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

"old_Boat" wrote in message
. ..
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?

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.

old_Boat wrote:
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

Find someone who will do a manual J.

--
Joseph Meehan

Dia duit

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.

LJ
"Dr. Hardcrab" wrote in message
news:XNaHf.605$Lr.4@trnddc01...

"old_Boat" wrote in message
. ..
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?

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.

the 11 PAGE PDF design answer is at :
http://www.buildingscience.com/resou...sizing_pro.pdf

lots more technical ventilation info index at:
http://www.buildingscience.com/resou...al/default.htm