Why DIY is necessary.

[paul42]

Thanks for the input!

I will check to see if there are places where I need to add some drywall nailers to the outside of the chase this evening. I added several to the inside of the chase while I was putting in the drywall liner.

The obstructions in the return air vent openings are not an issue. The return air duct at the bottom is a 14 inch flexible duct, or about 154 sq. in. The free opening area of the vent into the front room including the obstruction is about 170 sq. in. The free area opening into the other room is about 140 sq. in.

It would take some very restrictive grills to make that a problem.

The air velocity in that return air flexible duct is higher than I would like. I may end up replacing that flex duct sooner than later.

If I had gone to the trouble of specifiying things like the maximum allowable air velocity in the return air duct, I would have scared off all of the bidders (instead of just 6 out of 7).

[jpb2]

Paul, with all due respect, are you playing a joke on the folks here ? What it seems you have is a total wreck as far as a duct system goes.With a full basement why not run return trunkline and do it right? If the house is that tight why no discussion on an ERV ? There are a lot of issues to be addressed. No offense intended but I believe your " budget" dictated your choice in contractors and talking to the installers was next to impossible as they likely dont speak English. Best of luck.

[Shophound]

Thanks for the input!

I will check to see if there are places where I need to add some drywall nailers to the outside of the chase this evening. I added several to the inside of the chase while I was putting in the drywall liner.

The obstructions in the return air vent openings are not an issue. The return air duct at the bottom is a 14 inch flexible duct, or about 154 sq. in. The free opening area of the vent into the front room including the obstruction is about 170 sq. in. The free area opening into the other room is about 140 sq. in.

It would take some very restrictive grills to make that a problem.

The air velocity in that return air flexible duct is higher than I would like. I may end up replacing that flex duct sooner than later.

If I had gone to the trouble of specifiying things like the maximum allowable air velocity in the return air duct, I would have scared off all of the bidders (instead of just 6 out of 7).

Having now once again looked at the original photos and then of your modifications, I now see why the obstructions are there, and that they are pretty much immovable.

Nevertheless I think I would've just flipped the return grill collar to vertical vs. doing the fur-out/fur-down into the closet. I've seen plenty of rectangular return grills run with the long sides vertical. I don't see how much difference it makes in appearance, but in your case it would give you more free area, and you could lose the fur-down in the closet.

What is the tonnage of this system, again?

[paul42]

Paul, with all due respect, are you playing a joke on the folks here ? What it seems you have is a total wreck as far as a duct system goes.With a full basement why not run return trunkline and do it right? If the house is that tight why no discussion on an ERV ? There are a lot of issues to be addressed. No offense intended but I believe your " budget" dictated your choice in contractors and talking to the installers was next to impossible as they likely dont speak English. Best of luck.

No jokes.

When the total CFM for a 2,230 sq. ft. house is 800, a return trunkline is way past overkill. Not that I haven't already done some of that on this system.

In this part of the country, an ERV would not pay for itself before it wore out. They are just not very efficient at recovering the latent energy that is a very sizeable chunk of our energy costs. The total energy cost for this house should average well under $100 a month. An ERV isn't going to make enough dent in that to ever be worthwhile.

It turns out that the HVAC installers are all white Anglo Saxon English speaking men. They are just used to installing everything in the attic.

"BUDGET" is one of the realities I must live with. We do not plan on ever moving out of this new house - but we must also be intelligent and consider the unexpected. No decisions are made lightly.

Anything that is added to the house must have a reasonable payback, add appreciable comfort, add resell value, make a real dent in maintenance, or be a necessary part of our way of life. An ERV or a return trunk line were both considered, but they do none of the above.

[paul42]

Having now once again looked at the original photos and then of your modifications, I now see why the obstructions are there, and that they are pretty much immovable.

Nevertheless I think I would've just flipped the return grill collar to vertical vs. doing the fur-out/fur-down into the closet. I've seen plenty of rectangular return grills run with the long sides vertical. I don't see how much difference it makes in appearance, but in your case it would give you more free area, and you could lose the fur-down in the closet.

What is the tonnage of this system, again?

The central system is 2 ton.

The original work the installer left had even more obstructions than I ended up with. I removed a couple of studs and added others outside of the vent area appropriately.

I considered running the grills vertically, but the aesthetics of the horizontal won out. As you suggested, I did add a drywall nailer on the one side of the fur down. With a ten foot ceiling in that closet, the fur down doesn't cost any valuable space.

