What if pro wants to use existing size hvac?

SlamDunk

Hi all,

We have a house we inherited and it is time to replace the hvac-32years old Rheem.

The ac side is 2.5 tons and works very well at cooling the house all summer long The natural gas furnace is 120,000 btu "effective output".

I think the furnace is too large, although it warms up the house nicely. I have used the slant fin app, US boiler, and Alpine home air calculators and the heat loss is around 55,000/hr across the board. The house is a single story ,1950 brick ranch with a full unheated, unfinished basement., @ 1000sqft. In living area

When, I start calling for quotes, if the estimator wants to use same size that is there, should I go with it because I know it will work?

The cost of a 70 kbtuh output furnace vs 120k is negligible. But the long term operating costs arent.

What should I do?

mattmia2

Fine a real pro if they tell you to use the same size that is there but the heat loss says it is half that...

Give them a chance to explain with their own heat loss in case you are wrong, but it sounds like you are in the ballpark.

ChrisJ

It's pretty easy to tell if it's oversized.
On an the coldest nights you've seen did it cycle or run continuously?

If it cycled a lot it's oversized. If it ran nonstop or close to it, it's good.

JUGHNE

20 years ago I changed out a furnace, it probably was the size you have. It was a very heavy old 1950's Lennox. They thought they could smell fumes and just wanted to change.

The house is just as you described yours, maybe a little larger, but same 1950's construction.

I proposed an 80,000 input 90+%, which was more than enough BTU's. The owner insisted on the next size up. That was a 100,000 input....$47.00 more at the time. His wife was always cold and the house was drafty.

The old unit had no problems being the size it was.
I have since concluded it could handle a higher temp rise and the high limit would seldom open.....if it did that was not noticed.

So since the new install: numerous HSI failure from cycling on high limit.....eventually high limit failure several times.
3 years ago the final straw was adding a bypass humidifier that increased the temp rise because of the 6" SA going into the RA. Had to go back to thin cheap filters . Down fired the NG valve as much as possible. Increased the blower speed up to maybe high.

The real problem was the old existing ductwork. The existing AC was 2.5 tons.....I incorrectly, naively, assumed the duct sizing was adequate. It was OK for the high temp old furnace.

Just a situation I had experience with.........FWIW

BTW, this is northern Nebraska with -10 design temp and about 7000 HDD.

ratio

Protip: the ductwork is always undersized.

mattmia2

Especially when you add 60 years of paint to the grills.

HVACNUT

Are you thinking atmospheric or condensing?
Go with your numbers and think about a 2 stage heat and cool. Running a new 8 conductor thermostat wire shouldn't be a biggie.

And with what's going on, it might be a good time to look at a high MERV air cleaner, UV light, ionization generator, etc.

SlamDunk

ChrisJ said:

It's pretty easy to tell if it's oversized.

On an the coldest nights you've seen did it cycle or run continuously?



If it cycled a lot it's oversized. If it ran nonstop or close to it, it's good.

Fair question. I would say cycles 2-3 x per hr. We dont live there. Just visit several times per year. I never considered it was oversized because it warmed the house.

HVACNUT said:

Are you thinking atmospheric or condensing?

Go with your numbers and think about a 2 stage heat and cool. Running a new 8 conductor thermostat wire shouldn't be a biggie.

Atmospheric 80%. Because we're not there often enough

Didnt consider ductwork. I would say undersized. Originally, it was heat only. But , furnace is 1988 so it seems fine. I mean thirtytwo years without comfort issues......

ratio

I've run into a problem recently with changeouts from electromechanical controls (fan limit, relays, etc.) to electronic control boards. Many control boards implement 3 strike logic, meaning after a limit opens 3 times in a row the system will lock out. The old fan limit would happily cycle on high limit (until it fails, at least) until the stat satisfies, but the control board will shut the unit down & requires a reset.

JUGHNE

How is the chimney in the house? If not good then that makes a 90+ furnace a more attractive option with sidewall venting.
Maybe the same for the WH.

SlamDunk

JUGHNE said:

How is the chimney in the house? If not good then that makes a 90+ furnace a more attractive option with sidewall venting.
Maybe the same for the WH.

The chimney was in very good shape the last time I looked. I can see most all of it from the roof. I'd even say brushed clean since a mama raccoon raised five lil raccoons in the furnace' stack one summer. I got to know the chimney intimately that weekend.

I wouldn't have known a thing if the darn thing didn't go down the wrong stack at some point , opened the fire place flue and wandered around the house. Left a pile of scat on a kitchen counter and another on the bathroom counter. left soot prints everywhere. Fortunately, She found her way out and didn't damage anything. I cleaned up, ran to Lowe's for chimney caps, installed the chimney caps only to find out the family was still in the chimney!

So, I'd say it is in good shape.

JUGHNE

Does it have a tile liner?
Even the 80% condense somewhat.

SlamDunk

It does. rough guess, @ 8"x 8" square.

GW

2 1/2 ton AC with 120,000 BTU output? I would have to see that to believe it. That’s the most lopsided arrangement I’ve ever heard of, unless you’re in AK

1000 sq feet? Where are you anyway?

