Interim reality check

Jamie Hall

And most emphatically not for or against anything!

As many of you know, a heat pump system was put into an apartment (800 square feet wall area, 400 square feet to attic) which is part of Cedric's home (7,000 square feet wall area, 2500 square feet to attic) last fall. The weather gods gave us a nice chance to evaluate how things are going.

Conditions: 0 degrees F outside, holding 66 degrees inside in apartment, 62 in rest of building, 20 mph wind gusting to 40. Electricity is $0.30 per kWh. Oil is about $4 per gallon.

Cedric -- the boiler, for those of you who haven't met him -- had no trouble holding the main structure temperature (perhaps 25% on, 75% off).

The heat pump (52,000 nominal heat output) was able to keep the apartment at 66 (augmenting Cedric, which is still heating the apartment, but onl y to 62), but was running all the time full out, except for the defrost cycling.

Heat pump COP unknown, probably (generously) around 2.5

Bottom line. The heat pump kept working. However, the cost to run per BTU was higher than the cost to run Cedric (about $0.04 per 1,000 BTU vs about $0.03 per 1,000 BTU for Cedric).

Your mileage may vary. Draw what conclusions you like.

pecmsg

I shut down my heat pump at 20*. HW cast iron radiators take over. 

Can’t warm your winter cloths on forced air!

ChrisJ

pecmsg said:

I shut down my heat pump at 20*. HW cast iron radiators take over. 

Can’t warm your winter cloths on forced air!

That's literally how a clothes dryer works.

I know I know, not the point.........

ChrisJ

And @Jamie Hall You just started yet another thread on the same discussion that's now spanned like 10 threads.

mattmia2

Is the problem technical or political? Is there any technical reason the evaporator couldn't be designed to be efficient at -10 f? The evaporator in a conventional fridge seems to have no problem picking up heat at that temp.

Look at a 1.6 gallon toilet from the us from 1990 and a good one today. Same constraint on efficiency but when someone tried to make one that worked they succeeded.

hot_rod

Well that .30 per KW certainly makes the HP less viable. Maybe you should move Warmer climate, smaller home.

ChrisJ

mattmia2 said:

Is the problem technical or political? Is there any technical reason the evaporator couldn't be designed to be efficient at -10 f? The evaporator in a conventional fridge seems to have no problem picking up heat at that temp.

Look at a 1.6 gallon toilet from the us from 1990 and a good one today. Same constraint on efficiency but when someone tried to make one that worked they succeeded.

I've learned a huge amount from Monitor Tops but one thing was the bigger the temperature difference the lower the efficiency of a system.

No freezer has an EER of 10, I promise. It's been years since I looked but I seem to recall an EER of 4 typical. And no, that's not just 1930s technology modern ones run very much the same. A temperature difference of 80 to 100 degrees between the evaporator and condenser comes at a big cost.

Now, why that is I have no idea.
That falls under the category of I accepted it as "It's just how it is" and never dug further.

Sometimes things are cost prohibitive and just don't make sense in a practical application.

WMno57

ChrisJ said:

And @Jamie Hall You just started yet another thread on the same discussion that's now spanned like 10 threads.

No, this is different. An actual case study of a Heat Pump with data. I would like to read more of these. Also hope Wallies will share their experiences with A2WHPs and HPWHs.
Next year I will share some photos and data on my backup generator.

fentonc

@Jamie Hall - always great to see real-world data. What model of compressor and head(s) are you using? Did you measure actual power usage? If it's an inverter-style compressor, it should be running all the time once the heat loss is above the minimum output, right?

EdTheHeaterMan

hot_rod said:

Well that .30 per KW certainly makes the HP less viable. Maybe you should move Warmer climate, smaller home.

According to @hot_rod's map: The top 14 states to move to, in the continental US are: Alaska, Washington, Idaho, Montana, North Dakota, Minnesota, Wisconsin, Michigan, Ohio, Pennsylvania, New York, Vermont, New Hampshire and Maine.


Don't BLOCK me, I'll leave now!

WMno57

mattmia2 said:

Is there any technical reason the evaporator couldn't be designed to be efficient at -10 f? The evaporator in a conventional fridge seems to have no problem picking up heat at that temp.

Cold Climate heat pumps will always have low COPS. Below some temperature, you can make heat, but what is the cost to the lifespan of the heat pump?
MTBF drops when you work equipment harder.
Achieving lowest TCO is done by running resistance electric heat or burning traditional fuels in some low range of COPs.
Stamp out Heat Pump abuse. Use the right tool for the job.

ChrisJ

WMno57 said:

mattmia2 said:

Is there any technical reason the evaporator couldn't be designed to be efficient at -10 f? The evaporator in a conventional fridge seems to have no problem picking up heat at that temp.

