Do I understand this system correctly?

jawhittiSeptember 4, 2012

As I posted earlier I'm looking to install all-new HVAC to replace a 15 year-old York system that is having problems.

I am in St. Louis and am looking at a Trane dual-fuel singl-stage heat pump (probably XR15 or XL15i) mated with an 80% efficiency furnace with a variable-speed fan (e.g TUD2B080AV93?). Installation difficulties make the 90% not very arractive.

So tell me if I understand how a system like this will operate. If my summary below is true how would a two-stage heat pump change the equation?

Here's how I think I understand the system:

1) Around some Temperate Tmax (say, 110 degrees F) the A/C system and fan inside will go full-bore and barely keep up.

2) Cooler than that, but above, say 75 degrees and the A/C will run but the variable-speed fan in side will run slower? This will give good dehumidification when mated with a good thermostat (i.e. Honeywell IAQ).

3) Between 65 or so and the setpoint (around 25 degrees?) the heat pump will supply heat. The variable speed fan mostly runs at low speed. As the temperate drops the fan spends more time on high speed.

4) Below the setpoint the gas switches on. The furnace supplies much hotter air so once again the fan spends a lot of time running on a low-speed fan setting.

5) As the temperature drops below the setpoint the furnace runs more and more on the high fan setting.

6) below that we all freeze to death :)

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I don't think so.

With the variable speed, the unit will start with a low speed profile which will perform humidity extraction. After several minutes, the fan will start to speed up to it's highest speed. it will remain there until the setpoint is satisfied.

With heating, it will also start slow and then increase speed. But it probably does not go all the way to high with the heat pump.

You will have an outdoor temp sensor. You will set two points in the thermostat: the highest temperature for gas to turn on and the lowest temperature for the HP to operate. These two points would be set based on comfort and cost (cost of electricity an cost of gas). The first might be 38F and the second 28F. That means there will an overlapping area where both may be used (HP first, gas second).

    Bookmark   September 5, 2012 at 3:55PM
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Thank you. I guess what I would like to think is that if demand is not that high (it's not that hot outside) that the HP would run on a low-speed profile for a lonnng time versus on-off-on-off blasts of cold air. Ditto for the furnace. Is this possible? Is it something I can configure?

    Bookmark   September 5, 2012 at 4:11PM
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What you are wanting is 2 stages. The speed of the fan does not control how much cooling occurs. With a staged system, the lower stage runs at 75 outside and performs better dehumdification by having longer run times.

Variable speed can lower the speed to improve dehumidification but what you are envisioning is a staged system.

I hope that makes sense. Variable speed gives you some benefit but the real benefit is in staging. At least that is how I understand it.

On the heating side, there is a ramp up to avoid getting blasted with cold air. The cold air in the system is purged slowly and then when the air is hot, the fan has picked up in speed.

    Bookmark   September 5, 2012 at 4:16PM
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You're looking at a single stage system. The HP will run at one speed. The furnace blower will run at a variable speed so you don't get cold 'blasts'.

    Bookmark   September 5, 2012 at 4:21PM
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Residential cooling systems do not have a reservoir for liquid refrigerant beyond the pipes that move it from the compressor outside to the evaporator inside.

Some newer systems at least have staging to alter the system cooling capacity to match the cooling load, but more than two stages is uncommon.

Commercial systems can have actual variable capacity but they are expensive to install and maintain.

They can be designed with storage for liquid refrigerant, and actual variable metering (more than a simple thermostatic expansion valve now used in many residential systems) and have actual variable metering into multiple separate evaporator coils.

the compressor is run based on the storage reservoir, not the temperature.
The refrigerant volume and number of evaporators is then used to vary the output capacity of the system.
It gets complex and expensive to install, maintain, and run.

They often also cool the air year round to dehumidify, then heat it back up in winter, and add back water to a design level.
They even can have actual humidity sensors, and not just temperature sensors.
It is all about cost to design, install, run, and maintain (complicated systems break more often)

Even a staged system only can match the load at two points.

At every other point it is oversize and runs as a 'bang bang' controlled system (on or off, or maybe with on at lower capacity, or on at higher capacity if staged).

    Bookmark   September 5, 2012 at 4:59PM
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