Duct Question

[old_Boat]

I was talking to an HVAC professional tonight at a home improvement show. I have an oil furnace that I want to add a heat pump to so that I can set it up as a duel fuel system. The problem is that I have 4" supply ducts. I know, or from what I have read here think, that these are too small for the new heat pump systems as far as cfm. He told me that since the ducts are in the outside walls, that he would replace the 4" ducts as far as he could close to the end of the run with 6" and this would be OK. I think this is not correct and that leaving the end of the run 4" is not going to make much of a difference if any to the system. I think that you would need to replace all the 4"with at least 6". I am trying to do this right by getting professional answers. Another contractor at the show said that he would replace them all, but told me that after 30 years in the business, a manual J was not necessary as he knew what I needed. Any comments?

Thanks,

LJ

[Dr. Hardcrab]

4" ducts are fine........if you have enough of them.

You really need to have someone do a load calc for you. That way you'll know what size you need, or how many you need to add, or whatever.

[Joseph Meehan]

Find someone who will do a manual J.

[old_Boat]

That is what I thought, everyone wants to give me a free estimate, but when I asked them about a manual J they got a stunned look on their faces, like How do you know what that is. I will keep looking. By the way, I have 2 4" supply ducts in each room and 4 real big returns located in both ends of the house but not in each room and one in the basement.

[buffalobill]

the 11 PAGE PDF design answer is at :

lots more technical ventilation info index at: