Cold Climate Heat Pumps

[Snagglefoot]

The most important thing about the heat pumps is to fully understand what what your costs are and to realize that the costs are dynamic and change every year. Also, converting to a heat pump is more palatable if you were planning on adding or replacing an air conditioning unit or a gas furnace anyway. Finally, if you are a greenie and live in an area with cheap electrical power you can always play the low CO2 card.

 

[Cujet]

Some important points.

1) Heat pumps ALL suffer in performance as temperature declines.
2) Cold climate heat pumps simply use an oversized compressor and increase its speed as it gets colder, to help offset the performance decline, there is no magic here.
3) BTU output declines as it gets colder.
4) BTU requirements increase as it gets colder.
5) The Carnot limits (thermodynamics) are always in control and the fancy brochure wont change reality.

My point is this, don't be fooled by the COP numbers, look at the BTU output at 0F, and make darn sure it meets your needs. My childhood house in Wilton CT, needed more than the 175,000 BTU the furnace could provide on the coldest nights. In fact, it was quite insufficient, ran non stop and we needed to fire up a wood burning stove just to maintain 60F.

 

[brianl703]

2) Cold climate heat pumps simply use an oversized compressor and increase its speed as it gets colder, to help offset the performance decline, there is no magic here.

They also slow down the indoor fan to maintain higher discharge air temps (which actually increases head pressure and uses more energy).

 

[Number_35]

Some important points.

1) Heat pumps ALL suffer in performance as temperature declines.
2) Cold climate heat pumps simply use an oversized compressor and increase its speed as it gets colder, to help offset the performance decline, there is no magic here.
3) BTU output declines as it gets colder.
4) BTU requirements increase as it gets colder.
5) The Carnot limits (thermodynamics) are always in control and the fancy brochure wont change reality.

My point is this, don't be fooled by the COP numbers, look at the BTU output at 0F, and make darn sure it meets your needs. My childhood house in Wilton CT, needed more than the 175,000 BTU the furnace could provide on the coldest nights. In fact, it was quite insufficient, ran non stop and we needed to fire up a wood burning stove just to maintain 60F.

I assumed the newer units used super refrigerants with an extremely low boiling point to increase their low-temperature efficiency; didn't realize it was so simple.

 

[brianl703]

I assumed the newer units used super refrigerants with an extremely low boiling point to increase their low-temperature efficiency; didn't realize it was so simple.

No, they all still use R410A from what I've seen. R410A is to be phased out next year, yet systems that use the new replacement refrigerant are still very rare.

 

[Cujet]

I assumed the newer units used super refrigerants with an extremely low boiling point to increase their low-temperature efficiency; didn't realize it was so simple.

Nope! The bottom line is that it is possible to make a heat pump perform well in relatively cold weather through a few mechanical "tricks". There is more to it than simply speeding up the compressor, but that's the majority of it.

But we must remember, a COP of 2.0 only means it is 2x more effective than electric resistance heat. The COP will drop to below 2, as we trend beyond the below zero point.

Sooooo, burning natural gas, at 97% efficiency is still considerably more efficient from a fuel use standpoint than a COP of 2.0. As the grid is 33% efficient.

Put another way, at a COP of 2.0, the power plant is using more energy than simply burning gas in your home.


Yes, I know it may not matter to the homeowner's bottom line as energy costs vary, but it is good to understand that the efficiency is not over unity in cold climates.

 

[Number_35]

Nope! The bottom line is that it is possible to make a heat pump perform well in relatively cold weather through a few mechanical "tricks". There is more to it than simply speeding up the compressor, but that's the majority of it.

But we must remember, a COP of 2.0 only means it is 2x more effective than electric resistance heat. The COP will drop to below 2, as we trend beyond the below zero point.

Sooooo, burning natural gas, at 97% efficiency is still considerably more efficient from a fuel use standpoint than a COP of 2.0. As the grid is 33% efficient.

Put another way, at a COP of 2.0, the power plant is using more energy than simply burning gas in your home.


Yes, I know it may not matter to the homeowner's bottom line as energy costs vary, but it is good to understand that the efficiency is not over unity in cold climates.

We're > 98% hydroelectric here, so our electricity is cleaner than natural gas.

Natural gas heat is still much cheaper than conventional (purely resistive) electric heat here, but a heat pump narrows the gap considerably.

Geothermal would be cheaper than natural gas, but the capital cost is prohibitive. The interest on the borrowed money, or lost investment opportunity, would be far greater than the savings.

