# Matching AC size to furnace size



## crabjoe (Dec 18, 2015)

What size furnace do you have? Make and model would be helpful...

BTW, the furnace fan should have multiple speeds that are selectable, and you should be able to select one speed for AC and a different one for Heat. Oh.. A furnace can actually be to large for the AC..


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## Dixon12 (May 3, 2016)

It's Rheem Criterion II, model #RGPH-12EARJR, manufactured around 2001. According to one of the techs that gave me estimates it can't be set to the right speed for a modern AC. I don't know if that's true or they are just trying to sell me on furnace replacement, which I'm not gonna be doing--just too much $$, and the one I have works perfectly.


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## crabjoe (Dec 18, 2015)

If I'm reading the model number correctly, you has a 125K BTU furnace. Ummm... yeah, that's really over sized. Based on the model number, the slowest your furnace blower can go is 1800 CFM. That's bit to high. Not being in the business, I'm not sure how that will perform, but that's a lot of air coming though your ducts. I'm now wondering if your ducts are even sized correctly.

My understanding is that you want about 400cfms per ton for AC.


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## beenthere (Oct 11, 2008)

At .7" ESP, and the blower set to low speed, that furnace will only move 1175 CFM. A bit high for a 2.5 ton, but probably okay unless you live in a high humidity area.

No reason to go to 3 tons if your current unit is 2.5 tons and cooled fine.


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## Dixon12 (May 3, 2016)

It is 125K BTU. Also the ductwork is most likely undersized, but I'm really not looking into redoing the ducts at this point.

And I don't know whether the old 2.5 tons AC cooled the house well. Only bought the house recently and haven't used the AC.


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## crabjoe (Dec 18, 2015)

ESP is the static pressure. I'm not sure how that figures in, other than the less you have, the better the air flow. If the ducts are undersized, your going to have more..

Off the cuff, if the old 2.5 worked with it, I say leave it. That's what I would do. But when it came time to replace the furnace, I'd be looking at a much smaller furnace. 

I'm in a similar situation as you. My house is 2400sq ft and I have a 100K furnace. And being in MD, and as tight as my house is, I don't need one this large, but that's what the build put in. If I'm still here when it needs replacement, I'll downsize to an 80K furnace.


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## user_12345a (Nov 23, 2014)

Dixon12 said:


> It is 125K BTU. Also the ductwork is most likely undersized, but I'm really not looking into redoing the ducts at this point.
> 
> And I don't know whether the old 2.5 tons AC cooled the house well. Only bought the house recently and haven't used the AC.


It makes zero sense to oversize an ac to match an grossly oversized furnace.

You should downsize the furnace, it's probably cycling on limit.

Even 2.5 ton is a heck of a lot for a house of this size unless in an extremely hot climate. Unless you live in a dry climate it should be marginally sized for best dehumidification - sized so that it runs continuously in the most extreme heat you get. It's all about pulling out the moisture and even a high setting like 75 to 78f can be comfortable if it's dry.

You likely should get a load calculation done. Where i am if decently insulated with no attic/crawlspace ductwork detached house like that would get a 40 to 60k furnace and 1.5 to 2 tons of cooling. (with the 60 being on the large size - 95% efficiency assumed. get an 80% and a higher input is needed to get the right output)


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## sktn77a (May 11, 2009)

The furnace is too large for a 3 ton AC also. 2.5 tons sounds about right for your house size, although there at other factors that come into sizing. I'd be inclined to stick with the 2.5 ton sizing and run with the oversized blower until you can afford to replace it (sounds like you're working to a budget). You can reduce the blower speed on your present furnace somewhat, if the heating temperature rise remains within spec. You contractor should be able to do this properly. (Then, again, you might want to check your furnace installation indstructions and double check his work!)

:-(


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## supers05 (May 23, 2015)

sktn77a said:


> The furnace is too large for a 3 ton AC also. 2.5 tons sounds about right for your house size, although there at other factors that come into sizing. I'd be inclined to stick with the 2.5 ton sizing and run with the oversized blower until you can afford to replace it (sounds like you're working to a budget). You can reduce the blower speed on your present furnace somewhat, if the heating temperature rise remains within spec. You contractor should be able to do this properly. (Then, again, you might want to check your furnace installation indstructions and double check his work!)
> 
> :-(


If it's not a high humidity area, it'll with just fine. The OP will actually get higher capacity out of it. 

If you're going to check his work like he's suspect, then why even hire him? 

