you'll probably find that 17 SEER and higher systems may not be cost effective for you. I think 16 SEER is a nice balance of efficiency and cost.

 

[KrisZ]

If your brother works in refrigeration, he should know how to do a manual J calculation. It's not all that complicated.

 

[MParr]

It will take a 3 1/2 to 4 ton unit for that size home. And that’s going to depend on how well it’s insulated, what type and how many windows you have. It would be best to have a hvac contractor come out and crunch the numbers.

 

[Eddie]

3 ton unit is standard here in SW Florida for a 1,500 SQ FT home under air. Typical summer temps are 90-94 F and 80% humidity

 

[edwardh1]

I have been told manual J even oversizes, in that the people load is applied to each room.
anyone know?

 

[danez_yoda]

Just for for reference I have average insulated 2003 Perry home in Houston. 2500 sqft single story with a 4 ton ac. It will keep the house at 73 until the temp climbes to the high 90s. 104 degree days with high humidity it will hold 75 barely with all blinds closed and curtains pulled.

Standard 16" of blown insulation in the attic, not radient barrior. so-so (I think poor) roof ventalation (to few mushroom vents not whirley birds or power vents) Brick sides all around.

Heat of summer I'm peeking at ~2200Kwh/month keeping the house at 73.

 

[JHZR2]

Originally Posted By: KrisZ
If your brother works in refrigeration, he should know how to do a manual J calculation. It's not all that complicated.


+1. There are some free ones online. Don't know how good they are, but it's a start.

 

[maintenanceMan]

I Would go for the 3 ton. I bought mine off ebay and installed it myself. The only danger from oversize is that you don't remove enough humidity. When your units are running nonstop like that, you are removing lots of humidity.

 

[Nick1994]

Can't size a unit just by square footage and location.

Go create an account on hvac-talk.com and post in the AOP section, someone will write up a Manual J size for you for free. Nice guys over there. They'll ask you location, insulation info, window info, home age, etc. and tell you exactly what you need.