Single Stage vs. Dual Stage Heat Pump?

[chrysalishead]

I am in the process of selecting a contractor for a replacement furnace and air conditioner in Minnesota.

One contractor stood out as trying to meet our needs and sizing things properly (including looking at all supplies and returns). They are recommending a 3-stage Carrier 95% furnace and a 3.5 ton, single stage Carrier heat pump (which matches the size of the existing AC). The heat pump recommendation surprised me, because of all of the benefits touted by the other contractors about the humidity control that a 2-stage HP could offer.

We have two stories and a finished basement. The bedrooms in the upper story are usually too warm at night with the existing setup, even with the supply dampers closed in the basement. We have returns in each of the bedrooms. However, we have a large two-story great room, and there are no returns covering that space, as well as a similar entry-way space (about the size of two bedrooms in total).

The contractor's arguments for recommending a single stage instead of a dual stage heat pump are:

1) The single stage has a higher SEER and COP for most temperature ranges

2) The single stage is available in 3.5 ton, while the dual stage is only available in 4 ton, and our need is 3.5 ton

3) More importantly, he believes that if we went with a 2-stage and operated in low, the cold air would not adequately reach upstairs. On the heating side, they would rarely use the low setting on the heat pump. And, with the variable speed capabilities of the furnace blower, combined with the smart controller, we would get sufficient humidity control with a single stage.

Given this application, are there opinions about the logic and recommendation to go single stage instead of two stage for the heat pump?

[dash]

1.Savings from running in low stage may trump that.

2.Not a big issue,unless the ducts are undersized now for the 3.5 ton.

3.If that's true,ask how the hot air will get there,in the low stage of heating.

Two stage for humidity control while cooling is great.

[BaldLoonie]

Seems to me the higher efficiency is more important.

Heat rises so not as hard to get warm air upstairs.

While they say 2 stage dehumidifies better, that's not much of an issue in MN vs Florida.

Just a few things to consider on the other side of the fence!

[badtlc]

Seems to me the higher efficiency is more important.

Heat rises so not as hard to get warm air upstairs.

While they say 2 stage dehumidifies better, that's not much of an issue in MN vs Florida.

Just a few things to consider on the other side of the fence!

How is it not an issue in MN? It should be more of an issue in MN because it is can be humid and not as hot. There are 10,000 lakes in the state. I'd think 2-stage would be far more beneficial in MN than it would be in Florida where there would be much more heat to force the run times up.

[BaldLoonie]

Except on 1 Carrier model and 1 Trane model, low cool is 75-80% of high cool. Not much difference, run times won't be that much longer. One popular model of 2 stage has some high sensible heat ratios so on low it doesn't dehumidify well anyway. The TS equipped Carrier could do better with more like 60% on low. But any unit on low is using an awfully big coil compared to capacity, that's why the moisture sucking can be reduced.

While I never had trouble feeling comfy in MN with a conventional A/C system, if someone really wanted dry air they could go with a control that reduces blower speed to really suck out the moisture or since much of the time it is very humid, the cooling may be on so little that no matter what you do, the A/C doesn't dry the air enough, go with a whole house dehumidifier.

[motoguy128]

How is it not an issue in MN? It should be more of an issue in MN because it is can be humid and not as hot. There are 10,000 lakes in the state. I'd think 2-stage would be far more beneficial in MN than it would be in Florida where there would be much more heat to force the run times up.

While you do have 10,000 lakes.... Florida has the Gulf of Mexico. Not much of a comparison.

While MN does get humid days, the actual dewpoint rarely gets much above 65F thanks to cool overnight temperatures. You outdoor design tmperature will be lower so the equipment, if sized correctly, should have enough load to keep the humidity well controlled without needing 2 stage. With such a short cooling season, I think it's somewhat of a waste. Getting a VS blwoer and using dehumidify on demand to slow the blower for more latent capacity, should do just fine.