However, our federal government has put a significant carbon tax on fossil fuels, a tax which will climb much faster than inflation. At some point it will be more economical to run an air-source heat pump.

 

[Snagglefoot]

Some important points.

1) Heat pumps ALL suffer in performance as temperature declines.
2) Cold climate heat pumps simply use an oversized compressor and increase its speed as it gets colder, to help offset the performance decline, there is no magic here.
3) BTU output declines as it gets colder.
4) BTU requirements increase as it gets colder.
5) The Carnot limits (thermodynamics) are always in control and the fancy brochure wont change reality.

My point is this, don't be fooled by the COP numbers, look at the BTU output at 0F, and make darn sure it meets your needs. My childhood house in Wilton CT, needed more than the 175,000 BTU the furnace could provide on the coldest nights. In fact, it was quite insufficient, ran non stop and we needed to fire up a wood burning stove just to maintain 60F.

How’s 54,000 BTU per hour at 5 F. The curve above is for Florida heat pumps.

 

[Rand]

My point is this, don't be fooled by the COP numbers, look at the BTU output at 0F, and make darn sure it meets your needs. My childhood house in Wilton CT, needed more than the 175,000 BTU the furnace could provide on the coldest nights. In fact, it was quite insufficient, ran non stop and we needed to fire up a wood burning stove just to maintain 60F.

Was the barn door left open.. seriously that must have been some house.

My 1952 cape cod style house at -12f last year was running about 50% and its a 40k or 60k btu.

 

[Cujet]

Was the barn door left open.. seriously that must have been some house.

My 1952 cape cod style house at -12f last year was running about 50% and its a 40k or 60k btu.

From the web, but exactly the same. It was called a Deck House. About 3200 Sq Ft. Admittedly it was hard to heat.

 

[brianl703]

From the web, but exactly the same. It was called a Deck House. About 3200 Sq Ft. Admittedly it was hard to heat.

Looks like it was built in the era when energy was "too cheap to meter".

 

[Number_35]

From the web, but exactly the same. It was called a Deck House. About 3200 Sq Ft. Admittedly it was hard to heat.

I love the look of it, but would want the roof to be about 24" thick and filled with closed-cell spray foam.

 

[Rand]

That house looks very similar to the party house at a nearby condo building where some relatives lived.

Same type of roof/architecture but it was only 1 level.

 

[Fitter30]

Fujitsu mini XLTH is probably the best mini I've come across for capacity at low temps. All heat pump manufacturers will have btu capacity at different low temps just might have to contact them for that info. Not all hyper heat units are the same capacity at low temps.

 

[zzyzzx]

Geothermal heat pump for the win!

 

[Number_35]

Geothermal heat pump for the win!

For new construction I would look at it seriously. As a retrofit, it's prohibitively expensive.

 

[zzyzzx]

As a retrofit, it's prohibitively expensive.

How so? I mean if you have the land available and all.

 

[Number_35]

How so? I mean if you have the land available and all.

Factors for my situation when I checked out geothermal in 2006:

- Small yard, so the HDPE conduit would have had to be installed deeper.

- Landscaping in the yard would all have to be redone.

- Because a geothermal system does not produce as much temperature differential over ambient as burning natural gas does, a greater volume of air must be moved. This would have required ripping out much of my finished basement to have larger plenum and ducts installed. (If the ducts are not upsized, the air must be pushed much faster to compensate, resulting in whistling ducts.)

All in, I was looking at about C$35K in 2006. The payback just wasn't there.

 

[Snagglefoot]

The temp dropped and it’s 27.5 F. My old 3.5 ton heat pump is keeping up but will switch automatically to my propane condensing boiler with a heat exchanger in the plenum at about 27 F. This will probably occur in a few hours. I’ll check it tomorrow but I’m predicting I’ll use 50 kWhrs of electricity for the day, depending on when the propane kicks in. That will cost the equivalent of $5.00 US.

The gaps in the cycle are less than 10 minutes, meaning I’m coming to the end of what the heat pump can do before it runs 100% of the time. The temperature coming out of the usual duct I check is 78 F. This is a high velocity system that pulls the heat off the exchanger pretty quickly now allowing it to heat up to 100 F.

 

[Snagglefoot]

The system flipped over to propane. Here’s a shot of my Viesmann condensing boiler and the heat exchanger it feeds hot water to. It’s 20F outside this morning.