If it was Massachusetts, I’d be doing 40,000 of heat and 1 1/2 or two for cool, depending on some variables. If the ductwork is big, we kind a need to fill up the bag

ChrisJ

> @GW said:
> 2 1/2 ton AC with 120,000 BTU output? I would have to see that to believe it. That’s the most lopsided arrangement I’ve ever heard of, unless you’re in AK
>
> 1000 sq feet? Where are you anyway?
>
> If it was Massachusetts, I’d be doing 40,000 of heat and 1 1/2 or two for cool, depending on some variables. If the ductwork is big, we kind a need to fill up the bag

I have 3 tons for cooling and 125k for heat.

Actual loss is 72k at -8f as per the ecosteam. Cooling is sized dead on for my liking as per it's run times.

GW

That’s a little better, 125 in? Not 125 “net” What state?

ChrisJ

> @GW said:
> That’s a little better, 125 in? Not 125 “net” What state?

125 in, 104 out.

I assume the state is gas when the fuel goes in.

GW

Well that’s better, 3 ton and 104

State, New Jersey? Aren’t you in New Jersey guy?

ChrisJ

> @GW said:
> Well that’s better, 3 ton and 104
>
> State, New Jersey? Aren’t you in New Jersey guy?

Yessir.

I was trying to be funny.

GW

LOL, high pace day at work, I was sitting in my work truck, waiting for our system to get a good vacuum.

JUGHNE

4 tons, 2 X 2 ton ac's and 80,000 input mod con, it probably could be smaller.
I have 26 air supply runs.
Cool down with all 4 tons and then switch one off and keep it's fan running.
I did a little experiment with the return air ducting.
The return from zone "A" is connected to the zone "B" air handler and the return from zone "B" is connected to the zone "A" air handler.
Then the active AC zone has fan on auto, the inactive AC zone has the fan on constant.
Keep the inactive area within 2 degrees of the active one.

I also did this with a supermarket with the waste heat recovery.

BTW, I do without AC in my work truck.....it just spoils you....don't want to get back to work.

SlamDunk

> @GW said:
> 2 1/2 ton AC with 120,000 BTU output? I would have to see that to believe it. That’s the most lopsided arrangement I’ve ever heard of, unless you’re in AK
>
> 1000 sq feet? Where are you anyway?
>
> If it was Massachusetts, I’d be doing 40,000 of heat and 1 1/2 or two for cool, depending on some variables. If the ductwork is big, we kind a need to fill up the bag

This is why i asked. North Carolina. See photos. My mistake! 120k input!!!

GW

OK great I was speaking in gesture

The other HVAC people wanted to keep that same capacity? They must be on some serious wacky tobakky.

That’s 120000 input, still bonkers for a 1000 foot house. Especially, North Carolina. That is insane. Amana makes a two stage, 30,000 BTU input.

Keep calling other places until you find somebody that has half of a clue at least

HVACNUT

Even with your heat loss calculation, 1,000 sq ft at 55 BTU per is nuts by itself. You need the smallest furnace you can find. Rheem and others offers 50K input. And it will condense, even if you go larger. A lot has changed in 32 years. The chimney isn't going to like it. Get a liner.

JUGHNE

I have done 80% furnaces (80,000 input) with standing pilot water heater into 8 x 8 clay tile lined chimney without problems after 20 years. Maybe not to some codes, but we have no enforcement policies.....other than lawyers after the fact.

Also did that set up into an unlined chimney and could eventually see black "tar" coming thru plaster within the house. Stuck a "B" vent down on the next remodel.

Both these chimneys were in the center of the house....warm on 4 sides.

You said the furnace might cycle 3-4 times per hour....you would hear the blower, but the burner fire could have been cycling on high limit and you may not hear that with the blower running.
That was the case for the posting I did above. The exhaust inducer and air blower runs in the event of high limit opening, so one may not realize that the fire is cycling on and off.
FWIW

SlamDunk

I fudged where I could to get 55k. Like using outside temps @13 f (it happens) with inside temp of 75. Worse case windows etc.

I would direct vent out the wall with a condensing furnace before installing a liner. This system would probably be used a hundred hours per year( each on heat and cooling) so I'm not keen on spending more than necessary.

The place is three hours away so Iprobably wont be able to shop around. There arent many shops. I needed a gut check. My In-Laws didn't know better. They did what the estimator recommended. Im just trying to anticipate and figure out what to do.

JUGHNE

If you pull the furnace out of the chimney and have a gas water heater then you may have the "orphaned water heater" problem.
That is when the WH is vented into a chimney that is too large to properly vent the WH.
Perhaps with the limited use you may have, an electric WH may be more favorable. You could shut it off when not there and it may take 1-2 hours to recover when you visit.

SlamDunk

We do have electric WH.

SlamDunk

JUGHNE said:

You said the furnace might cycle 3-4 times per hour....you would hear the blower, but the burner fire could have been cycling on high limit and you may not hear that with the blower running.
That was the case for the posting I did above. The exhaust inducer and air blower runs in the event of high limit opening, so one may not realize that the fire is cycling on and off.
FWIW

THAT is a very good point! I never even thought about a high limit because the comfort level was fine. This makes sense. Thanks for that.

