Three stage cooling and Two stage heating.

[jjmarquez84]

I helped install a new residential multistage heat pump system that was equipped to provide three stages cooling with two stages of heating. To be honest I really don’t understand how all the stages of heating and cooling were to be accomplished by the t-stat. I guess it’s something that I never picked up, or really understood but what I don't understand clearly is how or what controls the stages? Any takers?

[motoguy128]

3 stages? How was that managed?

You might need one of those little PLC type programmable controllers with display that has analog inputs and at least 6 digital outputs.

[Txvman]

A digital t-stat with 5 relays could easly do it. Pretty easy really. Honeywell make 3H/3C t-stats. Not cheap thou.

http://customer.honeywell.com/honeywell/productinfo.aspx/T7350H1009

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[pacnw]

I do not understand how there would NOT be more stages of heat.

You should have the same heat and cool from heat pump(in theory) as well as at least 1 stage of back up, maybe more.

[Txvman]

Mind you this is a guess, 3 compressors and 2 reversing valves.

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[mark beiser]

What brand?

[motoguy128]

What brand?

I'm really curious on this. I'm having trouble seeing the value over something like a Carrier Greenspeed.

Maybe it's a two scroll unit with two refrigerant circuits and one of the scrolls unloads? But in heat, you only run with both loaded, since I think unloaded you don't gain any efficency in heating mode.

Or a custom air handler with a A coil that has 2 refrigerant circuits and one of them is a 2 stage condenser.

An interesting alternative when you think about it in applications where you want some redundancy and a sytem that rusn almost constantly or has a very small zone, but don't need more than 5 tons and are space limited to a single air handler. but it seems to me that an inverter unit would be better.

[jjmarquez84]

Your right looking at it agian its 3heat and 3 cooling.

[jjmarquez84]

Trane.Trane.

[54regcab]

Are you absolutely sure it isn't the much more common 2 cool/3 heat?

[billygoat22]

maybe controlling two units off one tstat? a/c units and furnaces?

I had a 3 cool, 2 heat a few yrs back. used a time delay for last stage.

[zacmobile]

This is quite common on geo systems and I would think on most air source HP's.
Generally you would have a two stage compressor with two stages of heat and two of cool & 1 back up. I'm pretty sure most t-stats i've used have a time delay or a combination of temperature change over time for the back up/3rd stage relay.

[motoguy128]

This is quite common on geo systems and I would think on most air source HP's.
Generally you would have a two stage compressor with two stages of heat and two of cool & 1 back up. I'm pretty sure most t-stats i've used have a time delay or a combination of temperature change over time for the back up/3rd stage relay.

But he's talking about 3 cool, 2 heat. The vision pro IAQ will do 4heat, 2 cool. But I haven't seen one that will do 3 cool, there's not much demand for it.

[beenthere]

But he's talking about 3 cool, 2 heat. The vision pro IAQ will do 4heat, 2 cool. But I haven't seen one that will do 3 cool, there's not much demand for it.

He corrected himself, and said its 3 heat 2 cool.

[martyinlincoln]

I helped install a new residential multistage heat pump system that was equipped to provide three stages cooling with two stages of heating. To be honest I really don’t understand how all the stages of heating and cooling were to be accomplished by the t-stat. I guess it’s something that I never picked up, or really understood but what I don't understand clearly is how or what controls the stages? Any takers?

Fairly simple for a good thermostat/thermidistat. First stage cool brought on by temp rise,second stage brought on by higher temp or time depending on how the stat is set up. Heating will be the same with an outdoor temp sensor or two in the mix to determin which stage to bring on.

Our house (has 4h2c) for example is set up to bring on 1st stage cool with a 1 degree rise and second stage cool with a 2 degree rise. If the humidity is above 50% then it runs second stage compressor but leaves the blower on first stage cfm. The heat pump runs seconds stage if the outdoor temp is between 32 and 50, first stage hp only if it's over 50,first stage gas if it's 31 or less and second stage gas if the stat falls 2 degrees below set point.

[motoguy128]

One simple solution for 3 cooling stages is to use the dehumidification terminal to drop out 2nd stage. Otherwise you normally cycle between 2nd and 3rd stages. The airflow would have to by managed by the air handler(s). But airflwo requirements would depend on how many refrigerant circuits it has and how the coils are configured.