[motoguy128]

Except on 1 Carrier model and 1 Trane model, low cool is 75-80% of high cool. Not much difference, run times won't be that much longer. One popular model of 2 stage has some high sensible heat ratios so on low it doesn't dehumidify well anyway. The TS equipped Carrier could do better with more like 60% on low. But any unit on low is using an awfully big coil compared to capacity, that's why the moisture sucking can be reduced.

While I never had trouble feeling comfy in MN with a conventional A/C system, if someone really wanted dry air they could go with a control that reduces blower speed to really suck out the moisture or since much of the time it is very humid, the cooling may be on so little that no matter what you do, the A/C doesn't dry the air enough, go with a whole house dehumidifier.

Yup.... and worse, to get higher SEER one might foolishly be inclined to oversize the coil on a 2 stage unit. Guess how cold a 4 ton coil gets with 2.2 tons of capacity of a 2 stage 3 ton unit??? Not very cold. I'm struggling with this problem. I'm comming to the conclusion that 2 stage units are somewhat overrated for dehumidification.

[Airmechanical]

Guess how cold a 4 ton coil gets with 2.2 tons of capacity of a 2 stage 3 ton unit??? Not very cold. I'm struggling with this problem. I'm comming to the conclusion that 2 stage units are somewhat overrated for dehumidification.

talking about coil temps;

the fact is, the blower speed is lowered as the compressor kicks into low speed

which in turn keeps the coil as cold as in high speed

low speed compressor and indoor blower is going to run longer than if in high speed

which will remove more humidity than a single stage system, when set-up properly



.

[larryb]

Nobody has mentioned quietness yet. Things are much quieter in low speed. I have a modulating furnace. I hardly ever hear the furnace run. Unfortunatley, my AC is single-speed and I do hear the system in cooling mode. A home is so much more enjoyable when the hvac equipment is quiet.

Larry

[DanW13]

[QUOTE=larryb;3749842]Nobody has mentioned quietness yet. Things are much quieter in low speed. I have a modulating furnace. I hardly ever hear the furnace run. Unfortunately, my AC is single-speed and I do hear the system in cooling mode. A home is so much more enjoyable when the hvac equipment is quiet.

If your system is that loud in cooling mode your duct work could be undersized?

The 3 stage Carrier furnace is a nice system, I have had mine for 1-1/2 yrs now and I love it. I'll be upgrading my 10yr old Payne 10 seer AC sometime this summer and I to am contemplating going with a 2 stage AC or HP but am still unsure if it's worth the extra money to do the HP so the jury's still out as far as that goes.

If your farther north than I am here in Wisconsin about a 100 miles south of Greenbay than I'm not sure your going to benefit from a 2 stage AC or HP as your payback will be somewhat non-existent. I would have someone run some numbers for you in your area to give you something to compare side by side with a single stage AC/HP and a 2 stage AC/HP so you can see just how they compare with that of area temps for your area.

Call your local utility company and see what they have to offer as far as a home accessment goes which would include a blower door test and other test in and around the home and see if you can't improve your house first then get a Load calc done and see where your at. I would bet a single stage AC/HP would suffice for you in your area ?

[chrysalishead]

Thanks to all for the thoughts on single vs. dual stage. While there is no clear answer, the debate has helped me lean toward going with a single stage. The same contractor also recommended a 14 SEER unit instead of the 16-17 quoted by the others, because it would not yield enough savings in our climate to warrant the extra cost. So, they are giving me the impression of selling me only what I need, and no more. They also have a great deal of experience in our area.

On the heat pump vs. AC question:

I downloaded 10 years worth of hourly climate data for the Twin Cities and found that we spend about 85% in heating season (< 70 degrees), and 15% in cooling season (> 70 degrees). In the heating season, we spend 30% of the time below 30 degrees (gas furnace range) and 70% between 30 and 70 (heat pump range). Given our energy costs and our heating gas usage (adjusted for a new 95% efficient furnace), the heat pump would save about an additional $300/year, or a 5.5 year payback given the increased cost for the heat pump. Not great, but probably enough to tilt me to the heat pump vs. the AC.