[jpb2]

Paul , your just wrong.

[paul42]

Paul , your just wrong.

My wife tells me that all the time

[pstu]

Paul, with all due respect, are you playing a joke on the folks here ? What it seems you have is a total wreck as far as a duct system goes.With a full basement why not run return trunkline and do it right? If the house is that tight why no discussion on an ERV ? There are a lot of issues to be addressed. No offense intended but I believe your " budget" dictated your choice in contractors and talking to the installers was next to impossible as they likely dont speak English. Best of luck.

I will second that opinion that an ERV does not do a whole lot in a hot-humid climate. Simple fresh air intake will provide ventilation with the AC having an opportunity to condition it better than an ERV would. If a dehumidifier could be put in the picture that would improve humidity guarantees, especially in mild spring and fall weather.

To Paul42, have you considered looking up a copy of ACCA Manual D and working on the duct design? I am a homeowner myself and don't have the experience, but can testify Manual D is not too expensive and is readable. It would also be very nice to obtain a Manual J heat load calc to know what airflows will be required. The software from this site is cheap enough and easy to use.

Best of luck -- Pstu

[Black Adder]

Return trunk too expensive? Flex duct in this situation is a waste of time and money. Return duct will give far better flow charactoristics. As far as an HRV or ERV, it is not there for return on investment but for comfort and livability in a tight house. How are you planning on ventilating the place? In our area if you used flex duct in a basement you would be laughed out of town. We use all sheetmetal, no duct board or flex. The chase is nice but way overkill, just have the clowns install a duct with two return air outlets on it if you don't want to use the chase unlined. Only about 15 minutes of work to install it once you make it up.

[paul42]

I have manual D and used it to design the duct work that I installed. I also have and have run HVAC calc on my house to calculate the necessary cooling and heating requirements.

The only flex duct work was installed by the HVAC contractor between the air handler and the ends of my sheet metal duct work. I knew I was going to be stuck with that, so I sized my work accordingly. The sheet metal supply ducts in my house are sufficient for about 1,800 cfm. So even with the flex duct connections, the total static pressure is going to be quite low.

I have run HVAC calc many, many times with different estimates on infiltration, etc.. So, I have a pretty good idea what areas of the house are important for me to spend my time and money on. According to HVAC, the total cooling load on my house is 16,137 BTUH.

There is a 6" fresh air intake into the return air of the central system for fresh air.

In a cooling dominated climate, it is preferable to place all of the supply and return vents up high. In 99.9% of the houses in this part of the country, that is through flex ducts in the attic with ceiling registers.

While waiting for the rain to stop so the foundation could be poured, I spent many hours in my shop building wall stacks so that all of the supply vents could be placed high up on the walls.

In a more northern climate, the ERV would make more of a difference in comfort than it does in North Texas. Different climates lead to different answers.

I prefer sheet metal duct work. But if I specified sheet metal duct work instead of flex, I would have to search long and hard for a contractor that was capable of doing the work, and the cost would be at least triple. So, lots of DIY.

[dcmcm5]

In a more northern climate, the ERV would make more of a difference in comfort than it does in North Texas. Different climates lead to different answers.

I am not debating your choice to not use an ERV, but in northern climates an HRV is a more likely choice. I believe the ERV was developed for the warmer climates to provide fresh air and recover the energy.

The concept is right on although the economics typically don't work. Maybe the ERV costs will come down to the point they will be worth installing.

[Shophound]

The only flex duct work was installed by the HVAC contractor between the air handler and the ends of my sheet metal duct work. I knew I was going to be stuck with that, so I sized my work accordingly. The sheet metal supply ducts in my house are sufficient for about 1,800 cfm. So even with the flex duct connections, the total static pressure is going to be quite low.

How low? Do you have a number arising from your calculations?

Ductwork capable of flowing 1,800 cfm is pretty generous for a two ton system that needs only 800 cfm. If you sized each supply outlet using a friction rate for 1,800 cfm vs. 800, you're likely to have insufficient throw and way too much drop at each supply register. Although each supply outlet will probably be whisper quiet, it will dribble cold air out into the room and generate cold draft complaints for occupants near the outlet. Other areas of the room may become too warm and stuffy.

