Sizing blower for different heating vs cooling ducts

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Old 06-10-17, 06:00 AM
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Sizing blower for different heating vs cooling ducts

Hi - I have an older home with hot water heat throughout the house that works great. I'm adding a bed/bath suite to the second floor and want to add A/C to the entire second floor. I want the A/c blowing in every room but the heat blowing in only the new room vents.

If I get baffles to seal up the old rooms in winter will I need to account for a different blower speed or something? Is what I'm suggestion possible?

It's a considerable increased cost to add hot water baseboard heat and just do A/C which is why I'm trying to go this way.
 
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Old 06-10-17, 07:10 AM
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I'm not sure if it's possible, the problem would be slowing the blower down enough with out tripping the high temperature limit.
 
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Old 06-10-17, 10:05 AM
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It depends on what type of heating system you are adding. Assuming heat pump you can get a two stage unit. And if you want fancy get powered dampers to open for a/c only. Have them set it up as a zoned system with two thermostats.
 
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Old 06-10-17, 07:51 PM
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I'm assuming that you want your equipment in the attic.

You should really look at mini-split heatpumps to cool and heat the second floor, cool the first floor. these are far more efficient than attic systems which lose 10 to 30% of their capacity to the attic.

in heating mode the mini-split heatpump performs far better than central units in cold weather - they are able to maintain capacity as it gets colder outside thanks to inverter technology.

To answer your original question, anything is possible if designed/engineered right. it's a question of how much money you want to spend and skill of your contractor.

If you get a furnace and ac each mode uses a different fan speed by default.

To do want you want, you would have to get a low capacity furnace, like 40000 btu or even better a 2-stage 40000 btu locked on low fire.

If you get the right model the fan would be large enough for up to a 2.5 or 3 ton a/c but use a lower speed for heat

The a/c would need more airflow for cooling than the furnace for heating in your case (because the ac is sized for the whole house and the heat just the upstairs) and in heat mode you would close all of the downstairs vents.

The main trunk ducts and branch lines going down stairs would be sized for cooling, the branch lines upstairs would get sized for whichever needs more air flow which will depend on climate.

It would be difficult to do this with a central heatpump unless you get a 2-stage and have it locked on low for heat.

most important thing to remember is that a certain amount of airflow is needed for heating; the upstairs branch lines have to be large enough so the furnace doesn't overheat when the main floor vents are closed.

You could accomplish this with multi-stage equipment and zoning too, having automatic dampers, not turning on the heat downstairs in winter (using the radiant systems t-stat instead) but at a much greater cost.
 
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