SlamDunk

You have all been very helpful in calibrating my thinking and preparing. When ready, I believe I'll ask for -if not offered- a 92% efficiency 60k input furnace and have it piped out of the wall.

This may be a huge help because I would have the option to let it run at low temp when not occupied for freeze protection. We tried it one winter with existing unit and it was very expensive so we stopped. Now, with the availability of wi-fi, I can deal with remotely and still protect the house and save money if I want to. These options are valuable!

Jellis

If you tell a contractor
"I believe my heating system is improperly sized, and I would like you to perform a heat loss calculation to properly size my new equipment"

Then you guys will be on the same page.

If they still want to size it off of the existing equipment then cross them off the list of contractors you would use and call the next guy!

mattmia2

If it is just a very small output and just for occasionally heating a vacation home, a console hater or wall furnace vented out the sidewall may also be an option to consider.

EdTheHeaterMan

GW said:

LOL, high pace day at work, I was sitting in my work truck, waiting for our system to get a good vacuum.

That sounds like an employee milking the clock.

When I'm on the job, the goal is to get the LineSet connected, check for leaks and get the vacuum pump running. After that,
*I would do the wiring, (then check the micron gauge)
*clamp the line set to the wall, (then check the micron gauge)
*or hangers to the joist,
*Then take a Lunch Break, (then charge with nitrogen and start a second vacuum)
*complete the plenum, (then check the micron gauge)
*seal the duct seams with mastic, (then check the micron gauge)
*then mount the thermostat on the wall (then check the micron gauge)
*Then fill with nitrogen and start the third vacuum)
*Install the condensate drain, (then check the micron gauge)
*Level the condenser, (then check the micron gauge ),
*check the paperwork to see if all included items are completed, (Then check the micron gauge)
*Then clean up the tools, (then check the micron gauge)
*throw the old equipment in the truck. (then check the micron gauge)
*and finally, weigh in any additional refrigerant, open the service valves, test the equipment, verify the pressures, temperatures, superheat, subcooling, airflow, and then go over the maintenance and operating instructions with the customer.

Employees seem to wait until the job is completed, then ar about 3:00 PM start the vacuum pump. Then check up on personal email, text the wife or girlfriend, and visit HeatingHelo.com. At 4:30 they call the office to get overtime authorization to finish up the job.

Does that sound about right?

LOL

EdTheHeaterMan

@SlamDunk

Your oversized furnace may be the reason you are so comfortable so quickly in the winter. That oversize will certainly heat up the house from a cold unoccupied state to an occupied state. (condition, Not New Jersey or North Carolina )

With the proper size furnace, the recovery will be somewhat longer. Airconditioning will also take longer to recover with a proper size unit. You may want to take that into consideration with the new equipment.

If this were my place and I was going to keep it as a vacation home for over 5 years, I might keep the 2.5 ton AC and split the difference on the heat. Maybe 80,000 input in place of 60,000.
FWIW

EdTheHeaterMan

Wifi thermostat will allow you to get a head start on the temperature before you leave your Regular Home to go the Vacation Home if you do choose the 60,000

SlamDunk

Yes, the 2.5 ton works fine, even now while low on freon.

I'll give the 80 k some thought. I'd rather be a little more the too little. Ill see what they say.

mattmia2

SlamDunk said:

Yes, the 2.5 ton works fine, even now while low on freon.

I'll give the 80 k some thought. I'd rather be a little more the too little. Ill see what they say.

You could always go with a 2 stage as well.

GW

Ok Ed I’m just a salary guy so whatever

HVACNUT

> @EdTheHeaterMan said:
>
> Employees seem to wait until the job is completed, then ar about 3:00 PM start the vacuum pump. Then check up on personal email, text the wife or girlfriend, and visit HeatingHelo.com. At 4:30 they call the office to get overtime authorization to finish up the job.
>


That's a real broad generalization, unless it's that bad by you. Who's teaching over there? BTW, do you get a chance to at least take a leak during your busy day?
In about 2 hours I'll be replacing the refrigerant for a wine room system with about a million dollars worth of grape juice. How do I get hold of you if I need you?

ChrisJ

That's a whole lot of checking the micron gauge......

The last system I did I think I checked the micron gauge twice.

EdTheHeaterMan

I guess I check to see if it is time to start the next step on a triple evacuation. That is the way I was taught. In 45 years I can safely say that I have never had a warranty compressor claim on my installs. I have completed warranty and non-warranty compressor change outs on ones that I did not install.

Just saying

Pretty good record

The guy who taught me triple evacuating did me and my customers a favor.

At least I don,t pressure test refrigeration piping for leaks with water. Knew a plumber that did, he had a lot of warranty claims until the manufacture questioned him about his methods. He insisted that his connections did not leak. Then the Manf. Tech Support finally went to see how he installed the equipment and he was so proud of the series of fittings he assembled to attach the water hose to the refrigeration lines

“Nobody ever told me not to use water. I never understood why you guys paid for nitrogen when water was free”


Pressure is pressure, right?