one duct system, two furnaces

[hank1]

Thanks for the T-stat question guys. My question is for a house of 2600 ft sq. and up in the midwest, N. Il, S. Wis. two story house. Wouldn't a duel duct/furnace system run more efficiently than one furnace and one duct system. My buddy is building a house and it is 3600 sq. ft. and his HVAC co. said it is better to have one duct system and two furnaces going into it.

Thanks in advance

Hank.

By the way, I am a sparky here in lake cty. taking hvac classes. This is a pretty cool web site. If you guys want any electrical info, try Mike holt he knows.

[t527ed]

2 units = 2 TOTALLY seperate duct systems.

[smadave]

This is not a black and white issue at the first reponse suggests. It depends on the situation. Its a larger house, so it is possible to twin two furnaces and run one duct system. Another situation is where a contractor may run two supply ducts and one common return. I am not going to sit here and say that it has to be one way or another, rather different situations call for different designs.


Dave in NJ

[hvac hero]

I see no advantage & twinning 2 systems to 1 duct system. The only time I have ever seen this is in commercial applications where they dont have 3 phase. I think it would be a lot more efficient to use 2 seperate systems & zone it. That way your not having to run 2 systems anytime you want some heat or cooling.

[t527ed]

it is black and white 2 units = 2 duct systems. especially for residential use. why have to run 2 heaters or 2 air conditioners every time the t stat calls for something?

[tommy graham]

I would not do that on residental applications, have twined twin furnace in commercial applications, would be very expensive on heating and cooling cost, not to mention added cost to instalation


“Tell me and I forget. Teach me and I remember. Involve me and I learn.”

[beenthere]

Twinning wouldn't cost much more then 2 seperate systems.

With 2 stage stat, and a twinning control, only 1 burner, or 1 condenser runs unless it can't handle the load, the same as a 2 stage furnace or A/C.

With 2 systems, you can have both burners or A/C units running at the same time, even under low load conditions, exspecially when using set back stats.



Whats right for your house, we can't tell from our chairs.

[tommy graham]

When furnaces are twined they both run at the same time, only way that they would run seperatly is if relays were added to use as second stage with out twinning and then you would have to use back draft dampers to make sure that the air flow went to the supplies and not thru the second furnce and recirculated thru the returns with no air reaching to the supply ducting









“Tell me and I forget. Teach me and I remember. Involve me and I learn.”

[trane]

Originally posted by tommy graham

When furnaces are twined they both run at the same time, only way that they would run seperatly is if relays were added to use as second stage with out twinning and then you would have to use back draft dampers to make sure that the air flow went to the supplies and not thru the second furnce and recirculated thru the returns with no air reaching to the supply ducting









“Tell me and I forget. Teach me and I remember. Involve me and I learn.”


Your right in the fact that both blowers run at all times but they can only be using one furnace for heat or one A/C for cooling at 1st stage. The better twinning kits will also alternate between the two furnaces every cycle so they both get the same run time.

[tinknocker service tech]

as beenthere has already said
if the furnaces are twinned together properly in some cases it can be cost affective depanding on the house and size of the units. saperate returns i do recomend and a stageing twinning kit.

this way both units get equal running time by switching units at first call