Cold Climate Heat Pumps

[Jay Rodewald]

What are you thoughts on the cold climate heat pumps? Once the temperature gets down to below 30 degrees and the electric resistant heat kicks in, I would think that would really cut your savings down depending on how many cold day you have. I am not sure they would be any savings for a climate like Wyoming.

[Martin Holladay]

Jay,
If you're talking a "cold-climate heat pump," then clearly you're talking about a unit that's designed to work at temperatures much lower than 30 degrees F.

For example, the Mitsubishi Mr. Slim Hyper-Heat unit (model PUZHA36NHA) has a nominal heat output rating of 38,000 Btuh. According to the manufacturer, at an outdoor temperature of -13°F, its heat output drops only 21%, to 30,000 Btuh. That's without any electric resistance elements kicking in.

Read more here:
http://www.greenbuildingadvisor.com/...nsulated-house

[Jay Rodewald]

Yes, Would they be anymore efficient than a 95% efficient furnace? How do the two compare?

[Martin Holladay]

As you point out, a gas furnace can be 95% efficient, while a heat-pump's efficiency might be in the range of 200% to 300%.

But so what? They use different fuels.

[charles]

I'd like to know whether ductless minisplits can be used in homes that do not have an open plan. I.e. a fan unit in each room. Or is a PTHP the way to go for this type of plan? If so, does anybody make really small PTHP's?

[NW Architect]

Its all a matter of optimizing the climate, sources and cost of fuels, equipment and controls.

I'm having to replace some HVAC equipment in my home and have been getting a good education lately. Top of the line option for THIS climate is a 2-stage heat pump, paired with an 80% efficiency, gas furnace with a variable speed fan.

We are mostly a heating climate - not real cold, hardly ever, but heating for a good 7-8 months of the year, then cooling for maybe 2 of the remaining calendar months.

Different combinations will come out "on top" elsewhere.

One interesting factoid I DID learn was that a 95% efficient furnace is not 15% more efficient than an 80% furnace. The presence of the heat exchanger in the 95% unit increases static pressure in the air stream and therefore electrical energy consumption - a factor not computed in the efficiency ratings. If one operates the fan continuously, that cost can add up.

There never is a simple answer that fits all, is there?

[Ted S.]

Its all a matter of optimizing the climate, sources and cost of fuels, equipment and controls.

I'm having to replace some HVAC equipment in my home and have been getting a good education lately. Top of the line option for THIS climate is a 2-stage heat pump, paired with an 80% efficiency, gas furnace with a variable speed fan.

We are mostly a heating climate - not real cold, hardly ever, but heating for a good 7-8 months of the year, then cooling for maybe 2 of the remaining calendar months.

Different combinations will come out "on top" elsewhere.

One interesting factoid I DID learn was that a 95% efficient furnace is not 15% more efficient than an 80% furnace. The presence of the heat exchanger in the 95% unit increases static pressure in the air stream and therefore electrical energy consumption - a factor not computed in the efficiency ratings. If one operates the fan continuously, that cost can add up.

There never is a simple answer that fits all, is there?

And condensing boilers are not always condensing, so they are not always 95% + efficient, the return loop and heat load must carefully be designed and verified in the field to produce conditions which condensing of the unit will occur. Many plumbers simply give options for either condensing or non-condensing, this can lead to an improperly designed system and lower efficiencies.

[Jay Rodewald]

Does anyone have experience with the Acadia by Hallowell International? The reason I am asking is I am looking at a home that has a air source heat pump. If I buy the home it will need to be replaced.

[ThingOfBeauty]

80% furnaces should be avoided just to save you the makeup air/pressure hassles and risks.
In a new construction situation (where there isn't an existing chimney in great condition), the 90% is probably easier to manage and comparable in price, because you can duct the exhaust sideways in PVC, instead of needing a vertical chase with an expensive metal flue inside.
A variable stage burner will also run much more efficiently, making a top of the line condensing furnace even more efficient.
I'd love to see the numbers on the "airflow restrictions" too, while that seems to make a certain amount of sense, when you look inside furnaces the 90% versions don't appear to block much more airflow.
I suspect you're being sold a bill of goods. But get us some data so I can learn that I'm wrong.

[WetCoast]

What are you thoughts on the cold climate heat pumps? Once the temperature gets down to below 30 degrees and the electric resistant heat kicks in, I would think that would really cut your savings down depending on how many cold day you have.