Cold Climate heat pumps will always have low COPS. Below some temperature, you can make heat, but what is the cost to the lifespan of the heat pump?
MTBF drops when you work equipment harder.
Achieving lowest TCO is done by running resistance electric heat or burning traditional fuels in some low range of COPs.
Stamp out Heat Pump abuse. Use the right tool for the job.

Is running an evaporator extra cold really working equipment harder?

I think it may be in a case where you're pulling the evaporator temperature down below where the compressor etc is intended to run. But otherwise does the equipment really know the difference or care?

Jamie Hall

As a bit of technical aside -- the problem with any heat pump (or similar -- refrigerator, air conditioner, what have you -- doesn't matter) if it is of the overall type which uses a refrigerant of some kind and a mechanical compressor is that the operating principal is the same -- you evaporate the refrigerant at some low pressure and temperature, compress it, and condense it at some high temperature and pressure. The power required to run it is that which goes into the compressor (one reason newer ones are so much better than some older ones -- better compressors!). The choice of refrigerant determines what the pressure and temperature relationships are -- and also the lowest practical temperature for the low side, as well as the highest practical temperature for the high side. The limitation on the low side usually is either the freezing point of the refrigerant or the difficulty of reaching and maintaining a low enough pressure (we're talking absolute pressure here, so in many cases one can also think in terms of maintaining a very high vacuum). The limitation on the high side is almost always a pressure limitation. There are very few non-toxic and non-hazardous refrigerants which have a really usable temperature range of much over 120 degrees F or so, which also presents a serious limitation (not so much for air to air, but for hydronic applications this is a real problem).

The reason the COP drops with increasing temperature difference is pretty obvious -- it simply takes more power to compress the refrigerant from the low side pressure to the high side pressure, but the BTU delivered per pound of refrigerant condensed doesn't change if the high side temperature doesn't change. Physics is a b___h, and you can't fight it.

There are also practical problems with decreasing low side temperature and pressure -- maintaining a leak proof vacuum. One thing which is quite detrimental to performance is the presence of any non-condensable gas in the system -- and air is, for the temperature ranges we are looking at, non-condensable. Pity...

Jamie Hall

ChrisJ said:

WMno57 said:

mattmia2 said:

Is there any technical reason the evaporator couldn't be designed to be efficient at -10 f? The evaporator in a conventional fridge seems to have no problem picking up heat at that temp.

Cold Climate heat pumps will always have low COPS. Below some temperature, you can make heat, but what is the cost to the lifespan of the heat pump?
MTBF drops when you work equipment harder.
Achieving lowest TCO is done by running resistance electric heat or burning traditional fuels in some low range of COPs.
Stamp out Heat Pump abuse. Use the right tool for the job.

Is running an evaporator extra cold really working equipment harder?

I think it may be in a case where you're pulling the evaporator temperature down below where the compressor etc is intended to run. But otherwise does the equipment really know the difference or care?

@ChrisJ -- see the last couple of paragraphs I just posted. It does make a difference.

Lance

There are two balance points to every dual fuel heating system; The BTUH capacity approaching full load, and the economic balance point between the two. Heat pumps have given this to us by being unable to meet a full demand load whereby a backup system was required. Today, we have heat pumps that can handle full load in heat without backup. But the rub is which is more efficient run; the heat pump or the alternate, non-electric fuel? The calculation to find the economic balance point for each degree and each changing efficiency point is not worth the effort. But coupled with the WAG method and the Multiple test method a general proximity to a setpoint can be deducted. My experience with a two speed heat pump of a 17Seer (AC rating), and if I recall a 2.9COP at a test lab set point which changes with temp drop, against an 83% Afue gas furnace I found, A) the heat pump could handle the load down to the lowest temperature and the furnace backup did not run. Complaint: cost went up instead of down. Solution: Find the economic balance point, "the outdoor temp", to turn off the heat pump where the gas furnace cost less to run than the heat pump. It turned out to be below 48F. The higher the fuel efficiency of the fossil fuel, the variable costs per Btuh for electric and gas considered, sets the economic balance point. We talk about the future energy needs and use, but we have yet to provide the duel fuel customer the ability to economically choose which appliance is least costly to use as fuel prices and the efficiency curves of the appliance change per degree of outdoor temperature.
Not unlike Miles Per Gallon which change depending on gravity, air and ground resistance, load, and operation between idle and go. Of course, if fuel is free, the choice is simple.

Hot_water_fan

Cedric: $4 x 1,000,000/138,000 / 85% = $34.10/MMBtu at 0 degrees outside. 