Another concern...if your duct system static is too low (a rarity, yes, but might be possible in your case), your indoor blower might actually move more air than desired...say closer to 500 cfm per ton vs. 400. For an arid climate like Arizona, 500 cfm per ton would net more sensible heat removal capacity, but we live in a mixed humid climate here in North Texas. Higher latent capacities come with lower cfm per ton rates...350-400 cfm per ton is about right for this area...but knowing the sensible heat ratio for the structure, derived from the Manual J calc, and designing the system around that factor is more desirable than a rule of thumb cfm per ton number, IMO.

Lowering static pressure on a blower will cause the blower motor to become more loaded. If the static pressure is phenomenomally low, the motor may continuously run higher than its rated or running load amperage, shortening motor life significantly. Sometimes, less is NOT more...i.e. if bigger ducts means lower static pressure, then even BIGGER ducts would mean even lower static pressure and better system performance...right? Not necessarily!

I have run HVAC calc many, many times with different estimates on infiltration, etc.. So, I have a pretty good idea what areas of the house are important for me to spend my time and money on. According to HVAC, the total cooling load on my house is 16,137 BTUH.

There is a 6" fresh air intake into the return air of the central system for fresh air.

Did you account for the additional sensible and latent load a 6" fresh air intake will impose on the cooling coil when you did your equipment size selection? Did you also derate the system nominal tonnage for your operating conditions, which are outside of ARI nominal tonnage ratings?

In a cooling dominated climate, it is preferable to place all of the supply and return vents up high. In 99.9% of the houses in this part of the country, that is through flex ducts in the attic with ceiling registers.

If 99% of the attics in North Texas had insulated roof decks or radiant barrier equipped roof decks, attic mounted ductwork would not be the sticky issue it now is.

While waiting for the rain to stop so the foundation could be poured, I spent many hours in my shop building wall stacks so that all of the supply vents could be placed high up on the walls.

High sidewall outlets are actually not as preferable in a cooling climate such as North Texas vs. ceiling diffusers/registers. While 99% of local residential installations use attic mounted ductwork with ceiling supply registers, almost none use round ceiling diffusers, which is almost ideal for a cooling climate. I now own a house with round ceiling diffusers...it's 48 years old. Those old dead a/c guys knew a few things about air distribution us younger contemporary counterparts could stand to relearn.

In a more northern climate, the ERV would make more of a difference in comfort than it does in North Texas. Different climates lead to different answers.

Actually, ERV's are for more humid cooling climates, vs. the HRV which is for heating climates.

I prefer sheet metal duct work. But if I specified sheet metal duct work instead of flex, I would have to search long and hard for a contractor that was capable of doing the work, and the cost would be at least triple. So, lots of DIY.

DIY that can lead to problems due to lack of experience and information. As for contractors that can run sheetmetal vs. flex...I can name some off the top of my head...R & D Sheetmetal, for starters. A strictly residential contractor won't bother with sheetmetal because commonly speaking all he knows is how to fling flex around like octopus tentacles. You want a good sheetmetal job, hire a commercial HVAC contractor or sheetmetal firm like R & D. You'll pay a bit more, but you'll get commercial grade work.

Of course that's probably water under the bridge for you, seeing you've already made up your own ductwork. Being that I can't see your system from here, I may be speaking out of turn regarding its sizing, but if you sized it for 1800 cfm like I think you may have, you may be in for some performance problems down the road.

[paul42]

I designed and installed the duct work before I had a copy of HVAC calc. I expected an energy efficient design, but not that low. But, I did install dampers in the duct work so that I have manual zoning control and can increase the throw at some registers by shutting others off.

Calculating actual numbers for the total static pressure seemed like a real waste of time. On the supply side, they had to use enough flex duct to connect up to all of my hard metal duct, so I know there is no problem there. The biggest restriction in the system is the 14" flex duct for the return air.

The variable speed blower means that a very low static pressure will just lower the energy requred by the blower.

I did include the fresh air intake on my HVAC calculations.

Besides the 2 ton central system heat pump, I also have a Fujitsu mini-split inverter driven system with variable capacity between 5,500 BTUH and 19,000 BTUH.

So, the total rated cooling capacity for the house is variable between 5,500 BTUH and 43,000 BTUH for a house that HVAC calc says needs 16,137 BTUH of cooling. I did not see any point in looking at derating factors.

I do have radiant barrier decking on my house. I VERY seriously looked at going with a hot roof and it was part of the plans for many months. I really wanted it to work for several reasons, but it just did not work for our design.

