What if pro wants to use existing size hvac?

ratio

I was looking for a meme that expressed my response to that. I failed to find one.

GW

Ed when I’m running low on nitrogen how do I get the Water pressure to 500?

EBEBRATT-Ed

Agree with @ratio , ductwork is ALWAYS undersized. WHY?

One reason is to save $$$

The other reason is if they size the ductwork to the heat loss.

Let's say your heat loss is 60,000 and the furnace is going to be oil fired.

So you put in 60k worth of duct and the smallest oil furnace is 80k of output now you have too much air flow for the installed duct

ChrisJ

> @EBEBRATT-Ed said:
> Agree with @ratio , ductwork is ALWAYS undersized. WHY?
>
> One reason is to save $$$
>
> The other reason is if they size the ductwork to the heat loss.
>
> Let's say your heat loss is 60,000 and the furnace is going to be oil fired.
>
> So you put in 60k worth of duct and the smallest oil furnace is 80k of output now you have too much air flow for the installed duct

I sized the ductwork for my AC based on 0.05" drop if memory serves. It was difficult to get a lot of it to fit. I did 9" drops to bedrooms.

And even after using stuff that everyone told me was way too big, with the largest Merv 8 airbear they make on a 3 ton I still end up with 0.5" total static half the time. It starts around 0.4 and I change the filter when the total gets near 0.6.

I have no idea what static most systems are running especially with 1" thick filters and undersized ductwork.

SlamDunk

I'm going to switch topics since I started this tread a year ago. Because of covid ( you know, was only going to be a short term thing), we essentially locked down and haven't reached out for quotes yet. I was going to make my first trip next week and get a quote or two.

I saw on the news recently that there is a shortage of units and long lead times. The reporter said this is causing the price of installation ( and repairs) to be higher than normal... Is this true? Or false?

pecmsg

Unfortunately TRUE

SlamDunk

Darn!

GW

I’ve seen oversized duct work. It happens 

pecmsg

GW said:

I’ve seen oversized duct work. It happens 

1 out of how many?

GW

100 

PC7060

ratio said:

Protip: the ductwork is always undersized.

I recently replaced a four ton carrier AC with a 1-1/2 ton unit with transitions to the original 14”x24” main duct that split through a tee into two 12x18 lines servicing the two sections of the first floor.  I had a local HVAC company run the new lines and set up the unit and they did great job.   

Unfortunately the sales guy came along and spent his time trying to convince me to replace my “oversized” duct work.  I politely declined. 

Fan noise is almost inaudible and balancing the dampers in the original ductwork is a breeze.

GW

Wow PC that’s quite a drop. Excellent. Is the air blowing well in all the registers? I’d think it would be hard to fill the bag. That’s some bigger ducting. 14/24 is yuge 

PC7060

Yes, @GW, those are monster duct but the system works well with pretty even air distribution. The envelope of the house was tightened significantly as part of a renovation so BTU requirement match up nicely with small HVAC.   The HVAC runs long periods and is effective at both cooling and pulling humidity out of the environment. 

Old 4 ton unit would run for 15 minutes and dump temperature down quickly but the short cycle times left a lot of humidity. 

The 3 ton unit on second level was replaced with a 2 ton two stage High velocity system with all new ductwork. Again good air sealing (home and ducts) were critical part of the project.  

SlamDunk

pecmsg said:

Unfortunately TRUE

Got my first quote. Wow. On to the second quote......I may have to, and can, wait until things settle down or my system dies and forces my hand.

pecmsg

SlamDunk said:

pecmsg said:

Unfortunately TRUE

Got my first quote. Wow. On to the second quote......I may have to, and can, wait until things settle down or my system dies and forces my hand.

Shortage of equipment is one thing, shortage of qualified techs is a bigger issue. To retain quality the contractor has to pay more which means YOU have to pay more!
Don't expect the pricing to come down any time soon.
Waiting until its dead and having to rush the job is never a good idea!
Panic buying leads to poor quality.

SlamDunk

The price quoted warrants additional quotes for comparison. It is always a good idea to get more than one quote. I learn something new with each visit.

ChrisJ

Are there shortages across the board, or are higher seer systems still plentiful?

SlamDunk

I decided to go with a high efficiency nat. Gas furnace when I do this replacement. I have a question about the combustion air intake. Does it have to be piped outside if I have foundation vents? It seems if intake temps are stable, combustion will be stable.

ChrisJ

If it was mine I'd be pulling from outside.

The less you pull through your foundation the better.

That's my opinion.

My dad's furnace frosts up the intake PVC when it's -20 outside.  Seems to burn plenty happy though.

JUGHNE

You say "foundation vents"....is this a basement or crawlspace?

Where is this located?

Here in NE, we treat either as a conditioned space with some duct runs into the areas.

ChrisJ

JUGHNE said:

You say "foundation vents"....is this a basement or crawlspace? Where is this located? Here in NE, we treat either as a conditioned space with some duct runs into the areas.

I've been trying to condition all of mine the best I can as well.  No vents, sealing up leaks etc

SlamDunk

It is a full basement with a partial crawl space thirty ft away. Three or four vents over there. Plus windows are steel frame tilt ins. Old wood door 15ft from furnace. Not an air tight basement at all. In NC.

JUGHNE

Does the ductwork extend into the partial crawlspace?

SlamDunk

Yes, actually, to the registers.