[gary_g]

Thanks to all for the thoughts on single vs. dual stage. While there is no clear answer, the debate has helped me lean toward going with a single stage. The same contractor also recommended a 14 SEER unit instead of the 16-17 quoted by the others, because it would not yield enough savings in our climate to warrant the extra cost. So, they are giving me the impression of selling me only what I need, and no more. They also have a great deal of experience in our area.

On the heat pump vs. AC question:

I downloaded 10 years worth of hourly climate data for the Twin Cities and found that we spend about 85% in heating season (< 70 degrees), and 15% in cooling season (> 70 degrees). In the heating season, we spend 30% of the time below 30 degrees (gas furnace range) and 70% between 30 and 70 (heat pump range). Given our energy costs and our heating gas usage (adjusted for a new 95% efficient furnace), the heat pump would save about an additional $300/year, or a 5.5 year payback given the $1600 increased cost for the heat pump. Not great, but probably enough to tilt me to the heat pump vs. the AC.

Good analysis.

Not sure if I would use 70F as the cut-off between heating season and cooling season, but I don't think that would change your end result.

Best of luck.

[motoguy128]

Thanks to all for the thoughts on single vs. dual stage. While there is no clear answer, the debate has helped me lean toward going with a single stage. The same contractor also recommended a 14 SEER unit instead of the 16-17 quoted by the others, because it would not yield enough savings in our climate to warrant the extra cost. So, they are giving me the impression of selling me only what I need, and no more. They also have a great deal of experience in our area.

On the heat pump vs. AC question:

I downloaded 10 years worth of hourly climate data for the Twin Cities and found that we spend about 85% in heating season (< 70 degrees), and 15% in cooling season (> 70 degrees). In the heating season, we spend 30% of the time below 30 degrees (gas furnace range) and 70% between 30 and 70 (heat pump range). Given our energy costs and our heating gas usage (adjusted for a new 95% efficient furnace), the heat pump would save about an additional $300/year, or a 5.5 year payback given the increased cost for the heat pump. Not great, but probably enough to tilt me to the heat pump vs. the AC.

Careful with that temperature data. You may spend 70% of the TIME above 30F... but you only consume maybe 30% of the total energy at those termpatures. Remeber you burn at least twice the energy maintaining 70F indoor temp at 10F than you do at 40F.

So 1 hour at 10F uses hte same energy as 2 hours at 40F.

I initially made that mistake when running calculations for a dual fuel system. The savings were about half what I originally estimated.

So you will get longer run times on the heat pump, and save energy. But it only outputs about 1/2 - 1/4 the heat of the furnace.

I'm still a fan of dual fuel for flexibility and cost savings, but the furnace still does the "heavy lifting".


I'd put it this way. In the north, it's a gas furnace with a heat pump. In the south, it's more like a heat pump system with supplemental gas heat instead of all electric.

[motoguy128]

Good analysis.

Not sure if I would use 70F as the cut-off between heating season and cooling season, but I don't think that would change your end result.

Best of luck.

I agree, I find my cut-off when keeping my windows closed is more like 60F. If anything, there's a dead band between 60-75F whre you don't need A/C or heat. It actually makes up almost 8 weeks out of the year in msot of the midwest. Don't underestimate how much heat you get from solar radiant heat and indoor appliances and activities.

[chrysalishead]

Great points. The dead zone is easy to fix, but the furnace energy consumption profile vs. outside temperature is something I didn't think think of, and I will try to model it too.

[Norml]

Even if the "pay back" amount or time frame is negative, or rather inconvenient, those of us who can afford to save energy at the modest increase cost of equipment (within reason both to our pockets and the putative extra energy spent building that equipment), I believe we have an ethical obligation to planet earth and our children's children to reach for the more efficient equipment.

I did. Both with HVAC and with a hybrid car (even though, eg., a Corolla is better than a Prius using total $ cost).

