[Bob Sanders]

I'm just looking at my performance charts and I'm noticing that once I get into the lower temperatures the COP is actually better on stage 2 than on one.
Just wondering what would save more... Long runs on stage one, or shorter runs on stage 2? (System rigged to bypass stage one and going directly to two at a set temperature)

Amps drawn at the lower temps is actually more on stage 1 than 2

 

[beenthere]

At lower temps, stage 2 requires a defrost more often then stage 1 does. So the increase in COP is often a trade off, efficiency wise.

Remember, COP is with a completely clean, frost/ice free coil.

 

[Bob Sanders]

Yeah, but at the lower temps (around these parts anyway) it's dry enough so that I'm defrosting for nothing more than oil return. I can go 5 hours at a time with absolutely no frost (on stage one)

Ahhh wait a minute.... okay.. I see what you're saying... I'll get a FASTER build up on stage 2 so it won't make much difference?

 

[beenthere]

Right. In second stage, the coil will be much colder, so it will build frost faster. Requiring defrost/more defrost cycles.

 

[supers05]

TBH, either I'm reading that wrong or misunderstand your statement, but I see that the second stage always has a higher COP... Although I do see a jump in difference at around 32*F.

Either way, your heat pump will likely be in second stage the majority of the time for the right half of that chart. (you have over a 50% reduction of capacity by that point)

And, I believe BT is right about the difference not being all that much anyways after accounting for everything.

Cheers!

 

[Bob Sanders]

TBH, either I'm reading that wrong or misunderstand your statement, but I see that the second stage always has a higher COP... Although I do see a jump in difference at around 32*F.

Either way, your heat pump will likely be in second stage the majority of the time for the right half of that chart. (you have over a 50% reduction of capacity by that point)

And, I believe BT is right about the difference not being all that much anyways after accounting for everything.

Cheers!

I don't start jumping in and out of stage 2 till about -17C (about 1F) and it goes full time on stage 2 at about -19 (-24 is when I lock out the condenser). Up until then it idles away on stage one. Right now it's about -14C (about 6 or 7 F) and it doesn't go into stage 2. It's on stage one at about 85 or 90% duty duty (on most of the time with just a little bit of cycling). During the day I'm defrosting not AT ALL for frost but for oil return (every 5 hours). Indeed all today I haven't seen an ounce of frost on the coils, although usually does get a bit frosty during the night.

I'm thinking on low humidity days at probably about 7F I should just lock out stage one (7F is about where amperage draw becomes higher on stage one than 2)

 

[Bob Sanders]

Had the 1st stage bypassed for a couple of days now. Makes a big difference. It's -20C right now (about -4F) and the HP is not on full time any more at this temperature. It's cycling a bit so it's having no issues keeping up. I have the defrost set up on a timer right now (defrosts every 5 hours for oil return.... no frost build up at all since the humidity is too low). Operating stage 2 only is definitely more efficient at these lower temps.

Right now I just have the bypass rigged with a manual switch, but I have a thermostat control on order so I will set it up to automatically bypass stage one at a set temperature.

 

[bfletcher7]

Nice feedback, Bob, and what a good discovery you made for yourself. It would be awesome if we had a web portal to control ALL functionality given us from our t-stats and third-party defrost boards, making it quick and easy to tweak on the fly

 

[user_12345a]

The compressor starting and stopping in very cold weather may shorten its life compared to just running.

You may get a slightly higher COP, but if the compressor fails at 15 years instead of 20 its not worth it.

 

[Bob Sanders]

The compressor starting and stopping in very cold weather may shorten its life compared to just running.

You may get a slightly higher COP, but if the compressor fails at 15 years instead of 20 its not worth it.

I would worry about it cycling in hot weather... but not so much in the cold. Head pressure is pretty low.

 

[user_12345a]

Would be curious to know the return on investment; you had electric forced air before, right?

 

[supers05]

Looking up the full manual for that model number, (and I could be wrong) but, if you look at all the other charts, the other models of that series do not follow the same pattern. I'm tempted to say it's a misprint. Maybe you can actually measure the current draw and verify.

I also found that they use a simple 2 speed psc condenser fan motor. Not the most efficient at the lower speed. On top of that, they use a single speed compressor, with a capacity control solenoid. While more efficient then single stage, not the most efficient multi capacity component (again, maybe I looked up the wrong manual) I suppose that there's a lower limit on its efficiency curve in relation to the 2 stages. (As the lower stage saves less % of power as the unit load falls off) You may find that you have more capacity if you switch to high speed condenser fan below a given temp. (low enough that you don't overload the compressor with high suction temps/pressures)

Cheers!

 

[user_12345a]

^No point in getting something more efficient if it has expensive ecm fan motor, inverter boards for variable compressor. It's more efficient until the repair costs come, the day after the warranty expires.

 

[Bob Sanders]

Would be curious to know the return on investment; you had electric forced air before, right?

When comparing this year with HP to resistive heat last year, so far I have save about 5000Kw. Here in Manitoba that would be $370 plus tax ($418).

At this rate I believe total savings for this heating season to be around $550 to $650... better than I expected.

 

[supers05]

^No point in getting something more efficient if it has expensive ecm fan motor, inverter boards for variable compressor. It's more efficient until the repair costs come, the day after the warranty expires.

I never said it had a better ROI, just in terms of energy efficiency alone.

FYI. Building code in Ontario mandates ecm furnace blowers on new installs. Also every recent ducless spilt (both the ones I've installed and the ones in stock at suppliers) are all inverter models. The future will eventually arrive, even if we get grey hair waiting. Lol.

Cheers!

 

[Bob Sanders]

Looking up the full manual for that model number, (and I could be wrong) but, if you look at all the other charts, the other models of that series do not follow the same pattern. I'm tempted to say it's a misprint. Maybe you can actually measure the current draw and verify.

Interesting. I'll look at that tomorrow or the next day and get back to you. I'm curious now.

You may find that you have more capacity if you switch to high speed condenser fan below a given temp. (low enough that you don't overload the compressor with high suction temps/pressures)

I in fact came to that conclusion a few weeks ago and bypassed the lower speed on the fan. The result is that I get an extra degree delta out of the deal in stage one.

 

[Bob Sanders]

I never said it had a better ROI, just in terms of energy efficiency alone.

FYI. Building code in Ontario mandates ecm furnace blowers on new installs. Also every recent ducless spilt (both the ones I've installed and the ones in stock at suppliers) are all inverter models. The future will eventually arrive, even if we get grey hair waiting. Lol.

Cheers!

ECM furnaces blowers make sense, but I have to say I agree with user... I don't see any real need for them on a condenser.... unless of course it's a variable speed machine

 

[user_12345a]

FYI. Building code in Ontario mandates ecm furnace blowers on new installs. Also every recent ducless spilt (both the ones I've installed and the ones in stock at suppliers) are all inverter models. The future will eventually arrive, even if we get grey hair waiting. Lol.

You mean in new construction or all new forced air system?

Either way, the road to hell is paved with good intentions.

I think it's linked to the silly energy star program, where governments spec out "energy star" everything for their codes and energy conservation rebates regardless of what is actually is.

The criteria keeps on changing.

Funny thing about new housing developments is the homes may be energy efficient but the savings get offset by extreme car dependency. They would be better off legislating for walkable communities than silly techno fixes.

sorry for going off topic.

 

[beenthere]

I would worry about it cycling in hot weather... but not so much in the cold. Head pressure is pretty low.

Does your compressor have a crank case heater.

 

[Bob Sanders]

Does your compressor have a crank case heater.

Yes, it indeed does.