My wife and I spent a lot of time looking at new houses while designing ours. We wanted to make sure that we spent our dollars in ways that added real value to our investment. The ONLY place we ever saw sheet metal duct work was in modular housing where the duct work was under the floor and sheet metal was required by code.

Houses that cost more than 10 times the amount we are spending on ours still had all flex duct. The only way that we could fit the sheet metal duct into our budget was for me to DIY.

[Shophound]

Out of curiosity, what was your reasoning behind using a basement for a house in north central Texas?

And by "hot roof"...do you mean tar and gravel?

Did you frame this house yourself or did you sub that out?

The blue strip seen in the photos winding around the top plate...is that to check infiltration or something?

[pstu]

>>Did you account for the additional sensible and latent load a 6" fresh air intake will impose on the cooling coil when you did your equipment size selection?

Paul42 has done a most impressive amount of design before building, but I would like to ask how he determined this airflow. I always thought it was highly dependent on return vacuum. A damper to adjust for whatever conditions? It is fascinating to read the back and forth discussion of design issues here.

>>If you sized each supply outlet using a friction rate for 1,800 cfm vs. 800, you're likely to have insufficient throw and way too much drop at each supply register... will dribble cold air out into the room and generate cold draft complaints for occupants near the outlet. Other areas of the room may become too warm and stuffy.

Shophound, why is this not a severe problem when certain 2-stage ACs are installed? Or is it, and we just seldom hear about it? If one planned to use a ceiling fan or other fan in the room, would that make the problem disappear, in your judgement?

>>While 99% of local residential installations use attic mounted ductwork with ceiling supply registers, almost none use round ceiling diffusers, which is almost ideal for a cooling climate.

Shophound, I would be very pleased if you would try to explain why. Is this something I will find if I finally buy the Manual T book?


Thanks to all who have contributed to this thread! -- Pstu

[paul42]

Out of curiosity, what was your reasoning behind using a basement for a house in north central Texas?

And by "hot roof"...do you mean tar and gravel?

Did you frame this house yourself or did you sub that out?

The blue strip seen in the photos winding around the top plate...is that to check infiltration or something?

A "cold roof" is a conventional ventilated attic with insulation on top the ceiling. A "hot roof" is where the insulation is either part of or just below the roof decking leading to a conditioned attic.

It is not a true basement, all of it is actually above grade. The tax appraisal district calls it an unfinished basement. It is actually a very tall conditioned crawl space with a concrete floor on 80%. The crawl space is 6 to 10 feet tall. The rest is a very steep slope of fill dirt under the garage / breezeway that is covered with a vapor barrier - which was a real pain to put in.

The house was built on a slope and to fit the house between the huge live oak trees, it was necessary to go with a pier and beam foundation. Fill dirt for a slab foundation would have covered the roots of the trees and killed them.

I found a very good framing contractor for my house - he was the most expensive bid, but my wife and I both believe that we saved money in the long run by going with him.

The blue strip around the lower top plate is a gasket to seal between the top plate and the drywall. It was another DIY project. The method was copied from a builder back east that does energy efficient houses - and does blower door tests to check their work. The gasket is made from 6" wide sill gasket. A simple jig made from a 2x6 and some utility knife blades was used to cut it into 4 strips the right width for this task.

I am also using duct sealer (for sealing electrical boxes) to seal the back of all of the electrical boxes. After the drywall is installed, I will use caulk to seal the edges of the electrical boxes and the gap between the drywall and the floor.

I will do some initial infiltration tests with my sawdust vacuum connected to one of the exhaust vents on the house. After I have found and fixed all the leaks I can find, I will pay for a blower door test - I've already found a contractor for that.

[Shophound]

>>Did you account for the additional sensible and latent load a 6" fresh air intake will impose on the cooling coil when you did your equipment size selection?

Paul42 has done a most impressive amount of design before building, but I would like to ask how he determined this airflow. I always thought it was highly dependent on return vacuum. A damper to adjust for whatever conditions? It is fascinating to read the back and forth discussion of design issues here.

6" fresh air return with a damper is a common way to install such a system. The damper allows a bit of fine tuning for the proper amount of make-up air needed. Make-up air is sized for recommended air exchange rates...if the house can't make up enough fresh air naturally it will need mechanical help...that's where a fresh air intake comes in.

