Length of in ground pvc ducting

[Sedgehammer]

Hello. need some inputs. Building new home. Deciding on either in ground duct work or above ground for 2 bedrooms. Will be in floor heat, so basically only needed for A/C. What's the maximum reasonable length one can run in ground PVC? Besides the elbow up to floor and going in, there would be 2 - 90° elbows. Bedrooms are about 300 sq ft including closets. In ground will also cool dining, living and master, but can run off different run, but from same plenum.

[BNME8EZ]

Hello. need some inputs. Building new home. Deciding on either in ground duct work or above ground for 2 bedrooms. Will be in floor heat, so basically only needed for A/C. What's the maximum reasonable length one can run in ground PVC? Besides the elbow up to floor and going in, there would be 2 - 90° elbows. Bedrooms are about 300 sq ft including closets. In ground will also cool dining, living and master, but can run off different run, but from same plenum.

What you are asking is all part of proper duct design. The size varies depending on length, how much air the room(s) need, what type and how many fittings are involved, and how strong the blower is. In the case of in ground duct insulation value plays a part. Finally the type of duct system will make a difference, it can be radial, perimeter loop or possibly a combination of the 2.

Your HVAC company should be able to explain all this to you, if not you might want to consider finding one that can. Price is always a consideration but the cheaper companies can't afford to take the time to do all the engineering to properly lay out a system. Depending on the layout of the house it may take as long for a proper sizing/layout as the time it takes for the cheap guys to do the entire install.

[Sedgehammer]

What you are asking is all part of proper duct design. The size varies depending on length, how much air the room(s) need, what type and how many fittings are involved, and how strong the blower is. In the case of in ground duct insulation value plays a part. Finally the type of duct system will make a difference, it can be radial, perimeter loop or possibly a combination of the 2.

Your HVAC company should be able to explain all this to you, if not you might want to consider finding one that can. Price is always a consideration but the cheaper companies can't afford to take the time to do all the engineering to properly lay out a system. Depending on the layout of the house it may take as long for a proper sizing/layout as the time it takes for the cheap guys to do the entire install.

I understand all that, but plans aren't done yet, so asking a rule of thumb type question so I can make some changes now, instead of after the plans are done, then the plans need to be changed.

[pecmsg]

There is no rule of thumb. To move X amount of CFM’s you need Y diameter pipe / total equivalent length!

[BNME8EZ]

I understand all that, but plans aren't done yet, so asking a rule of thumb type question so I can make some changes now, instead of after the plans are done, then the plans need to be changed.

If the plans aren't done then you are shooting in the dark. If you know everything I put in the first post you should know this also.

Are yo trying to get tube in the ground before the plans are done then make them adjust the plans to your duct system Like that would ever work.

If the plans aren't done how do you have anything laid out to change? With out the plans done how can you do a load calc to know air quantity to each room?

The only RoT that I have ever heard of is 1-6" run for each 100 CFM of air based off A/C size. The spread the runs around with at least 1 run per room with 2-3 in larger rooms. It has never worked in the past but if you are gonna be wrong at least you have a RoT as a reason. I don't see how this helps since 6" runs are the cheapest and there will be more 4,5,&7 than 6's so you will be bidding low but if that 's what you want, you got it.

The first house I did as a owner had 4 systems. I had to make over 4 changes to the duct system. They put beams in to carry weight that they hadn't figured on the original, so I adjusted. They moved the beams, so I adjusted. They moved walls to better match the beams so I adjusted. They got charged every time I adjusted. It all worked in the end.

[Sedgehammer]

I have plans that are being worked on, so there are rooms such as the mechanical I can move now rather than once i have a complete set, then have to hire the architect to go re-do them. ROT i was looking for is a run should be only x long due to ground temps changing the temp in the run too much.

[Sedgehammer]

There is no rule of thumb. To move X amount of CFM’s you need Y diameter pipe / total equivalent length!

I would tend to have to disagree. Run length shouldn't be over a set length i suspect due to temperature changes in the run being too great, thus losing efficiencies.

[firecontrol]

If you loose temperature in the run then the duct wasn't insulated properly when it was installed.

[Sedgehammer]

If you loose temperature in the run then the duct wasn't insulated properly when it was installed.

I can wrap my mind around that to a certain extent.

[teddy bear]

Avoid in ground ducts because of mold growth because the duct surface cool. Best kept in conditioned space.
Keep us posted.
Regards Teddy Bear

[micdundee]

heat gain on these ------ducts-- will not be much of an issue, if it is for cooling only. sizing on the other hand will be for a conventional system. As said, there are still too many unknows. You definitely need to finalize and get with a pro.

[Sedgehammer]

Avoid in ground ducts because of mold growth because the duct surface cool. Best kept in conditioned space.
Keep us posted.
Regards Teddy Bear

problem is we have a great room that can only be cooled by in-ground and the master bedroom attic area is a lean to type onto the great room, so not a lot of room to install ducting.

[Sedgehammer]

Avoid in ground ducts because of mold growth because the duct surface cool. Best kept in conditioned space.
Keep us posted.
Regards Teddy Bear

problem is we have a great room that can only be cooled by in-ground and the master bedroom attic area is a lean to type onto the great room, so not a lot of room to install ducting. The 2 bedrooms I started with i would like to have rough cut timbers and t&g ceiling decking exposed, so no place for duct work.

[teddy bear]

I would include a small whole house dehumidifier, like an Ultra-Aire 70H, in the a/c ducting to avoid moisture problems in the earth ducts. Heat loss/gain is not the problem. Keeping them +50%RH is the problem during high outdoor dew points and rainy weather.
Keep us posted on how it all works.
Regards Teddy Bear

[Sedgehammer]