4" Flexible Heating Duct Pipe

Hi digger,

I find it interesting that someone would install a heating pipe that is un-insulated in the first place.

But taking this at face value, if you're losing heat from the pipe and cannot retro fit another insulated pipe in it's place, sending an insulated 4" line through the 6" line is one solution. But you are creating other problems by doing this (and I think you realise this already), as the smaller line will restrict the flow and have a higher pressure drop (although it's only 20' straight run, so any increased DP should be minimal).

You may consider other solutions, such as running the line elsewhere or reconfiguring things to make a 6" line possible. I'm commenting blind, as I know not you're specific details.

In general to change the diameter of a line (especially smaller) will be problematic. As pumps and pressures are designed for the 6" line (not 4" line).

If you do proceed with the 4" line, you need to recalculate the system pressure drops and make adjustments to the supply pressures accordingly to ensure you don't encounter further complications downstream.

If you're supply pressure is maxed out, then you might consider a booster fan. I would consult manufacturers woth your specific system details (including flows, pressures, medium, system pressure characteristic, etc).

Good luck

Good Answer Ajwinemaker.

I think I am able to follow your intent, but to make sure I understand:

You are saying that if the 6" line in question were restricted it would cause backpressure in the system, causing airflow out any other vents to increase. And that if a booster fan were installed, it could cause a drop in backpressure that could cause lower flow out other vents (or no flow at all).

So, this could be compensated for by measuring flow out of all vents upstream and downstream and installing insulated 4" section then throttling the fan motor (or some kind of butterfly valve) until flow is approximately back to normal?

How accurate would the flow out of the vents need to be measured? Can it be done with some streamers and eyeball it or would an air speed meter be required?

Yep, by resticting flow (reducing the line diam from 6" to 4") the pressure drop along the line increases and therefore higher backpressure to maintain the flow rate.

But I have no knowledge of the system, I don't know if it is for air distribution. Can only assume it is air (or gas atleast) because the OP mentioned a fan (to boost the flow / pressure), rather than a pump (which would indicate a fluid).

It's a good point you make "if a booster fan were installed, it could cause a drop in backpressure that could cause lower flow out other vents" - it's definately possible, but assume the fan selection is chosen to maintain the system characteristic.

It's likely that the flow restriction will be minimal (assuming air flow at reasonable flowrates), but we really need the OP to fully define the system requirements (upstream and downstream of the line in question) to fully help him out.

The flexible aluminum hose connected to the back of

the clothes dryer is great at sending hot air outside.

Do you have any type of vent that could be tapped

for heat loss? If not,I would try a bypass of the entire

metal pipe prying it loose ,at the heat source then put

the end of the silver paper duct type in place there.

setting it right on top of the outside of the metal pipe and

then meeting up at the vent..

You plan to install 4 " flexible pipe to augment the existing 6" pipe if I have understood your posting correctly. It is an additional pipe. You have the flexibility to design it as per your requirements. I have observed many flexible pipes along with blowers used in automobile repair workshop to extract fumes and heat form automobile exhaust.

I would consider using a larger diameter pipe if possible with a more efficient insulator. Perhaps like the reflectix type foil backed bubble wrap that is only a 1/4" thick. If it is indeed a straight run then a snug fitting liner may be feasible. A 5 or 5.5" pipe woud be a dramatic increase in flow potential. An examination of the other ducting should be done to determine what sort of flow restrictions occur throughout the entire system. If the duct in question is a straight run and the others have multiple bends or other restrictions then a booster may not be necessary for that section of duct work.

I guess I should explain a little better what I'm trying to do, This is a hot air pipe from my hot air furnace. This was installed in the early 70's, using a metal pipe was common, I don't even think they had insulated heating duct pipe, if so I'm not aware. The pipe in question runs behind a wall that I can not get to . It is next to the block wall and it is cold out side and also next to the wall, I know in these days they do different things to insulate the walls, but that is not going to happen. This is cooling the air in the pipe before it gets to the room. So I'm thinking about using a insulated pipe 4" witch is going t be a tight fit, that is why only a 4" pipe. I did measure and we are talking 15ft. run. Thanks.

Digger, I will assume you have a typically sized furnace, maybe a 1200-1600cfm. Sizing ducts for about 1000fpm for supply air would give your 6" duct about 200cfm of air, or about 1/6th to 1/8th of the total airflow from the furnace. If this sounds reasonable, then the following applies as well.

The pressure drop for a 6" round duct with 200cfm is 0.3"WG per 100ft, or 0.045"WG. If you change that to a 4" duct, it will increase to 2300fpm, and 2.6" per 100ft or 0.4"WG. This is a tenfold increase in the friction pressure loss. A booster fan sounds reasonable. You can get a $30 fan for 65cfm to 140cfm. Install and see if things improve. Pretty cheap fix.

If you can not get at it with out ripping out a wall, you may want to consider drilling holes and pumping expanding foam insulation. Get some one who has some experience as you may pop of the wall covering.

Reducing the diameter of the run can lead to unexpected problems such as noise and loss of capacity.

if you do what your attempting to do you will end up frying the inline fan, you HAVE to re-route the line, you may have to build an insulated housing for it an then re-attach it but DO NOT reduce the line! What would help is a pic of the situation yet again I think the only true remedy is to build a seperate housing for it.