Sense has allowed me to evaluate something I have wanted to know
for some time. We hear how a heat pump running long periods of time
is more efficient than auxiliary or emergency heat strips.
I tested this tonight.
From 12 a.m. to 1 a.m. I ran just the heat strips with the heat pump locked out
During that time it came on a total of 17 minutes.
I then turned it back to heat and let the system decide to use heat strips to
supplement the pump. It ran for a total of 40 minutes during the 1 a.m. to
2 a.m. timeframe . It did not use the strip at all during that time. I was surprised
it didn’t being right at 20 degrees during the 12-2 timeframe.
At .085 KWH for electricity cost the strips used 4.9 KWH and use 15 KW.
The heat pump used 1.7 KWH.
So a heat pump uses, in my situation, about a third of the strips.
Two things surprised me about this little test.
The heat pump did need supplemental heat with 20 degree outdoor temperature
and was able to pull enough heat on its own to maintain the house at 70 degrees.
With as long as the heat pump ran (sometimes seems like it runs all the time)
that it was that much more efficient than the strips.
I was just curious, thought it might interest others.
Since I got Sense I did the same thing.
Heat pump Average 2,036W
Aux strips Average 9,844W
Run time will vary greatly for many reasons.
On my Nest thermostat, I used to manually configure outdoor temps settings for when heat pump and aux lockouts are active
Starting today, I configured my Nest thermostat to use the Hear Pump Balance control, which overrides my Aux Lockout based on inside temperature results.
My outdoor temperature today is 0F… I won’t see benefits until Fri or Sat when temps rise in the twenties and thirties.
I run my Nest in Balanced mode too. What’s disturbing is that I often see the heat pump running for an hour or so before it switches to heat strips. If it’s running the heat pump for any length of time and then switches to heat strips, the heat pump time is wasted energy. I’d imagine there’s a outdoor certain below which a heat pump simply isn’t going to produce enough heat to even run a heat pump. Still, Nest chugs away trying to get heat from cold air.
I’m currently having a problem with aux at my wifes store and
think it’s a defective Honeywell 8580 thermostat. She has gas
there and it won’t come on. I’m glad you posted about the Nest
and it’s ability to have such control over aux and has the ability to
use the outdoor sensor. It’s been difficult to find one with all those
features.
I haven’t found a way to compare HVAC efficiencies accurately, so many variables. I can see where they can do it in a lab but in the real world its a moving target once you really break down the testing conditions.
Interesting results, I recently went Geothermal heat pump vs LP (cheap LP right now) and the jury is out yet. They sure run a lot, but its hard to compare to LP usage because of the nature of the fills.
I need to do some deeper digging on this front, especially now that I have an Ecobee, but I hate my aux strips. When I see those on, I know my bill is going to be bad.
I keep them locked out until around 15 degrees. My heat pump is pretty decent up to that point and we’re really all about optimizing for cost savings and not so much comfort. Temps are supposed to plummet tonight and for the next couple days, so looks like it’ll be aux heat time again. You can probably tell from this pic what the coldest days have been this month.
Your right about way too many variables. that was part of the
reason I did this very limited test when I did. I had the same starting
and ending temperatures and they were of equal length of timing and
the outdoor ambient temperature was sustained for those time.
I was trying to take out as many variables as possible and make it as
level a plying field as I could.
We have a store too and it has a heat pump but with natural gas (cheaper
than propane) and I’d like to do a test there. Haven’t figured out how I’ll
do it yet as we don’t have a sense there and I don’t plan on installing one
in a rented building.
I’m thinking reading meters before and after is the best way but someone
might have a better suggestion.
I have my Heatpump Compressor Lockout so my compressor is used for temps above 28F. I truly believe this can be changed to lower. – I just don’t remember if I checked my owner’s manual for the plot points… so I’ll have to look again.
I found this information last year on how to correctly set your Lockout Temps for Compressor and Aux.
See the following video:
(adding video and screen capture from my phone in just a minute)
Based on the testing I’ve been doing over the last few very cold
days, I believe you 28 degree setting is to high. I had the belief
that a heat pump was ineffective below 32-40 degrees but that
is not the case. I’ve done testing all day today and when the heat
pump is running AND kicks in my 15 kw strips, it’s still cheaper than
strips alone. Yes, you read that right, the pup and strips will be on
at the same time for a few minutes.
