I am looking at heating and cooling options.

[MN Renovator]

Originally Posted by NiHaoMike

A furnace and A/C system are separate so each can be sized just right. A heat pump can indeed end up oversized in one of the modes. In that case, it makes sense to go with a multi stage or variable speed.

Can you find me the largest output air conditioner you can that will work with a 40k BTU furnace? If the load is a legit 4 tons because of a large house with lots of windows with no shading and it is in an area that doesn't get too cold, that may be the sizes you need. I can't find a 2 burner furnace that can put out the air flow required for a 4 ton. This is why heating ends up oversized in the south and why it is more critical to be sure you aren't installing more cooling capacity than needed or the heating gets even more oversized than it otherwise would be.

This is also why 2 stage furnaces get put in many homes in the south, they provide the air flow needed for the AC but allow a lower firing rate than otherwise would be available with single stage equipment that supports that volume of airflow for the AC.

An example explaining this with Bryant 2 burner furnaces.
80% efficiency 313AAV024045 44k input 36k heating output 800 CFM max airflow(2 ton AC)
96% efficiency 925TA36040E17 40k input 1200 CFM max airflow(3 ton AC)

3.5+ ton load, no go.

Another note on oversizing: If I put a 40k input unit that has a low stage output of 25k in my house(2100 sq ft in Minneapolis, MN), it would only need high stage when it is 80 degrees colder outside than my indoor temperature setpoint, figuring nobody has a door or window open too long and the thermostat is left as-is. The number of days we drop below -10f even in Minnesota is about 5 days and even then its only for a few hours except for when we drop to -20f which usually only happens every other year. I'd be comfortable with a 25k output furnace but since the smallest available is a 40k, its what goes in and I'm not going 2-stage because high fire is more efficient with gas and electricity(although only by a little bit).

[WyrTwister]

Originally Posted by NiHaoMike

In climates where heating and cooling requirements are highly asymmetrical (which would be most of them), I'm surprised they don't make furnaces designed just for that climate zone. Multi stage does mitigate it, but then you end up with what is effectively an expensive single stage.

True .

You could install two independent systems . Do not know enough to know how you would make that work , w/o having 2 sets of duct work ?

This would increase your installation cost . Do not know about the operating cost ?

I have worked on large systems where they had 2 furnaces twined together , to a single return air plenum and tied into a single supply plenum .

Both fans came on , any time heat or cool was called for . Can not remember if the gas valves were wired for two stage operation ?

Seems like the two condenser units were wired as two stage cool ( 2 separate A-Coils ) .

But I have only seen this done to serve large spaces .

God bless
Wyr

[WyrTwister]

We have our fair share of wind , both summer & winter . I suspect it increases the heat load & the coolinh load ? Especially in older houses that are not as " tight " as newer houses ? Or as well insulated ?

God bless
Wyr

[pinballlooking]

Originally Posted by WyrTwister

First of all , a mini split is simpler if installed on an outside wall . Is your bed against an outside wall ?

I have a window unit ( wall shaker ) in our bed room . This allows us to shut off all other HVAC , in the house , when we go to bed and only pay for the energy usage for the 6,000 BTU window shaker .

If it was not for the cost , I would replace it NOW . With a Mini split . It would be significantly quiter .

And I would position it right over the head of our bed .

God bless
Wyr



But it is still working and I will probably keep it until it gives up the ghost .

Yes my bed room has three outside walls. To mount it above my bed the outside unit will be south facing probably not the best place for AC but would be good for heating.

I ended up installing 3 single head mini splits and one duel head unit.
http://ecorenovator.org/forum/geothe...t-install.html

[BMR]

Originally Posted by Xringer

However, the older indoor unit has started to act up in cooling mode.
(Seems to be speeding up and slowing down).

I have a cooling-only unit that did this. I took it apart, cleaned the motor connector (fan speed sensor is integrated with the fan motor) and put dielectric grease on all of the pins. Did the same with the main chassis grounding wire, which had become quite corroded, and that seemed to fix it. This unit does not have a coil dry mode when it shuts off, so I think it stays fairly damp and promotes corrosion. I now try to run it fan-only for a few minutes before shutting it down.

[Xringer]

Originally Posted by BMR

I have a cooling-only unit that did this. I took it apart, cleaned the motor connector (fan speed sensor is integrated with the fan motor) and put dielectric grease on all of the pins. Did the same with the main chassis grounding wire, which had become quite corroded, and that seemed to fix it. This unit does not have a coil dry mode when it shuts off, so I think it stays fairly damp and promotes corrosion. I now try to run it fan-only for a few minutes before shutting it down.

Thanks for the info. I was thinking about checking out the terminals,
but we've been out of the cooling season for a while now.
Not seeing any problem in the heating mode. So far..

Not sure I want to take the whole cover off to get at the motor unit.
I'll wait and see if the problem comes back..
I've learned not to attempt repairs on machines that aren't failing
at the time of the repair 'attempt'..