HVAC help, anyone good with return duct sizing?

[92 5 oh]

Hi everyone,
Not too long ago I bought a house that needs a bunch of work. I'm pretty familiar with most things except HVAC. In 2011 the previous home owner had a new AC unit installed. It included the air handler/heat pump, which is located in the garage and the condensor unit outside. All the existing ductwork was left in place.
One night I was working on the house and heard the condensor fan outside cycle more than I expect. It wasn't abupt and didn't sound 'broken' or any noises out of the ordinary, just simply turned off and back on. So I monitored for a bit and I couldn't put an exact pattern on it. But the best way to descripe it is, the fan will turn on and run the heat pump for a while. Then the fan will turn off for anywhere from 30 seconds - 60 seconds. It will then turn on for about 2-5 minutes then turn off and the cycle repeats. It's not always like this, but something along these lines. I noticed more cycling when it is colder inside. For example, if the inside temperature is 60 and I set the thermostat to 68 it cycles more then if the inside temp was 66 and the thermostat is set to 68, in my experience so far. So I checked the filter (it's one of those cheap fiber ones), pretty much spotless. I then thought maybe the freon was low and causing it to cycle. Called an HVAC guy to come look at it. He said the freon levels were right on the money, so he looked around some more. He ended up calling his boss and they determined that there was not enough return air and it was choking the system out. The reason the condensor fan is shutting off is because it is going in to a 'protect mode' and it's not causing any damage. To be safe I haven't been running the unit. But I am wondering if this could be causing damage? I am somewhat sketical about his answers because he had consult someone else for almost everything. I think the job was a little over the technicials knowledge.
The unit is a Trane 3 ton electric heat pump with aux and the condensor is a Trane XR13. I don't have the exact model of the air handler on me but I can get it of need be. The house is two stories and is almost all rigid duct between the floors. There is are two returns as well. The return downstairs is a straight shot to the air handler plenum. This return is 12" diameter. The return upstairs starts out as 10" rigid duct in a hall, then goes in to the attic and makes a U turn down a wall between two closets. When the 10" duct meets the first floor it changes to 6x12 square rigid (I think because of the tight spaces with the supply duct. The 6x12 then enters the garage and connects to the air handler plenum. I've done my best to draw out the supply (red) and return (blue) and how the second floor return snakes through the house. I don't see any way to upgrade the second floor return because of closets and wall construction. Even if I could, it runs in to the 6x12 return and that would need to be upgraded too. I was thinking about upgrading the return downstairs from the 12" to 16".
A lot of things I read indicate I should 400 CFM per tonnage for return, therefore I would need 1200 CFM. Mathematically I should be seeing around 765 CFM, but realistically I think it is a tad lower because of all the bends on the second floor return. I get this arrive at this number because the 10" duct upstairs should flow at 240 CFM and the 12" duct downstairs should flow 525 CFM. My thought was to change over to 16" flexable duct (the air handler plenum and return plenum are not perpendicular or I would use regid) which flows 1000 cfm. Between the two returns I should have a little over the needed 1200 cfm return air.
My questions is, do you agree that the reason the condensor fan is shutting off is because the supply is too low? Do you also agree that this hasn't caused damage?
Next, do you think I am going about this the correct way or is there a better way to do this?


If anyone has anything to offer I would greatly appreciate it.
Steven

 

[kimcrwbr1]

If your saying the fan on the outdoor unit is cycling off and then on and the compressor keeps running the defrost control board would be suspect. The first thing you allways check is air flow indoors and the indoor coil is clean. For a three ton heatpump you need 1200 cfm airflow across the indoor coil. Pull the front off of the coil in the air handler. I use a long bristle bottle brush to break the big stuff loose and carefully shop vac it out. Then with a pump bottle with simple green rinse the coil down good with fresh water. If it is upflow the bottom of the coil needs cleaned on a down flow the top needs cleaned. Use a wet vac to suck up the water and to clean the condensate trap/pipe. Once your sure you have proper air flow across the indoor coil. Are you getting frost on the outdoor coils in heat would be a sign of being undercharged. When the outdoor fan turns off and the compressor keeps running check to see if the emergency heat strips are on the thermostat should say em-heat or aux-heat. Anyway if it keeps cycling defrost the large copper tube will get cold it could be the temp senser out door of the defrost control boad going goofy. Give me the model numbers of the air handler and heat pump I can get you part numbers. I have been installing and servicing trane equiptment for 10yrs now!

 

[kimcrwbr1]

To move 1200 cfm at .05 static pressure you need 17 inch round pipe or 10X25 rectangular duct. A 16 inch round pipe would be sufficient. That is .05 inches water column on the furnace side of the air filter.

 

[kimcrwbr1]

Looking at your third diagram are you saying it has a 12 inch round off ths side into the bottom floor and a 10 inch pipe up in the attic? If so it is definately high on static pressure and not pulling enough air across the indoor coil. If you can increase the return downstairs to 14 inch and 12 inch in the attic.

 

[kimcrwbr1]

Especially for the summer months the more return you add upstairs the cooler your house will stay so keep that in mind.