Cheers!


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## Dixon12 (May 3, 2016)

supers05 said:


> If it's not a high humidity area, it'll with just fine. The OP will actually get higher capacity out of it.
> 
> If you're going to check his work like he's suspect, then why even hire him?
> 
> Cheers!


I picked 3 contractors from Angie's list, all A+ and whathaveyou. I don't think these ratings mean much since they are most likely based on typical installs. IMO, there is no way to know how any of them will handle a trickier situation, like what I have. Also, I'm on a budget, really don't plan to live in the house for very long, and there are other things that need to be fixed. So spending another $3,000 or so on furnace isn't an option.

As it is, here're the proposals I got:
1st HVAC tech diagnosed major leak and basically said that the 26 years old AC system I have should just be replaced because repair would cost as much as a new one. (Having done some research, I figured that it's really the case.) Proposed 13 SEER Rheem 2.5 ton for $4.4K Price is an obvious ripoff, so they are out.

Next guy (A++++++ on Angie's list, family owned company (as if that should matter for some reason:vs_worry, etc) proposed 3 ton 13SEER Trane, claimed 3 ton is a better match for the oversized furnace. Said 16SEER would be $600 more and doesn't make sense because requires variable speed blower. $3.5K

Next guy (same background as the first) -- proposed 16SEER Bryant, said doesn't require variable speed blower. After checking furnace specs said there is no way to make both AC and furnace work right.

Next guy (same Angies List ratings, etc)-- 16SEER 2.5 ton Bryant, 3 ton coil, $3.5K

So given that they all say different things, it's hard to know whom to trust. Was thinking of getting a couple more estimates, but I doubt they'd be much different price-wise or gonna tell me anything new.


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## sktn77a (May 11, 2009)

supers05 said:


> If it's not a high humidity area, it'll with just fine. The OP will actually get higher capacity out of it.
> 
> If you're going to check his work like he's suspect, then why even hire him?
> 
> Cheers!


I wasn't saying that it wouldn't work, just that a 3 ton condenser isn't going to be matched to that furnace either.

As to your last point, I think president Reagan said it best...... _"Trust but verify!"_


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## user_12345a (Nov 23, 2014)

> I picked 3 contractors from Angie's list, all A+ and whathaveyou. I don't think these ratings mean much since they are most likely based on typical installs. IMO, there is no way to know how any of them will handle a trickier situation, like what I have. Also, I'm on a budget, really don't plan to live in the house for very long, and there are other things t


Get the properly sized a/c even if it's 2 ton.

The fan can be slowed down to what a 3 ton needs.

You can use a restrictive filter, close dampers to get the airflow down.

In a situation like yours I would consider getting the unit topped off and leak stop put in, unless the leak is very large. I'm not fond of doing that, but it's far better to do the furnace and ac as a set.

If it was my choice i would forgo ac for a while and put the money towards dumping that very grossly oversized furnace, then do the ac later (and if it's the same age as the ac u would save a lot of gas).

But oversizing cooling to match an oversized furnace is nuts. 

With a load calc u may find that the house will cooling with 1.5 or 2 tons. Putting in the same as as before just because it worked is silly. (it may have worked but burned through more kwhs than necessary, or u may have had to crank the t-stat down to removed humidity)


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## Dixon12 (May 3, 2016)

As I understand it, topping it off to even find a leak would cost upwards of $1000. At least that's what the HVAC tech that diagnosed the problem told me. It runs R22, and according to the guy that's what it would've cost to just fill it up to find the leak. He may have been just lying, since I used a $40 diagnostic service coupon from Angie's list, and may have just wanted to recover/make some money. Was thinking of using a $99 leak detection coupon from another AL company, but figured even if they find it, the repair would be way expensive, plus the $1000 to fill it with R22, and would end up being about the same as new unit.

As far as the furnace, it's not as old, and didn't cost me that much over the winter. My worst gas bill was around $150 for a month. So I'm reluctant to do anything with the furnace and really hope I won't have to. Figured I'd call up 2-3 more companies and see what they have to say.


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## beenthere (Oct 11, 2008)

A 1000 bucks to fill/top off a 2.5 ton. Dang, I'm way too cheap on my R22 prices.


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## crabjoe (Dec 18, 2015)

beenthere said:


> A 1000 bucks to fill/top off a 2.5 ton. Dang, I'm way too cheap on my R22 prices.