That is the spirit behind the government's energy plan!

[chrysalishead]

Finished the easy part:

Percentage of hours in the last 10 years in the Twin Cities of Minnesota:

<30 degrees (gas furnace heating): 24%
between 30 and 60 degrees (heat pump heating): 41%
between 60 and 75 degrees (dead zone): 26%
>75 degrees (AC): 9%

So, during the heating hours, 37% is gas furnace heating and 63% is heat pump heating.

[motoguy128]

Even if the "pay back" amount or time frame is negative, or rather inconvenient, those of us who can afford to save energy at the modest increase cost of equipment (within reason both to our pockets and the putative extra energy spent building that equipment), I believe we have an ethical obligation to planet earth and our children's children to reach for the more efficient equipment.

I did. Both with HVAC and with a hybrid car (even though, eg., a Corolla is better than a Prius using total $ cost).

That is the spirit behind the government's energy plan!

True... but a heat pump is all about saving money when compared ot a furnace. A heat pump wastes far more energy when you factor in the power plant efficiency (the majority of pwoer in US is from coal) and then efficiency in power distribution.

The same problem occurs when you look at things like biodiesel and ethanol.

you want to help the next generation... have less than 2 children (to reduce the population to a more sustainable number) and build smaller more efficient homes to conserve energy. I can have a low efficiency system and still use far less energy if my home is half the size.

Wood fireplace and wood furnaces have also gotten somewhat ignored as practical, renewable energy source for those living in small towns and more rural areas with sustainable population densities.

Sometimes the best solutions are the simpliest ones.

Ethics and the American lifestyle don't really go together well. We're a very wasteful society. I'm not really complaning. But some "solutions are really just a drop in the bucket.

I'm not against the stimulus. I do think it was however poorly impelmented and too confusing and the details could be improved. I go think that in term of stimulating hte economy, investing in things like energy efficincy, infrastructure and research, put us in a good position when we recover.

[Norml]

Good points. I do care about the TOTAL impact of my footprint.

May I infer HP is only a compromise in year round use?

Doesn't HP compare favorably to A/C by using less electrical energy in summer?

Now I regret not doing geothermal. It seems geoT is better both for heating and cooling; using minimal electricity for fans and pumps, right?

I did go with hybrid Nat Gas and HP on the belief it was the better choice for the world, without the effort chrysalishead is doing in math. I respect him for being precise.

Maybe if Obama CO2 "tax" were based on the final cost for energy use, we would make logical and cost effective choices?

[chrysalishead]

I have some early numbers that account for motoguy128's point about needing to account for the outside temperature in the "heat pump vs. AC question", but I want to play with them some more before posting them. But, here is my methodology, for critique:

motoguy128's point is that just looking at the percentage of time spent in the furnace and heat pump temperature zones does not account for the fact that the furnace must take on much more than this proportion of the total heating load because of the high temperature differential in the furnace heating range compared with the heat pump heating range. So, as the outside temperature increases, the required heating energy decreases, which means that the heat pump takes on less of the heating load than I originally assumed, which means less cost savings from the heat pump.

I first took a year's worth of heating bills and took the monthly therm reading and subtracted out the lowest monthly value (mid summer) so I could could capture the therms per month dedicated to heating (as opposed to water heating). Then, I got the corresponding data for each monthly reading period "heating degree days" by taking 60 (I know that the official definition is 65, but I wanted to be consistent with the other info I posted) minus the average daily temperature (throwing out the negative (or cooling degree days) values. Doing this, it looks like my current furnace spends about .2 therms/heating degree day (and it was fairly consistent each month). Then, I found out the total heating degree days between 30 and 60 degrees for the year. Multiplying this by my .2 therms/heating degree day factor yields the number of therms spent in the heat pump temperature zone. I reduced these therms by the factor .78/95 to account for the new furnace, and finally it is these therms that are subject to the heat pump savings over the furnace. Whew!

Numbers in a day or so.