 

[92 5 oh]

Kimcrwbr1,
Thank you for all the info! I'll pull the cover off the side of the unit and check the coils this weekend and while i'm there i will get the model information.

Regarding your 3rd reply, that is correct. There is a 12" return off to the side and a 10" in the attic. Ideally I would like to increase the return in the attic but I may not be able to because of the surround walls.
What would be the biggest downfall to not upgrading the attic return? a warmer upstairs?
I have never seen any frost on any of the lines. The larger line on the outside condensor (high side?) gets very warm when the unit is in heat. I'm not sure about how things are in AC, I haven't yet had to run the AC.
Given the size of the returns, do you believe this is what's causing the unit to cycle? Is there a way to tell if it is going in to defrost?

thanks a lot for you help.

 

[cvar]

Our house is almost exactly same layout as yours (50'x27' plus a storage bsmt).

We recently replaced our HVAC unit, cuz it leaked freon and then the indoor coil kept freezing up. I verified the leaks with my R134A A/C manifold gauges (slightly different hose than truck). So we got a new 2.5 ton HVAC for our house. We asked for a 3 ton replacement, thinking bigger is better, but the pros said no, that would be worse at removing humidity (mildew).

Our central main ducts are 10x16" (rectangular) and the branches to each window are about 8" dia (round). Our returns are 16x16. So we have bigger ducts than you (maybe 1.5 times bigger), and the pros all said ours are too under-sized. Yep, I agree. I want more air movement, esp. in summer when the problem is humidity. Winter is fine. Instead, they just added more returns for slightly greater air flow.

Apparently, the indoor A/C coil freezes up (ice plugs it up) when freon is low or when air flow is too low. Do you have ice there? That's bad.

The pros here (doing estimates) said we should increase our ducts, but that's cost prohibitive cuz walls would need to be ripped open, and so they told us "it's not worth it". A stronger fan with helper fans along the way might help, but the furnace blower fan cannot be increased either. Darn! We're outta luck.
Are you?

Or is your fan motor failing (separate from the compressor)? Does the fan contact-switch work OK? It's a set of contacts that turns the fan on/off. We had it go bad once, so fan stopped turning, and got thick ice on the outdoor coil of our heat pump (in winter).

BTW, I googled for HVAC online calculators that figure out your ideal duct sizes. They're too complicated for me. The pros did gut-feel estimates.

 

[kimcrwbr1]

My calculator is sitting right in front of me For your 2.5 ton unit you need 14X16 inch duct for 3 ton you need 14X18 That is for ideal air flow at 100 ft of duct. It is real important to have as many warm air vents wide open as possible. Depending on you air handler/furnace there are different speed taps on the blower motor usually four speed or if variable you just change the dip switch settings.
http://www.ebay.com/itm/Trane-CNT03715-Defrost-Control-Circuit-Board-21C140501G33-/251509989170?pt=LH_DefaultDomain_0&hash=item3a8f29df32
For the trane defrost control board while it is running in heat mode there are three pins in a triangle FRC DFT-Test common and TST. While in heat mode use a jumper wire or I just use a nut driver to short the DFT and test common until the reversing valve switches. The fan should shut off and the compressor stays running. That takes the hot gas coming from the inside and thaws out the outdoor coil basically A/C without the fan. At the same time it turns on auxillary heat in the air handler so your not cooling the house down at the same time. There is a sensor down on the coil tube lower left with two yellow wires that switches back to heat when the liquid temp reaches a set value. Somtimes it is obvious if the defrost control is bad look for little critters on the back of the board. But somtimes it is a leap of faith after you check air flow indoors. Check indoor delta in heat or heat rise. Aster running 15 minutes check the temp of the closest heat run and the return air goin in. You want to see a minimum of 20 degrees difference between the two.

 

[kimcrwbr1]

Dont leave the jumper on the pins pul it away as soon as the reversing valve switches.

 

[kimcrwbr1]

Is there a way to tell if it is going in to defrost?
The compressor stays running and the fan stops outside and the thermostat should say auxillary heat!

 

[kimcrwbr1]

Yours being a upflow you may need to either cut a access panel under the furnace to get up there or possibly pull the coil out if the lineset can be bent out enough the drain can allways be refit. I am on call this weekend PM me and I can give you my phone number. You cant increase airflow upstairs can you increase return downstairs to 14 inch that will give you around 1000 cfm across the coil.

 

[92 5 oh]

Yours being a upflow you may need to either cut a access panel under the furnace to get up there or possibly pull the coil out if the lineset can be bent out enough the drain can allways be refit. I am on call this weekend PM me and I can give you my phone number. You cant increase airflow upstairs can you increase return downstairs to 14 inch that will give you around 1000 cfm across the coil.

You have been a wealth of knowledge, thank you for your help so far. I got the model numbers from the air handler and from the condenser.
First thing this morning I've got some flashing to fix while I've got some nice weather. This afternoon if time permits I will double the coil and make sure it is not icing up. The weather has been pretty nice lately so I haven't been running the unit too much. I'll try to get some more details this afternoon and post them up.

thanks again!