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Question DetailsAsked on 5/15/2015

will adding ducts to a furnace reduce air pressure?

My understanding is when a furnace is put into a house (with permit) it must be able to support the house.

Does adding more ducts afterwards compromise this?

Contractor added 4 more ducts to my basement. I asked before installment if this would reduce my air pressure form the vents. They said no. after install I am feeling very little air pressure.

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2 Answers


If no other changes are made, adding 4 supplies, will definately reduce the air flow to other registers.

However, you should go more by, how's the temperature in therest of the home. Before the vents were installed in the basement, the floor above was cold in the winter, now it's warm, so less air flow is needed. In the cooling mode ,you may need to close the vents in the basement, a little.

The fan speed could be increased, if the furnace temps will allow, ask a Pro.


Answered 5 years ago by BayAreaAC


As BayAreaAC said, the more openings from the ducts, the more freely the air from the furnace flows, so the less noticeable the flow from each vent, relative to what you had before.

However, as he implied, you are looking at two different things here:

1) overall furnace heating capacity relative to house size - yes it does have to be designed to meet the heating (or cooling, for A/C) load of the house as you said. Your house size and total load have not changed much, so unlike adding new ducts and vents in an addition where you are adding new load or capacity requirement, your furnace's "heating load" is about the same, so the addition of more vents in the house does not affect that. I say it has not changed "much", because likely the basement will be running at a warmer temp than before, so you will be losing somewhat more heat through the foundation and slab than before, so your heating load will have gone up a bit - but not dramatically assuming the basement is as airtight as the rest of the house so you are not "losing" that hot air. The most pronounced effect will be in the start of the heating season, when the basement heat has to warm up the foundation and nearby soil from the ambient summer soil temperature to a higher temperature, if the basement is allowed to cool down (or does so by itself due to lack of heat) in the summer. This process can take several weeks to a month depending on your soil temp.

2) However, there is also the issue of balance - as BayAreaAC said, more basement vents now means that area gets a greater percentage of the total output from the furnace, especially if the furnace is in the basement so those vents are also the closest to the furnace (shortest duct run, so least airflow friction in the ducts) . Therefore, the basement will heat up much faster, and while a warmer basement will heat the first floor both by warming the floor more and by leakage up through the floor and around piping and ducts and probably at the basement door, you will get less airflow to the other rooms than before.

As he said - the key is to balance the flow between the various areas, either with manually or thermostatically adjustable dampers in the ducting itself (look like this in round ducting) -

or by adjustable register gratings like this (picture is of a wall/ceiling vent, but floor registers with less pronounced vane stick-out are readily available too)-

that can be adjusted to control the opening size, reducing the airflow to those rooms that are getting more than their share of the flow. So, if one area is feeling like it heats/cools more slowly open that register up more, and if fully open than restrict the flow at one that is getting more than its share. Also, if one room or area that does not have the thermostat (say basement) heats up too much (or cools too much) before the thermostat cuts off, restrict airflow to that area so it heats/cools to about the desired temperature at the same time as the area with the thermostat.

This leads into one other side-effect of putting multiple levels on one duct, which is what it sounds like your case is, is that different floors have different ambient conditions - like the basement loses a lot of heat to the surrounding ground so it is cooler and takes a lot of heating, especially in the fall, to heat up the surrounding ground. So initially in the heating season it may seem cool, but then after the foundation heats up may feel warmer than the upstairs floors which are seeing the cold outside air so need more energy to heat the area. And as BayAreaAc said, in the summer, especially if you have A/C, the basement may feel too cool because it is getting more air from the A/C plus is cooled by the cool ground surrounding it. That is a main reason (aside from the two-floor airflow balance issue in previous paragraph) why basement vents should either be on a separate duct with an adjustable damper (or thermostatically controlled electric damper controlled by a thermostat in the basement) or at least have manually adjustable registers, to allow balancing of the load between areas.

In summary, if you are feeling very little airflow upstairs but strong airflow in the basement, it will heat/cool much quicker (and in that case, overheat/overcool before the thermostat upstairs cuts off), so you will need to throttle the airflow to that area somehow so the heating/cooling is more evenly distributed between floors.

Answered 5 years ago by LCD

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