[chrysalishead]

Wow, motoguy128's point makes a huge difference in the heat pump vs. AC payback period. With some hesitation on the validity of my methodology, here are some preliminary results:

Assumptions:

- Average daily temperature data for St. Paul, MN for the last 10 years
- Cost of electricity: $0.0829/KWH (heating season)
- Cost of natural gas: $1.022/therm (heating season)
- Furnace efficiency: 95%
- Average heat pump COP (from contractor) in our climate: 3.25

(So, the cost for 1,000,000 BTUs for the furnace is $10.45 and for the heat pump is $7.48, so the heat pump yields a 28% savings in the 30-60 degree temperature band)

- therms/heating degree day (with a base of 60 degrees) for my house = .2 and it is linear (not true, but hopefully close enough)

Considering DAYS spent in each range:

<30 degrees (gas furnace heating): 24%
between 30 and 60 degrees (heat pump heating): 41%
between 60 and 75 degrees (dead zone): 26%
>75 degrees (AC): 9%

So, considering only heating days, 37% are gas furnace heating days and 63% are heat pump heating days.

Now, considering motoguy128's point:

Looking at the heating degree day distribution (with a base of 60 degrees), and using the .2 therms/heating degree day factor, there are an average of 474 therms per year used when the outside average daily temperature is between 30 and 60 degrees, and these therms are subject to the heat pump savings FOR MY HOUSE.

And, I estimate that over the last year, I used 1759 therms for heating, so although we spend (10 year average) 63% of the heating season in the 30-60 degree range, for this last year we spent only 27% of the therms in this range, and this latter number is the one that relates to the cost savings.

These 474 therms are for my old 78% furnace, so if I had a 95% furnace, they would be reduced to 389 therms. With gas furnace heating only, these 389 therms would cost $398. But since a heat pump would save 28%, I would save $113/year with the heat pump. But, given the additional cost of the heat pump over an air conditioner, the payback would be about 14 years. In my view, this isn't good enough to justify a heat pump!

Interestingly, two contractors gave me a publication from the Minnesota Heating & Cooling Association called "Do Heat Pumps Work in Northern Climates?". The only payback case study it discusses involves propane! So, my conclusions for me are:

1. Yes, heat pumps work :)
2. Yes, they save money each year
3. No, they are not cost effective with 14 year payback

If anyone lasted to this point in my posting, I welcome critiques on my methodology, assumptions, and conclusions. This is such a hard question to analytically answer in the face of all of the general sales and marketing information on heat pumps.

[motoguy128]

My payback period was improved because my utility rebates covered almost half the price difference.

Beenthere has talked about using "temerature bin data" and "heating degree days" combined ot more accurately reflect costs of operation.

No a 14 year payback probably isn't worth it. Your electrical isn't cheap enough. Dual fuel has much better paybacks for propane and oil or larger homes and in more mild climates.

[jkish]

- Average heat pump COP (from contractor) in our climate: 3.25


If you are only considering the HP performance between 30 and 65 degrees, the average COP is probably better than 3.25.

I quickly looked up the COP ranges for a 4 ton Amana ASZ18 HP on low stage:

Outdoor Ambient Temperature
65 60 55 50 47 45 40 35 30 25 20 17 15 10 5 0 -5 -10

COP
4.79 4.63 4.46 4.27 4.13 4.04 3.84 3.63 3.24 3.07 2.90 2.78 2.71 2.50 2.28 2.05 1.80 1.52

This is hard to read, but at 30 degrees, the COP is already 3.24. At 47 degrees (halfway), it is 4.13.

Still, this adjustment might be minor in terms of the savings.

[jeepsterman]

What about a geothermal unit with a 30% tax credit?

[motoguy128]

What about a geothermal unit with a 30% tax credit?


For most people, drilling the wells represents to large of a cash outlay even with a 30% credit. In many residential areas, you need to use verticla wells, which are more expensive.

Geo makes a lot of sense in the rural retrofits or in new construction, but I'm not sure about retrofits in town.


