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emarlin 04-17-2010 11:08 AM

Adding basement supply's to trunk line

Is it possible to add basement ceiling supply's directly to the main trunkline. Have a heat pump system. basement is about 900sq ft. Am planning on adding a cold air return and a damper.


beenthere 04-17-2010 08:38 PM

Your basement will still be cold.
The cold air from the first floor will fall into the basement. And the warm air from the basement will rise to the first floor.

emarlin 04-18-2010 11:37 AM

What is your suggestion to this problem.


beenthere 04-18-2010 03:45 PM

Few basements need cooling. Most need a dehumidifier.

So electric baseboard heat. And maybe 1 supply from the central system. If its a basement that will need some cooling in the summer.

If its a basement that is going to be used a lot for entertainment. Then a zone system to condition it off of the central system. That way, the supply ducts are closed off from the rest of the house.

PA_DIY'er 04-18-2010 06:34 PM

there are a few things to consider:

1. the size of the register you are thinking on installing
2. the size of the trunk piece you are cutting into
3. the current load of the house, and whatever spare cfm you may or may not have

any cfm you let flow to the basement is cfm not going where the duct work was designed to put it. for example, if you install a register that has a flow of 600 cfm at the beginning of a piece of ductwork that only handles 600 cfm, dont expect to get much, if any airflow to the branches from that piece of trunk or further down the line.

i would recommend going with beenthere's advice and get an electric baseboard. i dont recommend cutting into your trunk line without knowing the info above. no sense making other spots in your house uncomfortable.

sktn77a 04-19-2010 01:26 PM

Your existing system is probably not sized to condition an additional 900 sq ft of living space, so simply adding a couple of registers in the basement isn't likely to be an acceptable solution.

emarlin 04-19-2010 09:57 PM

Thanks for everyones input, additonally there were two registers that were on the trunk line in the basement when we moved in. Never have use them, kept them closed. But have noticed that when the heat was on, basement was warm. we have 2 heat pumps, one for the 1st floor and a second one on the 2nd floor.

Does this alter your recommendations?

Please advise.


beenthere 04-19-2010 10:33 PM


How warm is it gets warm?

Same temp as the first floor?
Does it stay warm until the first floor calls for heat again?

emarlin 04-20-2010 08:16 AM

Stays within 5-8 degrees of 1st floor between heat calls, this was before I insulated the wall with r-15 and top plate spaces. the ceiling supplies that I want to install have butterfly dampers that can be closed or opened in winter and summer. Moisture has not been an issue. Basement has a walk out.

beenthere 04-20-2010 10:59 AM

Basements always have a different heat loss/gain rate then the rest of the house.

You'll find that it will still tend to cool down quicker then the rest of the house. For both that reason, and because the cooler air of the first floor will always be dropping/falling to the basement through the registers. And the warmest air of the will rise to the first floor.

While making the supplies, and or return all low in the basement will prevent the warm air from rising to the first floor through the duct system. It won't prevent the cool air from the first floor from falling to the basement.

If your worried about electric baseboard heaters raising your electric bill. Install the supplies from the duct system also. And use the baseboard heaters to maintain temp when the central system isn't running.

emarlin 04-20-2010 11:11 AM

Do they make 1 way damper inserts for supplies that might help if supplies have to be on the ceiling? This might help prevent some of the rise in heat.


beenthere 04-20-2010 11:19 AM

They do make less then 2% leakage backflow dampers.

At more then 100 bucks per. They are a waste for a residential system.

They also require you to run larger duct to get the same air flow.

And yes, they do make cheaper ones. But they also restrict air flow. And require a larger duct to move the air you need/want to heat the area/basement.

Some of them develop a squeaking noise when they open or close. Very annoying.

Don't forget. When its 65 and slightly above outside. Your first floor won't need any heat. But your basement will. So are you going to over heat your first floor to make your basement warm?

emarlin 04-20-2010 11:42 AM

Good point. Will need to explore either baseboard or alternate form of temp control.

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