This screenshot shows what it looked like running in that state.
Very alarming looking. Hard to believe tbis was cheaper but it
is.
I truly believe that, @Samwooly1… with the Nest Heat Pump Balance setting, I believe what you described is what will just happen. I can set the Heat Pump to Time-out after 5hrs (default setting) to where that determines the heat pump isn’t doing the job… but Nest recommends that as Max Savings.
Hopefully tonight, I’ll be able to determine my plot points for that chart and set my Compressor lockout from 28 to something that’s more correct.
I’m doing another hour long test right now and temperature is a steady 8 degrees.
I will see if the heat pump has any effectiveness on it’s own. I suspect not.
I just finished with heat strip only for the 11-12 hour and used 3.2 KWH
according to sense and I believe it to be correct. It values my strips at over
15kw and that could be because the fan running and being picked up as the
same device.
The heat pump shows to be 2700 when running and I think its off by a little
right now and is not including the fan but that’s only a couple hundred watts
if that’s whats happening. Not enough in my opinion to skew the data far.
Absolutely. There definitely seems to be some engrained institutional knowledge that heat pumps just don’t work below 35 degrees, despite any data that says otherwise. Even at 15F, my heat pump is rated to push out 17k BTUh @ 2kw. Regardless, the tech who installed my system had it hard locked out with an outdoor temp sensor below 40 degrees.
I went to amazon for tp link hardware, it’s going to be a slow process
for me to build on this system. I’m not very old (45) but live on the
dreaded “fixed income” otherwise known as social security due to
past years and ongoing major health issues.
The few things I have now are generic or smart things.
So there are a couple of reasons to lock a heat pump out below a particular temperature…
The efficiency drops right off as the difference in temperature between “desired” and “outside” increases, and the dropoff is pretty substantial - at 50 degrees outside with a target of 68 degrees my heatpump is well over 400% efficient (Trane 3ton 14 SEER), but once it drops to 30 degrees the efficiency is barely above 100%, which means that the only heat input is the energy being put into the compressor - not really enough to match the heat losses.
As the temperature drops, the refrigerant condenses out, and accumulates in the bottom of the compressor under the oil - and when the compressor starts and the pressure suddenly drops, it forms a very thin foam, which has pretty much no lubricating ability, and wears the compressor out in no time flat.
Hope that helps
I was surprised when we had some really cold temperatures last week. My heat pump was maintaining our house at 73 when outside temps
We’re at 15 during the day. When it got below 15 it had to help help and the strips turned on.
My wife has a store and that heat pump is effective to about 32 so I have it locked out at 35.
Bother systems at Trane and mine is a year old while hers is about 8-10 years old.
I know what you mean about the oil. The Freon has to carry the oil through the system otherwise it sits wherever it is.
Your one is probably R410, hers is likely to be R22 so there will be some significant differences in pressure and boiling point of the refrigerants, as well as the heat loss of the building - if your building is pretty tight, it can probably manage with a bit less than 2kw of heat input, where the store loses heat much faster and needs rather more…
They are both r410 but it’s very poorly installed. It’s 5 ton and they used 12” pipe for a return with a 20 foot distance and 2 90’s. I’ve had to do a lot of modifications already and not done yet.
The main problem with the 5 ton there versus the 3 ton here is airflow. Each ton has to have. A certain CFM I’d air in and out. The way the 5 ton was installed has plenty of duct the correct size to go out but the return with 12” duct had about a third the capacity it should have. It works like electricity and water, if you have a 20 amp breaker but the device pulls 30 then there isn’t enough “flow”. If you have a 1/2 pipe and then go to 1” then the bigger pipe is starving to get full. It’s working hard but starving for air.
Sam,
The breaker analogy does really work. I think you may have meant cable.
A 20 Amp breaker with a 30 Amp load will just trip the breaker.
Fluids and electricity don’t really behave the same.
I agree that A/C units are notoriously over sized and under ducted. I have a crappy old 4 ton unit with a 14" diameter 12’ long return duct. I believe it is getting only about 1/2 the air it needs. This causes a lot of problems with older, fixed power, units. The modern, variable power, units cope with it better but are still wasting energy if not sized correctly.