A fresh air intake must also have provision for exhaust or the house will not see sufficient air exchange rates...and will become pressurized to boot. An ERV could be used to accomplish this need, but so could a whole house dehumidifier that dries the incoming fresh air before it is injected into the house environment. A low cost method for exhaust with a system using fresh air intake only is to tie a kitchen or bath exhaust fan to energize on a call for heating or cooling...as long as the equipment runs there will be an exchange of air.

>>If you sized each supply outlet using a friction rate for 1,800 cfm vs. 800, you're likely to have insufficient throw and way too much drop at each supply register... will dribble cold air out into the room and generate cold draft complaints for occupants near the outlet. Other areas of the room may become too warm and stuffy.

Shophound, why is this not a severe problem when certain 2-stage ACs are installed? Or is it, and we just seldom hear about it? If one planned to use a ceiling fan or other fan in the room, would that make the problem disappear, in your judgement?

With properly installed two stage systems, we're talking about ductwork sized for the high stage...and there is not a significant CFM change between stages, typically. That is, provided the system in question varies blower speed with staging the refrigerating capacity (compressor). If Paul designed and built a duct system capable of flowing 1,800 CFM of air, and will only be flowing approx. 800 CFM, that's a significant difference in dynamics being imposed on the duct system and air distribution outlets.

The only use I have for a ceiling fan is when I'm in the room with it. When I leave, it gets shut off. It is also handy should I walk into the room after either being outside on a hot day, or being active indoors and wanting more air movement to feel cooler.

With a properly designed air distribution system, the room is enjoying the benefit of evenly distributed cooling even when I'm not in the room, so when I walk in, if I haven't been active to where I feel hot, I may not need the ceiling fan at all. The air and the objects/surfaces in the room are sufficiently cool to give me a feeling of comfort.

>>While 99% of local residential installations use attic mounted ductwork with ceiling supply registers, almost none use round ceiling diffusers, which is almost ideal for a cooling climate.

Shophound, I would be very pleased if you would try to explain why. Is this something I will find if I finally buy the Manual T book?

Manual T states the benefits and drawbacks of each air distribution method clearly. It includes drawings for each type that show in basic form how each supply air introducton method affects air movement throughout the room. When I first read this info, it was a big eye opener for me. In the house where I was living prior to purchasing my own (my in-law's place) I used the info gained in Manual T to modify the supply registers throughout the house from the stamped steel cheapo crap to curved blade registers, aiming the airflow to flow along the ceiling and wash the exterior walls.

This is accomplishing to a degree what a round ceiling diffuser does...an attempt to mitigate the natural upward convection that occurs at windows, being window surfaces are in summer warmer than the room air and will set up a "stack effect" of upwardly rising air in front of each window. Left unchecked, this air will accumulate at ceiling level, combined with the ceiling material being warmed from above due to an overheated attic. The ceiling diffusers...or a high sidewall register aimed between 5 to 15 degrees toward the ceiling, will move this air and cause it to mix with the rest of the room air before the return outlet removes it from the room to be conditioned.

In my new house, the round ceiling diffusers are not smack dab in the middle of the room. They are consistently set a bit off-center, toward the windows, where the discharge toward that side mitigates upward convection from the windows. The rest of the diffuser distributes air throughout the entire room, washing every wall. With the location favoring the windows, the exterior walls tend to get more "washing", which they should.

When we get settled I will possibly add transfer ducts to assist in return air volume, provided the door undercuts are not sufficient. Since we removed all of the carpet throughout the entire house and are installing laminate bamboo, the door undercuts may be sufficient in this case. Only time and testing will tell.

I would suggest to you, since your curiosity about HVAC system design remains perenially high, that you visit the ACCA web site bookstore and purchase Manual T for your own perusal and enjoyment. Read it through thoroughly, even for systems not like your own. If I ran a contracting shop I'd have every technician and installer in my employ read that book, and pass a test on it. I wouldn't jump their case if they didn't get it right away...for many it takes awhile for the concepts presented in air distribution literature to sink in. It's not emphasized nearly enough in our trade...but needs to be. Paul would not be tossing around the DIY coin if he had found someone who "got it" regarding air distribution design and installation.

[Lash]

I am also using duct sealer (for sealing electrical boxes) to seal the back of all of the electrical boxes. After the drywall is installed, I will use caulk to seal the edges of the electrical boxes