Bwahahahaha!! The best quote my buddy got was $100 a lb for R22. He was told if it's completely empty a 2.5 ton system would take up to 8 lbs.

He says he doesn't want it fixed, just fill until he can save up enough for a new system. I told him I'd fill for less than 1/2 + beer and pizza since I still have some R22 here and my current system is R410a. I just need enough to cover my drive and a new tank of R410a.

BTW, I recently priced out a tank of R22 at the local supply house and the price jumped almost $200 from the beginning of last fall. I thought it was expensive before but it's getting crazy now.


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## crabjoe (Dec 18, 2015)

If you're going to replace the system anyways but need some freon to hold you over .. Have you looked into something like MO 99? It's not as efficient as R22, but it's a drop in replacement for it. I still wouldn't mix it with R22.

I also don't think you need a EPA card to handle it (purchase), but you still need to follow EPA rules on handling refrigerants.


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## Dixon12 (May 3, 2016)

I'll look into MO 99. Looks like it's about $250 for 25lbs on ebay. Would be great if it can carry me through the summer. 

Would it be possible to do with just a set of gauges? How much do these usually run--I seen some for ~$100, but have a very faint idea about how to use these and no idea what to look for. Also a quick search for MO99 says to replace filter dryer and elastomeric seals. Where would I look for these and how difficult or necessary is it do to this?

Regarding R22, I don't have the quote in front of me, but it was something like $100/lb x 5 lbs plus evac of old refrigerant plus service charge and whatever fine-tunning may be needed.


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## crabjoe (Dec 18, 2015)

I don't think any respectable person is going to say just add it, but there are people doing it. You can google it and see for yourself.

The proper way would be to evacuate the R22 then refill with MO99. Meaning you're going to need recovery equipment and have an EPA card or call someone in to do it.

If it were me, I would pull all the R22 out then refill with MO 99. The problem you're going to have is if you put a vacuum on a leaky system, you could pull air into the line...

If you add it to your existing R22 system and end up with a mix, I don't know what will happen.... But I do know one thing, assuming it works, you'll need to tell the person recovering the refrigerant later that it's a mix of R22 and MO 99. If you don't and they add it to tank that has R22 in it, it's going to contaminate their tank and they'll have to pay to have it disposed of instead of being able to sell it for recycling.


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## beenthere (Oct 11, 2008)

Dixon12 said:


> I'll look into MO 99. Looks like it's about $250 for 25lbs on ebay. Would be great if it can carry me through the summer.
> 
> Would it be possible to do with just a set of gauges? How much do these usually run--I seen some for ~$100, but have a very faint idea about how to use these and no idea what to look for. Also a quick search for MO99 says to replace filter dryer and elastomeric seals. Where would I look for these and how difficult or necessary is it do to this?
> 
> Regarding R22, I don't have the quote in front of me, but it was something like $100/lb x 5 lbs plus evac of old refrigerant plus service charge and whatever fine-tunning may be needed.


If he is just topping off, no need to evacuate the system. Sounds like they are just trying to make the bill high enough you replace it.


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## user_12345a (Nov 23, 2014)

mixing refrigerants may pose problems when they recover it to replace the machine.

topping off without dealing with the leak is bad but apparently legal in the us.

seems like no one one to keep anything running any more, just replace.


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## Dixon12 (May 3, 2016)

So what would be a fair price to charge some 5-7lbs of R22? And would a tech agree to put stop leak in there if I ask for it?


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## beenthere (Oct 11, 2008)

Dixon12 said:


> So what would be a fair price to charge some 5-7lbs of R22? And would a tech agree to put stop leak in there if I ask for it?


Just about what ever a contractor wants to charge, and a customer is willing to pay for it. Lots of companies are charging around 100 bucks a pound.

Some will put it in, others won't. Best to ask on the phone before you have someone come out.


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## sktn77a (May 11, 2009)

Dixon12 said:


> So what would be a fair price to charge some 5-7lbs of R22? And would a tech agree to put stop leak in there if I ask for it?


Ranges from $50 a pound from the Craigslist hacks to $100 a pound from the top price places. Given your situation (short term "gas 'n' go" fix) you might want to check Craigslist.


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## user_12345a (Nov 23, 2014)

I must say that if the machine has no gas, it must have a large leak. large leaks shouldn't cost much to fix.

there's no point of gassing up a unit with nothing in it.

*they can pu*t nitrogen with a little r22 in for a fraction of the cost of a full charge.

alternatively just get a window unit to get by.


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