Help with single stage or 2 stage unit

[tjdorrie]

Hello,
My AC unit is 11 years old, failing and needs to be replaced.
Since I moved in three years ago the unit never did a good job with humidity and now is having issues cooling. I have no idea how well (or not) the unit worked when it was new.

I have had a few dealers out to give me quotes.
Nobody in my area seems to do the manual-J they just look over the unit, ducts, returns, and house size.

The house is single story & 1775 square foot.
Existing unit is a 2 ton carrier AC unit (38TRA024320) with a 3 ton coil.
Most dealers have told me that it was sized too small and a 2.5 ton unit is needed.

Here is where I need some help.
The carrier dealer has recommended the following:
Infinity gas furnace (58MVC080-14)
Heat Pump (25HNA636A003)
Evaporator Coil (CNPHP4221ATA)
Infinity Thermostat (SYSTXCCUID01)
Honeywell Media Filter (F-100)

While I like the idea of a 2 stage for getting the humidity out, however, isn't stage-1 (1.5ton) to small and stage-2 (3ton) to large if he felt a 2.5 ton was needed?
I was also quoted a 2.5 ton performance unit from another carrier dealer.
Need help trying to figure out which unit would be better.
Infinity 2 stage or performance 1 stage.

Thanks.....

[dan sw fl]

Hello,
My AC unit is 11 years old, failing and needs to be replaced.
Since I moved in three years ago the unit never did a good job with humidity and now is having issues cooling. I have no idea how well (or not) the unit worked when it was new.

I have had a few dealers out to give me quotes.
Nobody in my area seems to do the manual-J they just look over the unit, ducts, returns, and house size.

The house is single story & 1775 square foot.
Existing unit is a 2 ton carrier AC unit (38TRA024320) with a 3 ton coil.
Most dealers have told me that it was sized too small and a 2.5 ton unit is needed.

Here is where I need some help.
The carrier dealer has recommended the following:
Infinity gas furnace (58MVC080-14)
Heat Pump (25HNA636A003)
Evaporator Coil (CNPHP4221ATA)
Infinity Thermostat (SYSTXCCUID01)
Honeywell Media Filter (F-100)

While I like the idea of a 2 stage for getting the humidity out, however, isn't stage-1 (1.5ton) to small and stage-2 (3ton) to large if he felt a 2.5 ton was needed?
I was also quoted a 2.5 ton performance unit from another carrier dealer.
Need help trying to figure out which unit would be better.
Infinity 2 stage or performance 1 stage.

Thanks.....

A/C selection is somewhat similar to the old Real Estate adage.

What is your Location?

Gas rate $_._ _ / per therm ?
Electric Rate $0._ _ / kW ?

[RyanHughes]

I too would want to know the location. If you're getting a heat pump, a 95% 3-stage gas furnace for backup may not be cost effective.

Regarding your concerns with the 2-stage unit, the idea is that it runs long cycles in its first stage (because of its capacity being less), which allows it to dehumidify well. Chances are it will spend most of its time in low stage maintaining temperature/humidity. However, you do have extra capacity (high stage) when you want to change the temperature rapidly or when its very hot outside.

[tjdorrie]

Thanks for the replies.
Locations is Southern Maryland, and it can get very humid here.
I currently have an electric A/C and gas Furnace, dealers all recommend the heat pump option with the gas furnace. I will need to look up the rates.

That is exactly how one of the dealers described the 2-stage unit, stage-1 is great for removing humidity. I guess my main question was if 2.5ton is recommended and this unit is a 50/100 split; 1.5ton stage-1 & 3ton stage-2. 1.5 ton is smaller then what I have now, 2.5 ton was recommended, and 3ton is larger then what was recommend. So does a 1.5/3ton unit work better then a 2.5ton unit? By work better I only mean comfort (humidity removal is important).
I would hate to get something that is both to small and to large.
Thanks...

[dan sw fl]

Thanks for the replies.
Locations is Southern Maryland, and it can get very humid here.
I currently have an electric A/C and gas Furnace, dealers all recommend the heat pump option with the gas furnace. I will need to look up the rates.

That is exactly how one of the dealers described the 2-stage unit, stage-1 is great for removing humidity.

1. So does a 1.5/3ton unit work better then a 2.5ton unit?
By work better I only mean comfort (humidity removal is important).
I would hate to get something that is both to small and to large.
Thanks...

1. Yes.

3-ton 2 stage is my recommendation for the several 93'F + days to handle higher than the actual outside air design temperature.

Controls will handle the differences in cooling and heating loads with
'staging' (on/off switching ) of the lower and higher capacities.

Use of Gas furnace may prove to be more economical below ~ 28'F given southern Maryland rates and HP COP characteristics.
( I know electric rates in MD have risen significantly in the last ~ 2 years).

[dash]

Without a load calc.,your experience in previous summers is the best gauge for sizing.

If it was satisfactory ,then two tons is correct.

Plus maybe add some attic insulation,if less then R30 now.

[gary_g]

Thanks for the replies.
Locations is Southern Maryland, and it can get very humid here.
I currently have an electric A/C and gas Furnace, dealers all recommend the heat pump option with the gas furnace. I will need to look up the rates.


I live in Harford County.

Not sure if you have BGE, but our electric rates are high (about 15 cents per kw-hr delivered). When checking your electric rate, make sure you get the delivered rate. Delivered = generation + transmission + distribution.

I switched from BGE to Washington Gas and Electric (WGES) a few months ago and locked in at 10.9 cents per kw-hr generation for 1 year.

I have a 3-ton heat pump in an all-electric home. Same size house as yours but a split-foyer.