Heat pump: $.3/kwh x 1,000,000/3412 / 250%(from Jamie) = $35.17/MMBtu. A 3% difference! 

mattmia2

Jamie Hall said:

As a bit of technical aside -- the problem with any heat pump (or similar -- refrigerator, air conditioner, what have you -- doesn't matter) if it is of the overall type which uses a refrigerant of some kind and a mechanical compressor is that the operating principal is the same -- you evaporate the refrigerant at some low pressure and temperature, compress it, and condense it at some high temperature and pressure. The power required to run it is that which goes into the compressor (one reason newer ones are so much better than some older ones -- better compressors!). The choice of refrigerant determines what the pressure and temperature relationships are -- and also the lowest practical temperature for the low side, as well as the highest practical temperature for the high side. The limitation on the low side usually is either the freezing point of the refrigerant or the difficulty of reaching and maintaining a low enough pressure (we're talking absolute pressure here, so in many cases one can also think in terms of maintaining a very high vacuum). The limitation on the high side is almost always a pressure limitation. There are very few non-toxic and non-hazardous refrigerants which have a really usable temperature range of much over 120 degrees F or so, which also presents a serious limitation (not so much for air to air, but for hydronic applications this is a real problem).

The reason the COP drops with increasing temperature difference is pretty obvious -- it simply takes more power to compress the refrigerant from the low side pressure to the high side pressure, but the BTU delivered per pound of refrigerant condensed doesn't change if the high side temperature doesn't change. Physics is a b___h, and you can't fight it.

There are also practical problems with decreasing low side temperature and pressure -- maintaining a leak proof vacuum. One thing which is quite detrimental to performance is the presence of any non-condensable gas in the system -- and air is, for the temperature ranges we are looking at, non-condensable. Pity...

Perhaps it is time to look at 2 stage systems.

SlowYourRoll

How good is Cedric at cooling the apartment in the summer? [just kidding around, please don't hate me]

EdTheHeaterMan

mattmia2 said:

Perhaps it is time to look at 2 stage systems.

I always wanted to be on the stage. I was thinking of stand up comedy, but that went out the window when I was confined to a wheelchair (I can't stand up for those who didn't get it)

My friends in the local Community Theater Group said "Ed ,You should be on the stage". When I agreed, they told me that "the afternoon stagecoach was leaving in 20 minutes."

hot_rod

Of course if you had to buy oil tomorrow, at over 5 bucks a gallon🧐

hot_rod

looks like your heat pump is mostly fuel oil powered anyways No wonder your rates are the highest in the country.

SlowYourRoll

 im assuming that such a frequent contributor to the HeatingHelp board runs a finely-tuned and optimized boiler that probably isn't representative of the efficiencies most folks have. so i think this info is useful to others here who've taken the time to maximize efficiency, but if we're talking about 33% cost reduction (3¢ vs 4¢) when comparing to Cedric, I'm guessing it's closer to dead even for most people.

Hot_water_fan

@SlowYourRoll it’s not 33%, it’s 3%. $/1000 btus makes small differences seem larger due to rounding. $/1,000,000 btus tells the story more clearly. ~$34 vs ~$35/MMbtu. 

Jamie Hall

It's a penny per 1,000 BTU, @Hot_water_fan -- so for Cedric operating at full power (400.000 BTUh) it's actually a difference of $4 per hour in Cedric's favour.

I'll happily grant @SlamDunk 's comment, though -- Cedric and the system he powers has been worked on rather carefully over the years!

Hot_water_fan

Cedric: $4 x 1,000,000/138,000 / 85% = $34.10/MMBtu at 0 degrees outside. 

Heat pump: $.3/kwh x 1,000,000/3412 / 250%(from Jamie) = $35.17/MMBtu.

Just depends where you round @Jamie Hall . It’s a penny if you round $.03410 down and $.03517 up but it’s not $10/MMBtu different, it’s 1/10th that. 

EBEBRATT-Ed

Not only does a heat pump suffer the colder it gets outside due ti increased compression ratios caused by lower outside temp and (70 deg we hope) indoor temp the heat pump also pumps less refrigerant the colder it gets outside due to the dropping suction pressure available to fill the the cylinder or the scroll.

EdTheHeaterMan

I always get messed up with decimal points. I just don't get the point of it all. I wish someone could point me in the right direction.

SlowYourRoll

i didn't follow all the other threads so I don't know if this was posted or not, but this article is pretty great and it gets into a lot of the stuff we're looking for here. 

https://learnmetrics.com/heat-pump-efficiency-vs-temperature-graph/

Jamie Hall

My intention was not to determine which was less expensive, nor to advocate for one over the other -- simply to put down what I found in the most recent cold spell.

A LOT more goes into the discussion of relative cost, never mind relative benefits. But that wasn't my point.

As they used to say on a TV show long before most of us were around, "just the facts, sir, just the facts" (and yes I used to watch it -- bonus points for identifying it).

Hot_water_fan

Just a minor rounding quirk, that’s all. 

