Fresh Air from Combustion Air Ducts

[RandallIja]

We need a new gas FAU, AC, and coil. There is no Fresh Air (FA) intake. 35+ years old. FAU is in a closet off the hallway in a 700sf condo with other condos above, on both sides, and behind. We live in southern California. The return air is directly below the FAU. There are two outside combustion air vents, high and low (Outside 6x14" louvered with inside 6x14" 1/4" screen).

We do open windows but it gets noisy (horns, alarms, conversations, big trucks, Harleys, etc.) on our busy street especially during the sleeping hours. During winter and summer the house stays buttoned up. I do have bouts of asthma.

I have gotten estimates but only 2 were written and they do not want to discuss options without signing a $ contract first.

I have spent days researching and now have many many questions. I have enjoyed your web site and have gleamed many great ideas from the responses.

Assumptions:
1) fresh air change every 5 hours when occupied.
2) Media filter (4-5") MERV 11-13 depending on static pressure drop.
3) Variable speed fan for comfort and to cycle air for filtering only mode at a low setting.
3a) 2 stage FAU so more time in low mode for more filtering.
4) Gas averages $15/month; Electric $70 when hot, $45 when cold. So $75/month of which 50% goes to temperature. So a 15% savings (80-95% AFUE) only yields a savings of $6/month ($90/year possible savings).
5) Line set 30' (5/8 and 1/4).
6) AC on flat roof on 2nd story.
7) 4" exhaust pipe. No straight shot for PVC for 95% AFUE FAU.
8) 15' of duct work in a dropped hall way ceiling with 4 registers.
9) Microwave fan over gas stove venting to outside, electric dryer, bathroom fan, and tub access panel opens to space between condos.
10) Gas water heater located in outside enclosure.
11) In zone 6, with minimal duct work in a conditioned space so I do not need the HERS tests.

Specs:
1) 14 SEER 1.5 ton AC unit.
2) 80% 2 stage FAU variable speed (ECM motor?) upflow.
3) 4" Media filter 11 MERV (at lease) the larger area the better (16x25 is about as big as I dare as the FAU bottom opening is only 14x24).

Questions:
1) Can I tap into the ducts routing air for the combustion air and use a portion for fresh air? I think I have enough extra combustion air to steal some for fresh air. It is very difficult/expensive to route a new duct to the outside for FA.
1a) If not, can I tap into the bathroom fan duct and install a one way damper so if FAU and bath fan on at the same time the moist air is not sucked into the return air?
1b) What is the suggested % for FA into return air? 10%?
2) Should I add a prefilter rack before the 4" Media filter (AprilAire) rack for a 1" prefilter (rock catcher) and a 2" charcoal (for smell/VOCs)? What about the static pressure drop?
3) Ever heard of installing a static pressure measuring device (amazon.com/gp/product/B009PAN3C8/ref=ox_sc_act_title_2?ie=UTF8&psc=1&smid=ATVPDKIKX 0DER) to check when the filter(s) need changing? How much is too much static pressure. I asked Maytag for their specs and they replyed with "No filter is needed". I read somewhere that each 0.1 in static pressure drop is equal to about $60 in extra power per year?
4) Should I just go for the All-In-One (4" Media Filter, UV-C Sterlization, and UV Photocatalytic odor control) $ unit (hvacsolutionsdirect.com/products/UV-Duct-Light-Systems/Residential-UV-Light-Lamp-Systems/Fresh-Aire-UV-PURITY-Whole-House-Air-Cleaner-Purifier-SKU2026.html)?
5) Should I replace the 35 year old lineset? I would prefer a new lineset that uses large radius 90 degree turns and larger lines (3/4 opposed to 5/8 and 3/8 opposed to 1/4)?
6) Should I have a liquid line dryer installed on a new system? I think so and I think it is better installed inside the closet rather than next to the AC on a hot roof. Should it be threaded so it can be easily replaced?
7) Should I install pressure taps on either side of the line dryer? 2 PSIG drop across the filter-drier signals replacement? Or elseware? Or Gauges for maintenance?
8) Should I install a sight glass for monitoring?
9) Can the AH fan be set to lower speeds to reduce the static pressure drop and run the unit longer for more filtering time (i.e. no short cycling, remember this is a small, almost tiny, home)?
10) Is there a connection that can be installed inside the closet, opposed to on the roof next to the AC, to check and fill refrigerant?
11) A UV-C light can not be installed (nor replaced) in the coil area due to a wall 6" away on either side so I am looking at installing one in the return air; more exposure time due to the larger area and slower air movement. But I can not find a UL approved unit for inside the return air area sealed by ductboard.

I know this was a lot but I am hoping others have these questions as well. We do not make much $ and hope to never do this again. But we want a good quality, easily maintainable system. Better to do it right the first time. You guys, and gals, are great and I appreciate any good advice you have.

[pacnw]

I have gotten estimates but only 2 were written and they do not want to discuss options without signing a $ contract first.
.

I would tell these companies to go pound sand!!!