Not sure if a heat pump is cost effective with our electric rates. I don't know the delivered price of gas.

If you do go heat pump, you have a choice of heating fuels if the price of one of the fuels should jump in price. Choices are a good thing.

Take care.

[jerryd_2008]

...

That is exactly how one of the dealers described the 2-stage unit, stage-1 is great for removing humidity. I guess my main question was if 2.5ton is recommended and this unit is a 50/100 split; 1.5ton stage-1 & 3ton stage-2. 1.5 ton is smaller then what I have now, 2.5 ton was recommended, and 3ton is larger then what was recommend. So does a 1.5/3ton unit work better then a 2.5ton unit? By work better I only mean comfort (humidity removal is important).

I would hate to get something that is both to small and to large.
Thanks...

I'm a HO and not a pro, but aren't they telling you that the 1.5 ton will take out the humidity by running long cycles at greatly reduced power. The 2nd part is that the larger stage 2 will then be able to rapidly take out the sensible heat when and if stage 1 fails to do that.

Another consideration for the larger stage 2 is that it will probably provide more heat in the winter, right?

Anecdotally, I changed out a 4-ton 10 SEER split AC/furnace for an 18 SEER DFHP with a 5-ton coil, the new configuration is probably over sized since the old unit keep up on hot days even in our larger home. The new system is capable of keeping up and dehumidify while running at stage 1 which is about 70% of stage 2 capacity for my York system. That's a ton less more efficient capacity with same cooling and dehumidify thrown in. A smart tstat will also help manage the stages.

PS: Do that Manual J even if you have to pay for it. Does your utility company offer a reasonable cost energy audit with this included? Mine cost only $100.

[tjdorrie]

Without a load calc.,your experience in previous summers is the best gauge for sizing.

If it was satisfactory ,then two tons is correct.

Plus maybe add some attic insulation,if less then R30 now.

I agree, but that is the problem for me because I felt the that AC was not working right since I moved in 3 years ago (got really bad this year), but it might have been working great for the original owner. So far nobody in my area that I have called seems interested in doing a load calc, they just look it over and then tell me what they think I need based on their experience. An since the unit is now struggling to cool the house (because its has problems) I think that is probably why most of them are going from 2ton to 2.5ton.
This is crazy, I certainly was not expecting this issue when I called for quotes. I feel like I am flipping a coin.

[tjdorrie]

I'm a HO and not a pro, but aren't they telling you that the 1.5 ton will take out the humidity by running long cycles at greatly reduced power. The 2nd part is that the larger stage 2 will then be able to rapidly take out the sensible heat when and if stage 1 fails to do that.

Another consideration for the larger stage 2 is that it will probably provide more heat in the winter, right?

Anecdotally, I changed out a 4-ton 10 SEER split AC/furnace for an 18 SEER DFHP with a 5-ton coil, the new configuration is probably over sized since the old unit keep up on hot days even in our larger home. The new system is capable of keeping up and dehumidify while running at stage 1 which is about 70% of stage 2 capacity for my York system. That's a ton less more efficient capacity with same cooling and dehumidify thrown in. A smart tstat will also help manage the stages.

PS: Do that Manual J even if you have to pay for it. Does your utility company offer a reasonable cost energy audit with this included? Mine cost only $100.

That is exactly what they said about stage-1, will do better with humidity and stage-2 will be for those really hot days. I guess my concern was that stage-1 may be so small that it can cool and will always need to run in stage-2 which could be way to big.
Going to try and call a few more dealers and see if I cant find somebody that will do the load calc.

[tjdorrie]

I too would want to know the location. If you're getting a heat pump, a 95% 3-stage gas furnace for backup may not be cost effective.

Regarding your concerns with the 2-stage unit, the idea is that it runs long cycles in its first stage (because of its capacity being less), which allows it to dehumidify well. Chances are it will spend most of its time in low stage maintaining temperature/humidity. However, you do have extra capacity (high stage) when you want to change the temperature rapidly or when its very hot outside.

I guess this is the main question that I have. How do I know that the system will run in stage-1 for a long time and dehumidify? What if stage-1 is too small would it not just switch up to stage-2 to cool properly? If stage-1 can not maintain temp would it not just switch to stage-2?

[tjdorrie]

A/C selection is somewhat similar to the old Real Estate adage.

What is your Location?

Gas rate $_._ _ / per therm ?
Electric Rate $0._ _ / kW ?

Southern Maryland
Not sure how to do the math so I just took the total and divided by therm and/or kWh used.
Electric Rate $0.16
Gas rate $2.05
The Gas rate seems to go up and down, was as low as $1.40, the electric rate was rock steady.

[dan sw fl]

Southern Maryland
Not sure how to do the math so I just took the total and divided by therm and/or kWh used.
Electric Rate $0.16
Gas rate $2.05
The Gas rate seems to go up and down, was as low as $1.40, the electric rate was rock steady.

Gas 100,000 BTU = $2.25 @ 90% efficiency
HP 100,000 BTU =~$1.57 @ C.O.P 3.0 near 28'F

So use gas sparingly by-design ( < ~ 20'F)

[DanW13]

That is exactly what they said about stage-1, will do better with humidity and stage-2 will be for those really hot days. I guess my concern was that stage-1 may be so small that it can cool and will always need to run in stage-2 which could be way to big.
Going to try and call a few more dealers and see if I cant find somebody that will do the load calc.

If you can't find someone in your immediate area willing to work with you or supply what your asking for,then I would simply pick up the phone book and start calling companies outside the immediate area and ask the question over the phone, that way they won't waste your time or you won't waste there's if they tell you up front that they do not need to run any load calc's !!