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Question DetailsAsked on 3/29/2014

got 2 bits for a gas furnace for a 1800 sq.ft. home.only one suggested air ducts. Does that sound right?

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4 Answers


What was the rationale for the suggestion?

Doesn't mean that either person was wrong...just trying to see the basis for their suggestion.

Answered 5 years ago by Guest123


Did either of them go through the HVAC Manual calculations ? Manual J to determine heating load, Manual S to size the furnace and fan, Manual D to determine duct sizing and runs, and Manual T to evaluate vent/damper needs in each room ? Without that, one would not know whether new or resized ducts or flow control dampering were needed, unless one of them recommended duct work just due to general physical deterioration of the ducts like rusting or deterioration of the plastic in round flexible ducts.

I would start with asking the one why he included duct work - why it is needed, in his opinion. Make it clear you are not questioning his judgement, but you got one bid without duct work and want to do what is right to get a properly operating and efficient system so you need to be educated. Then see if he makes sense or just threw it in hoping you would buy off on the added work.

Clearly, before you make a decision on who to use, you need to have confidence they know what they are doing, and make sure all the contractors are bidding on the same scope of work. It is SO rare that competingbids are actually for the same identical scope of work, and commonly homeowners end up with bids way more or less then they budgeted for more because the scope was not fixed than because different contractors actually had radically different prices. This failure to define scope works two ways too - contractors not understanding exactly what the customer wants, and sometimes the contractor mis-scoping or omitting things in his bid, resulting in a bid that is out of whack from the get go.

Answered 5 years ago by LCD


50% of the systems we replace need renovations of the qriginal duct system , as it was usually undersized by the low bidder when the home was built. I'd look at going with the one that found isssues with the duct system.

I also would not go back to the one who missed the duct issues and trust that they actual know how to correct them, if they really knew, they would have found them the first trip. We see that a lot, our price is higher and includes duct renovations and the home owner goes back to the low bidder and he says he will correct the issues, but seldom does, homeowner doesn't know the difference.


Answered 5 years ago by BayAreaAC


Good point by BayAreaCool on the undersized ducts. As he said, they are sometimes undersized because the low bidder got the job by cutting materials cost by downsizing the ducts or by using a single trunk system when it should have had two (or more).

Also, in older homes, the new energy efficient furnaces get some of their efficiency rating by downsizing the fan horsepower - this requires larger ducts to get the same airflow. I remember in the 70's it was common to have several hundred pascals positive pressure in a heating system - today 25-75 at the fan is more normal I would say, and some run as low as 5-15 Pa - not much more pressure than natural stack ventilation in a house which is commonly around 5, and equivalent to the pressure applied to the outside of the building in about a 10 mph wind or by heating a room up about 20 degrees. Using a low-pressure fan unit with small duct will cut your airflow below the design level, causing not only less efficient heating than planned, but possibly dangerous backpressure on the system which can cause the flame to seek another exit out of the firebox rather than through the heat exchanger - call rollout - a vdery dangerous thing to have happen. That is why a Manual D calculation is critical not only for initial system installation, but also when a furnace is changed out.

Answered 5 years ago by LCD

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