SlowYourRoll

i appreciate the data @Jamie Hall

hot_rod

Jamie Hall said:

My intention was not to determine which was less expensive, nor to advocate for one over the other -- simply to put down what I found in the most recent cold spell.

A LOT more goes into the discussion of relative cost, never mind relative benefits. But that wasn't my point.

As they used to say on a TV show long before most of us were around, "just the facts, sir, just the facts" (and yes I used to watch it -- bonus points for identifying it).

Dragnet, Joe Friday, maybe?

JakeCK

You keep it at 62? Burrrrrr

I demand 72f be kept by my boiler regardless of what it's doing outside. :P And 72 it was while -4, with -35 wind chills outside. Had a 50% run time to accomplish that. Design for this area is 7f I believe. Just a little oversized.

Now I await the gas bill. 

WMno57

ChrisJ said:

Is running an evaporator extra cold really working equipment harder? I think it may be in a case where you're pulling the evaporator temperature down below where the compressor etc is intended to run. But otherwise does the equipment really know the difference or care?

Two concerns:
One is the ambient temp the compressor lives in. Crankcase heaters and running the HP every half hour might make this a non issue. If power goes out for a day and the compressor does a cold start immediately after power comes back on that might be a problem. Software could prevent this.
Two is duty cycle and number of starts.
Not sure how cold it would have to be for either of these to become an issue. Might be lower than most HPs will ever see. You can make something idiot proof and then later someone invents a better idiot.

ChrisJ

WMno57 said:

ChrisJ said:

Is running an evaporator extra cold really working equipment harder? I think it may be in a case where you're pulling the evaporator temperature down below where the compressor etc is intended to run. But otherwise does the equipment really know the difference or care?

Two concerns:
One is the ambient temp the compressor lives in. Crankcase heaters and running the HP every half hour might make this a non issue. If power goes out for a day and the compressor does a cold start immediately after power comes back on that might be a problem. Software could prevent this.
Two is duty cycle and number of starts.
Not sure how cold it would have to be for either of these to become an issue. Might be lower than most HPs will ever see. You can make something idiot proof and then later someone invents a better idiot.

The colder it is, the less starts there should be if any with an inverter.
Starting would happen far more, even with a normal compressor when it's mild out. To me, starting is the hardest on the compressor and running non-stop is preferred, power consumption aside.

The crank case heat situation is an interesting one, because now the evaporator is surrounding the compressor and should always be at a similar temperature even if completely off and the CCH is off. I'm curious on how this would actually behave because obviously the condenser does have some quantity of refrigerant that could migrate. I just don't think it's anywhere near as much as the evaporator.

Also scrolls will generally tolerate some slugging even though it should be avoided.


The points Jamie and others mentioned are good ones though. As suction pressure drops it would put more of a load on the pump and it's sealing points. Not sure how this effects a scroll or it's wear, if any.

I know of compressors out there with 200,000+ hours on them. That's enough for what, 20 years running continuously? These were some of the best hermetic compressors ever made, so I know it's not likely to get something similar in a heat pump, just that it is technically possible. And I'm sure many on here have seen 1970s and 80s compressors on air conditioning still running fine after all these years. No, they didn't run continuously, but we know they have a lot of hours on them.

Hermetically sealed compressors in general are pretty damn impressive as far as failure rate and lifespan.

Jamie Hall

@hot_rod gets the bonus points prize!

pecmsg

EdTheHeaterMan said:

I always get messed up with decimal points. I just don't get the point of it all. I wish someone could point me in the right direction.

And your Point is?

pecmsg

Jamie Hall said:

My intention was not to determine which was less expensive, nor to advocate for one over the other -- simply to put down what I found in the most recent cold spell.

A LOT more goes into the discussion of relative cost, never mind relative benefits. But that wasn't my point.

As they used to say on a TV show long before most of us were around, "just the facts, sir, just the facts" (and yes I used to watch it -- bonus points for identifying it).

Actually it's "Just the facts mam, Just the facts"

I'll give a hint
Col Potter was in it!

Jamie Hall

I know it''s just the facts, mam -- but I think the object of my comment is a sir, not a mam -- though nowadays one can't be sure...

ChrisJ

Jamie Hall said:

@hot_rod gets the bonus points prize!

Never cared for Jack Webb.

Now, Perry Mason, that's a show.

mattmia2

So basically we are trying to doll up a system that was designed for about a 30 degree differential to work at a 70 degree differential and wondering why it doesn't work very well instead of designing components for a 70 degree differential.

If you run in a vacuum eventually you no longer have air as an insulator and the motor shorts so probably picking a refrigerant that is still around atmospheric pressure at your low temp is important. Or using a 2 stage system with a low temp and a medium temp refrigerant.