Heat pumps and cold weather

[My Old Tools]

In Texas, load calculations are done on the cooling side as we are a cooling dominate climate. A unit sized for your cooling load may be inadequate for the 5 days a year when it gets really cold. A delta T of 20 degrees between return are and output air is pretty reasonable for most heat pumps. That Delta T diminishes as the temps fall below the design of the unit, it gets less efficient at transferring heat. If your house isn't well insulated (and most in Texas aren't compared to our northern friends), then it can't supply enough heat to keep up. My house is quite inefficient as it has a ton of glass as well as being 30 years old. I went with a geothermal heat pump tied to our lake. The lake never drops below 44 degrees in the winter nor more than 80-85 in the summer, so a heat pump works pretty well. Our previous system was a conventional heat pump with propane backup. With the price of propane now, the geo will pay for itself before it dies.

 

[danski0224]

In Texas, load calculations are done on the cooling side

Huh?

If I run a calculation, it does both heating and cooling. I have no option to do only half of the job.

 

[dcg9381]

If I run a calculation, it does both heating and cooling. I have no option to do only half of the job.

Does it account for substantially decreased BTU output as ambient temperature goes below 25 degrees or so? I don't remember this being taken into account.

 

[u2slow]

According to the Nest thermostat, we had a day where it ran for 21 hours.
Debating if it’d be worth it to add a propane furnace. House is all electric now, I wouldn’t mind a gas range. Not sure what it’d take to add gas to an existing house.

Figure out what your propane extortionist charges first. It's not the same everywhere. I have done the homework and stay on electric.

 

[ericm]

Does it account for substantially decreased BTU output as ambient temperature goes below 25 degrees or so? I don't remember this being taken into account.

This is my understanding, not being a professional:

The Manual J calculation says how many BTUs you need at the design heating and cooling temps (98th percentile lowest and highest... or is it 99th?) to maintain target indoor temps. Then you plug those into the Manual S calculation which factors in latent and sensible cooling and some other stuff to get the output BTUs you need at the design temps.

For heat pumps you would then look at the specs for heat pumps to see what meets the BTU requirement from Manual S at the design temps. Some companies like Mitsubishi show BTU outputs at a range of temperatures. A lot don't, but at least should show the BTU output at the listed high and low design temps of the unit. Usually if it's say a 2 ton unit it'd be 24k btus at 15 degrees or whatever, and 24k btus at 110 or whatever the high is for cooling.

 

[danski0224]

Does it account for substantially decreased BTU output as ambient temperature goes below 25 degrees or so? I don't remember this being taken into account.

That information is in the product specifications.

Heating output at 47 degrees is plugged into the balance point calculation, along with the Manual J heat loss.

Evaporator coil selection makes a big difference both for airflow restriction and capacity.

If "5 tons" of cooling is needed, better watch the evap and condenser combination... and airflow.

 

[mm08822]

That information is in the product specifications.

Heating output at 47 degrees is plugged into the balance point calculation, along with the Manual J heat loss.

Evaporator coil selection makes a big difference both for airflow restriction and capacity.

If "5 tons" of cooling is needed, better watch the evap and condenser combination... and airflow.

It's all about energy transfer. The season only determines the direction of transfer.

From a creature comfort perspective:
In the winter, if the dwelling heat loss is > than the heat pump production, aux heating is needed. It those 2 sources together can't keep up, they will run continuously trying to reach set point.

From a creature's wallet perspective:
You always want to use the lowest cost energy source BUT have to consider the conversion efficiancy for the true cost. As the temperature drops, there is less and less ability to draw energy from the outside while the KwH for the compressor and blower keep consuming electricity while less BTUs are being provided to the interior.

Component selection is critical for each region's design temps and construction quality. Fuel type availability is the next consideration.

 

[PopcornSutton]

In Texas, load calculations are done on the cooling side as we are a cooling dominate climate. A unit sized for your cooling load may be inadequate for the 5 days a year when it gets really cold. A delta T of 20 degrees between return are and output air is pretty reasonable for most heat pumps. That Delta T diminishes as the temps fall below the design of the unit, it gets less efficient at transferring heat. If your house isn't well insulated (and most in Texas aren't compared to our northern friends), then it can't supply enough heat to keep up. My house is quite inefficient as it has a ton of glass as well as being 30 years old. I went with a geothermal heat pump tied to our lake. The lake never drops below 44 degrees in the winter nor more than 80-85 in the summer, so a heat pump works pretty well. Our previous system was a conventional heat pump with propane backup. With the price of propane now, the geo will pay for itself before it dies.

Thanks MOT, for recognizing the 20 degree delta across the coil. Seems many don't but it is a fast and accurate determination of ho the performance is going. When I replaced my first heat pump here, I considered two stage, inverter variables, etc. and decided to just use a standard unit that has much less features to fail. Over this past winter (even though we aren't done) I think I switched over to emergency heat maybe 8 nights. I like wood heat and will keep burning wood as long as I'm able to. In my area, people who put in a geo system are stuck with having to drill well holes usually 200 feet each due to rock. If I had a pond I would have done that in a heart beat!

 

[pcmeiners]

It all makes sense if you can follow this example:

That chart is at least 9 years old based on old technology, does not correspond with today's heat pumps.

 

[mm08822]

The purpose was to simply show the limitations of energy transfer as a function exterior temp. and when supplemental energy is needed.

So what makes it so outdated?

 

[pcmeiners]

Technology advances allowing lower temperatures without supplemental heat sources. The chart is only good for 10 year old technology.

 

[mm08822]

Technology advances allowing lower temperatures without supplemental heat sources. The chart is only good for 10 year old technology.

It's the concept. Exact temps, heat loss, etc. is situation specific.

 

[danski0224]

That chart is at least 9 years old based on old technology, does not correspond with today's heat pumps.

Not true.

 

[PoorUB]

Technology advances allowing lower temperatures without supplemental heat sources. The chart is only good for 10 year old technology.

The cross over numbers may have gottne lower with the hyper heat models, but they all reach a point where straight resistance heat may cost the same.

Like he said, the concept is the same.

If you want to nit pick it, the chart isn't accurate for all manufacturers and all models, even ten years ago. Every manufacturer made models of various efficiencies, and the lower temps they would operate efficiently was all over the map.

Ten years ago I sold units that were done heating at +20F and units that were done at -20F. The balance point was higher. Even Mitsubishi's hyper heat gives up at some point.

 

[PWC Repair]

Easy peasy......I have a 20+ year old heat pump and, as stated, they don't heat well below freezing and they run A LOT. I really just don't care much for the excessive wear and tear on the system. Simply use a thermostat that can handle an outdoor sensor. Set it to "dual fuel" and set switchover point to 30*. Mine has been set up like this ever since I installed it. works like a champ and just switches itself over to strip heat so I get nice warm air blowing out when it's 0* outside.

 

[My Old Tools]

Huh?

If I run a calculation, it does both heating and cooling. I have no option to do only half of the job.

And the cooling number will be the biggest for most of the state.