The point of cold climate heat pumps is that they don't need electric backup until well below 30. For example, a 4 ton Acadia will put out 50,000 BTU at 10 and 25,000 BTU at -30.

The Acadia has been discussed a few times on hvac-talk. Usual hvac-talk disclaimers apply.

[dgbldr]

As you point out, a gas furnace can be 95% efficient, while a heat-pump's efficiency might be in the range of 200% to 300%.
.

The efficiency goes down at low temps. You should look at the manufacturers' charts.

More importantly, the HEATING CAPACITY of the system goes down with low temps. So it doesn't really matter how efficient your system is at 10F. If the house needs 50k BTUs and the system can only put out 20k at that temp, you're running on electric heat. That's not efficient at all.

You can oversize the heat pump, but that has its own set of issues.

[Martin Holladay]

DG Builder,
The Mitsubishi Mr. Slim Hyperheat model PLA-A36BA has a COP of 1.9 at 5 degrees F. That's 90% better than electric resistance heat.

[mrd999]

Air source heat pump capacity and efficiency vary with outdoor temperatures (and the indoor set-temp too, though this can be ignored.) I think it's safe to say you're going to see both the capacity and efficiency drop along with temps.

Heat pumps with variable compressor speed - inverter or digital scroll systems - are more complex. At a given outdoor temperature, they have a certain capacity range, and the efficiency depends on where within that range the system is operating at. As you vary the outdoor temperature, the range and efficiency also varies. This additional complexity caused a mess with AHRI certified ratings. How do you compare the efficiency/capacity of variable systems - how to compare between variable systems, and how to compare them to traditional fixed-speed systems?

I don't know details of how this has been/is being resolved, but I do know a new standard was created specifically for these variable systems, AHRI Standard 1230. It would be nice to know more of how these systems are tested and rated.

For heating performance, I look at the HSPF. This is supposed to be essentially an average COP over a typical heating season, so it should reflect comparative heating costs. It is an average COP, but in terms of kBTUs. You can convert it to average electrical efficiency like so: HSPF / 3.413 = average COP.

To compare a heat pump HSPF with other fuel sources isn't too difficult. A good source of prices here: http://www.eia.doe.gov/emeu/steo/pub/contents.html The simplest way is to convert everything into terms of cost per million BTUs.

Let's say I'm paying $0.14/kwhr, my HSPF is 10.3:
$ per million BTU = elec_cost / HSPF x 1000 = $0.14 / 10.3 x 1,000 kBTU = $13.59/million BTU
The 1000 is there because there are 1000 kBTU per million BTU

Now for natural gas, let's say I'm paying $12.60/mcf, and the furnace has an efficiency of 95%
$ per million BTU = gas_cost / (eff x 0.9804) = $12.60 / (0.95 x 0.9804) = $13.53/million BTU
The 0.9804 is there because there are 0.9804mcf per million BTU

These numbers assume the HSPF accurately reflects system operation. If your capacity doesn't coincide with the tested capacity, it's possible the numbers could differ quite a bit. This is why I'd like to know more about AHRI 1230, and why manufacturers should publish comprehensive performance data.

Also, I've seen specs for new systems available overseas with heating COPs surpassing 4. It seems the manufacturers aren't in any rush to get new systems imported to the US because of their low popularity. An average COP of 4.5 translates to an HSPF of 15.3, plug it into the earlier equation and you get a cost of $9.15/million BTU! I wonder how much higher COPs will go..

[Dancing Dan]

Does anyone have experience with the Acadia by Hallowell International? The reason I am asking is I am looking at a home that has a air source heat pump. If I buy the home it will need to be replaced.

Even thought they're a local (to me) company, I know the mechanical guys whose judgement I trust around here are all far more keen on the various Japanese companies.

[dgbldr]

DG Builder,
The Mitsubishi Mr. Slim Hyperheat model PLA-A36BA has a COP of 1.9 at 5 degrees F. That's 90% better than electric resistance heat.

Yes and the average residential I do is 70-90kBTU per furnace. Does Mitsubishi make a unit that covers that range? I need 3 of those Mitsus to replace one furnace, at a cost of $20k, right? Even ignoring installation cost, those units will cost me like $20k vs a $4k gas furnace/air conditioner combo.

The heat pump has no SEER advantage over the average high-eff A/C. So you are going to amortize the cost difference just from the winter use. How many hundreds of years is it going to take to amortize that kind of difference?