Now in a "green" city, I suppose a city or developer could bury loops under city streets and offer access to the loops as a utility service... and/or have the water loops going to auxillay cooling/heating towers located near the water treatment plants. Its' an interesting thought. That would make it relatively affordable. That would eliminate the guesswork in line capacity.... since you would be billed for the total BTU's transferred.

[chrysalishead]

jkish,

For fun, I plugged in a COP of 4.13 instead of 3.25 and the payback changed from 14 years to 9.

[jeepsterman]

The "green" city approch is used on several military bases, it works well








For most people, drilling the wells represents to large of a cash outlay even with a 30% credit. In many residential areas, you need to use verticla wells, which are more expensive.

Geo makes a lot of sense in the rural retrofits or in new construction, but I'm not sure about retrofits in town.


Now in a "green" city, I suppose a city or developer could bury loops under city streets and offer access to the loops as a utility service... and/or have the water loops going to auxillay cooling/heating towers located near the water treatment plants. Its' an interesting thought. That would make it relatively affordable. That would eliminate the guesswork in line capacity.... since you would be billed for the total BTU's transferred.

[chrysalishead]

jkish,

For fun, I plugged in a COP of 4.13 instead of 3.25 and the payback changed from 14 years to 9.

Also, I looked up the proposed heat pump (Carrier Performance 25HPA542H30 3.5 ton, single stage) and the furnace (58MVC100-20), and if I read the chart right, the "high temp" (whatever that is) COP is 3.58 and the "low temp" (whatever that is) is 2.64, giving an average 3.11.

[motoguy128]

Also, I looked up the proposed heat pump (Carrier Performance 25HPA548H30 3.5 ton, single stage) and the furnace (58MVC100-20), and if I read the chart right, the "high temp" (whatever that is) COP is 3.66 and the "low temp" (whatever that is) is 2.64, giving an average 3.15.


High temp is usually at 47F. low temp is normally 17F. So depending on the location, and being dual fuel, you'd have to weight the COP by bin data and also know you balance point.

[Jack2007]

The contractor's arguments for recommending a single stage instead of a dual stage heat pump are:

1) The single stage has a higher SEER and COP for most temperature ranges

2) The single stage is available in 3.5 ton, while the dual stage is only available in 4 ton, and our need is 3.5 ton

3) More importantly, he believes that if we went with a 2-stage and operated in low, the cold air would not adequately reach upstairs. On the heating side, they would rarely use the low setting on the heat pump. And, with the variable speed capabilities of the furnace blower, combined with the smart controller, we would get sufficient humidity control with a single stage.

Given this application, are there opinions about the logic and recommendation to go single stage instead of two stage for the heat pump?
Thought I would give my two cents on my experience.

As the pros always say, "your duct work is always key".

I had a Carrier system installed;
Split System: Heat Pump with Remote Outdoor Unit-Air-Source
Outdoor Unit Model Number: 25HNA648A30
combined with Indoor Unit Model Number: CNPV*4821A**
Oil Furnace 58VMR120-20
Variable speed blower.
(EDIT) TP--PRH thermostat

The numbers all said I needed a 3.5 ton unit but the 4 ton two stage unit was installed. I am told the 25HNA648A30 in first stage is 32,000 BTU. My original system was a 5 ton unit so the duct work was more than adequate.

My electric rate is 0.16 cents/KWH (including all taxes and fees), but my electric company gives a substantial discount for using a heat pump. Now the electric rate is supposed to increase Jan 1, 2010 but the so called "transition charge" will go away and hopefully will cancel out each other.

So far my electric usage went up but because of the discount my actual cost went down. I'm also using less oil, so more savings.

My system runs about 80 to 85% of the time in first stage or about 2 1/2 tons for both HP and AC and my humidity issues with older system have disappeared.

I live NE of Philadelphia. Just my two cents. ;)

BTW; There is about a two to three degree difference between first and second floor.
.