[beenthere]

Assumptions:
1) fresh air change every 5 hours when occupied.
2) Media filter (4-5") MERV 11-13 depending on static pressure drop.
3) Variable speed fan for comfort and to cycle air for filtering only mode at a low setting.
3a) 2 stage FAU so more time in low mode for more filtering.
4) Gas averages $15/month; Electric $70 when hot, $45 when cold. So $75/month of which 50% goes to temperature. So a 15% savings (80-95% AFUE) only yields a savings of $6/month ($90/year possible savings).
5) Line set 30' (5/8 and 1/4).
6) AC on flat roof on 2nd story.
7) 4" exhaust pipe. No straight shot for PVC for 95% AFUE FAU.
8) 15' of duct work in a dropped hall way ceiling with 4 registers.
9) Microwave fan over gas stove venting to outside, electric dryer, bathroom fan, and tub access panel opens to space between condos.
10) Gas water heater located in outside enclosure.
11) In zone 6, with minimal duct work in a conditioned space so I do not need the HERS tests.

Specs:
1) 14 SEER 1.5 ton AC unit.
2) 80% 2 stage FAU variable speed (ECM motor?) upflow.
3) 4" Media filter 11 MERV (at lease) the larger area the better (16x25 is about as big as I dare as the FAU bottom opening is only 14x24).

Questions:
1) Can I tap into the ducts routing air for the combustion air and use a portion for fresh air? No.

I think I have enough extra combustion air to steal some for fresh air. It is very difficult/expensive to route a new duct to the outside for FA.

1a) If not, can I tap into the bathroom fan duct and install a one way damper so if FAU and bath fan on at the same time the moist air is not sucked into the return air?No.

1b) What is the suggested % for FA into return air? 10%? 0

2) Should I add a prefilter rack before the 4" Media filter (AprilAire) rack for a 1" prefilter (rock catcher) and a 2" charcoal (for smell/VOCs)? What about the static pressure drop? Only if your return duct/or return path is large enough to allow unrestricted air flow.


3) Ever heard of installing a static pressure measuring device (amazon.com/gp/product/B009PAN3C8/ref=ox_sc_act_title_2?ie=UTF8&psc=1&smid=ATVPDKIKX 0DER) to check when the filter(s) need changing? Yes.

How much is too much static pressure. Varies with the filter used.

I asked Maytag for their specs and they replyed with "No filter is needed". I read somewhere that each 0.1 in static pressure drop is equal to about $60 in extra power per year?
4) Should I just go for the All-In-One (4" Media Filter, UV-C Sterlization, and UV Photocatalytic odor control) $ unit (hvacsolutionsdirect.com/products/UV-Duct-Light-Systems/Residential-UV-Light-Lamp-Systems/Fresh-Aire-UV-PURITY-Whole-House-Air-Cleaner-Purifier-SKU2026.html)? Personal preference.


5) Should I replace the 35 year old lineset? I would prefer a new lineset that uses large radius 90 degree turns and larger lines (3/4 opposed to 5/8 and 3/8 opposed to 1/4)? What ever size the manufacturer recommends


6) Should I have a liquid line dryer installed on a new system? Not if it already has one.


I think so and I think it is better installed inside the closet rather than next to the AC on a hot roof. Should it be threaded so it can be easily replaced?
7) Should I install pressure taps on either side of the line dryer? 2 PSIG drop across the filter-drier signals replacement? Or elseware? Or Gauges for maintenance? Won't hurt, but not needed.


8) Should I install a sight glass for monitoring? No, at times it should have bubbles, and other times not. So you'll just be worried when you look at it.


9) Can the AH fan be set to lower speeds to reduce the static pressure drop and run the unit longer for more filtering time (i.e. no short cycling, remember this is a small, almost tiny, home)? Has to be set high enough to prevent limit trips.


10) Is there a connection that can be installed inside the closet, opposed to on the roof next to the AC, to check and fill refrigerant? Yes, but that can cause the condenser not to be checked for problems and caue more trouble down the road.


11) A UV-C light can not be installed (nor replaced) in the coil area due to a wall 6" away on either side so I am looking at installing one in the return air; more exposure time due to the larger area and slower air movement. But I can not find a UL approved unit for inside the return air area sealed by ductboard.

I know this was a lot but I am hoping others have these questions as well. We do not make much $ and hope to never do this again. But we want a good quality, easily maintainable system. Better to do it right the first time. You guys, and gals, are great and I appreciate any good advice you have.

See answers in quote.

[Hrdworkingacguy]

OP must be an engineer...

[RandallIja]

How can you tell :-). Yes, but not an HVAC engineer. So now I am looking for an HVAC engineer to see what my options are because HVAC companies are more about 'Remove and Replace' than planning for the install using current knowledge.
P.S. My girlfriend thinks I'm nuts.

[RandallIja]

Thanks for the answers.

I think this post is way too long and should be broken into several smaller to-the-point posts which I will attempt to do now.

[Hrdworkingacguy]

I have been doing this way to long.

[2old2rock]

I hope this poll has cleared everything up for you.

[vstech]

all of this can be done to provide information... but seasonal service will give better comfort and provide opportunity to discover problems before they show issues your monitoring would.