I live in BC, Canada. The house is 3800 square feet and 40 years old (meaning that the insulation may not be good enough). I would like to replace the very old furnace with a new heat pump for both heating and cooling, but I am worried about Vancouver In winter, if the heat pump power enough without any electric auxiliary heating , can the indoor temperature reach 21 degrees Celsius?

Hello Steve,
Are the two furnaces ducted in parallel – share common supply ducting? If so, the ductwork should be sized to handle the blower load of both – there may not be a penalty in using a two stage thermostat to stage the two furnace burners even if you operate both blowers on stage 1.
If you operate the two furnaces independently, you will need to outfit them with backdraft dampers if they share common supply ductwork. You don’t have to run both burners all the time they are on.
Huge limitations to forced air systems…to my mind.
Dale

Separating them into two independent systems will allow you zone it much better (assuming you can’t get by with just one unit). I never understood the logic behind “twinning”. You lose a thermostat and the ability to better control temperature in different parts of the building.

You’re right, it’s an old church that was built in 1892, so certainly not a modern enclosure or modern duct design.

The only places I’ve seen big twinned furnaces are in old homes with big ducts. These old homes were often very leaky and drafty, but with heating, bigger ant necessarily better. You’ll end up with hot spots and cold spots in the home, and the thermostat will satisfy, but the building is never going to be comfortable. If they’re 25 years old, they’ll probably need replacement soon enough. My suggestion would be to seal and insulate the building as best as possible and get the new equipment properly sized for the new load.

Thanks for corroborating my theory.

They’re almost 20 years old and I’ve only owned the building for 3, so I have no insight into whether a Manual-J was done, but it certainly doesn’t feel like they were adequately sized.

Steve, it sounds like you will effectively be running the twin furnaces in a “heat pump” mode, i.e. lower supply T, longer cycles.

Sounds to me like your twinned 120’s are oversized if they’re only running for 14 hours/day on even the coldest days, and 7-9 hours/day on a typical cold day. Was a Manual-J done when they were installed in the first place?

It does this exceptionally well: “With high-temperature air, the furnace blasts you with hot air for a short time and then goes off”.

Heat and fan are independently run to relays which trigger both furnaces simultaneously, and both furnaces the common ducts for supply and return.

My thought, as an experiment, is to flip the fan wire to the heat terminal at one of the furnaces.

When heat is called in this modified configuration, one furnace should run its heat cycle, while the other would just blow its fan, ensuring the same amount of air flow, but with reduced heat at the registers.

I’d be able to see how much heat is called for each day as well as a visual representation of each cycle runtime (via Nest).

I’m curious about the comfort observations, but also what this does to efficiency (would a 7 hour runtime double to 14 hours on days with similar weather?).

Will post again if/when I run this experiment, but I’m all ears if anyone has comments about the scenario I outlined.