HVAC change-out questions

In My Humble Opinion
1

The Central AC evap coil is leaking and it is uneconomical to repair given its age, so we will change out the entire AC system.

We are near the Gulf Coast, so hot and humid summers, but very mild winters. The HVAC system is 18 years old; we have been in this house 16 years. The AC evap coil is attached directly downstream of the gas furnace.

The 55,000 BTU/Hr gas furnace typically only runs about 6 to 8 hours on a “cold” day, maybe 12 hours when it is really cold for here.

Should we replace the furnace also now solely due to its age, or keep it? I have had to replace the hot surface igniter myself 2 or 3 times and cleaned the flame sensor every couple of years. Other than that, it is all original and works fine.

If I do replace the furnace, is 55,000 BTU/Hr too much given how little it runs?

The current AC is a 4 ton (48000 BTU/Hr) 14 SEER. We are very miserly in its use. Last August, it ran on average less than 11 hours per day, and perhaps 16 hours on the hottest days. It may be properly sized for an average user (who tends to spend 1.5x what we do per month to run the AC), but given our usage and the fact we will be in this house another 8 years or so, should or could we go with a 3.5 ton?

Do the 18 year old copper lines connecting the AC condenser to the evaporator need to be replaced if there are no obvious signs of deterioration?

We are having HVAC people come to inspect and bid on the change-out, and I’d like to be well informed.

Thanks

2

You may have no choice in changing the lines. With the ‘new’ and ‘improved’ refrigerants these days, the lines need to be changed also.

The furnace is probably getting close to the end of it’s life also. Have you looked into heat pumps? Check with a couple professionals, the good news is that with higher efficiencies your operational costs may go down.

3

Get a heat pump!
You are in the perfect heat pump climate, and even with cheap gas, it has to be cheaper to run.

4

There is a web discussion board called HVAC Talk. On it you can get non-DIY information from professionals. I have found it very useful.

5

There’s a lot of really good discussion there, however IME, they’re not typically all that keen on DIYers asking questions. Understandably, I guess, since DIYers are taking their business away. I’ve found that if you are planning to post on that board, it’s really helpful to at least sound like you know what you’re talking about.
I typically search for what I’m looking for rather than start a new thread and risk getting shot down.
But still, it’s a great resource (and while I’m at it, TerryLove is good for plumbing questions).

Okay, that’s out of the way. First off, whoever installs these new units will make recommendations. They should take some measurements first (how well insulated is your house, how many windows do you have and how drafty are they, how much heat gain do you get from the sun etc), they might not, but technically they should. Listen to their recommendation and compare that to what you currently have.
If what you have works fine, you probably don’t need to change the size. The is especially true for an AC. If it cools the house well and you go to a larger one, it’s likely to cool the house too well. That is, it won’t run for long enough to dry the air as well. Too small and it’s going to run longer than necessary.
As for the furnace, it’s not affecting the humidity so that’s not a concern. However, you also don’t want to make it too small and have it not able to maintain the temp you want.

As for if you should replace them or not…personally, I wouldn’t, but that’s me. You can clearly do the minor repairs yourself, so you can likely keep it running until something major happens, like the heat exchanger cracking.
Have you don’t the math to see how much gas (or electric, for the AC) is actually being used to see if how long the payback period would be?

For the AC, even if you replace the furnace, you can still keep the AC if you want. You’re not required to swap them all at the same time. If you go that route, they’ll likely have to disconnect the lines to make the swap, so in the process you know you’ll have a new filter/dryer, no leaks and the refrigerant will be full.
And, whether they do it or you do it, glance at the evap coil and see if it needs to be cleaned. You’d hate to replace the furnace just to find out the problem was 20 years worth of dust and grime caught in the evap coils you’ve never looked at.

Having said all that: If raindog is still hanging around these parts, he’s the guy you want to talk to.

6

where is the system located?

upflow, downflow, or crossflow?

what’s style heat exchanger do you have? (clamshell or a tubular)

is the condensing unit 18 years old? near the coast?

*that’s probably the smallest btu furnace that was available with a 4 ton drive blower to meet your a/c demands.

*if you’ve made any efficiency improvements to the structure, it’s likely you can go to a lower tonnage unit. the ideal way to determine this is thru a load calculation.

*smaller systems run longer but typically this is desirable in coastal regions as humidity is kept in check better this way.

*your linesets will typically be fine.

*make sure your contractors installers know what a micron gauge is, and what subcool and superheat are. and that they plan on using these tools and fine tuning charts at your place

7

Thanks for the responses. New AC (4 ton 16 SEER) and furnace going in tomorrow.

8

Can I ask why you didn’t go with a heat pump?

9

TBH, I started researching heat pumps but got distracted and forgot to go back to them.

10

LOL
Been there, done that.