Heat Pump Costs Less to Run??

[joeyjoey]

I have heard that heat pumps cost less to run in areas with moderate winters. Assuming the strips never come on, the heat pump still has to run the condenser to operate doesn't it? If so, shouldn't that cost be then be the same to run the a/c in the summer - that's pretty expensive.

[amd]

For every unit of energy consumed, a good heatpump can move 3-4 units of energy. Compare that to electric heat strips with a 1-1 ratio.

Electricity tends to cost more per unit of energy compared to gas or even oil. You can only compare it to straight electric heat strips, not gas or any other fuel.

Even in a mild climate, supplemental heat is required during the defrost cycle. Dual fuel is also an option to consider if you have gas or oil readily available. (During cold weather when the heatpump is not economical to operate, it can be set up to shut down and let the furnace take over)

Generally, it really depends on your electricity rate.

[deq1269]

Its cheaper to run compared to a gas furnace. During the summer what do you compare the ac to?

[rifter1]

I have run into this a LOT this year.

A typical (please consult the extended performance data) heat pump works like this:

At 40 degrees or above outdoor temperature (ODT) the heat pump produces 100% of its design rating.

In other words, a 3 ton produces 3 tons of heat.

At 32 degrees F ODT it produces about 75% of its rating.
ie: a 3 tons (36,000 BTUs) produces about 27,000 BTUs.

At 10 degrees F ODT it produces about 50% of its rating (or less).
a 3 ton will produce about 18,000 BTUs (a lot of this comes from the compressor's heat of friction from operation at this point).

Heat strips produce 3143 BTUs per KW. (roughly 15,000 BTUs per 5 KW)

So. If your average outdoor winter temperature never drops below freezing, then a 5KW will more than overcome the 25% drop that a 3 ton heat pump loses due to the 32 degree ODT.

In other words, a 36,000 BTU unit becomes a 27,000 BTU unit at 32 degrees (consult your specific manufacturer's extended performance data... this is kind of average)... which means you lost 9,000 BTUs. How can you get it back?

Install a 5KW heat strip (draws 21 AMPS - expensive, requires 30 AMPS per National Electric Code at your box)... and besides that... it produces about 15,000 BTUs.

So.

The next problem:

Thermostats. How do thermostats work?

Well, your average stat does this:

Say you set the heat for 70F.
It will bring on the heating device at 68F. (2 degree drop).
It will bring on the SECOND stage heat at 67 F. (3 degree drop).

That's nice. BUT What if the heat pump is at a point where it will run forever and keep the home temperature at 68F? That's stupid. Stage 1 only - never kicking on stage 2? That HAPPENS A LOT. If it's at its exact balance point, the supply temperature of a heat pump is also likely less than a 25 degree split.

ie: normally a 25 degree split is about right for a heat pump.
ie: 70F in the home, expect 95F supply air.
Well your skin is 98.6? So the air feels COOL!!!!!

This is objectionable for most people.

ESPECIALLY if you were stupid and removed an existing gas furnace (typical syupply temp of 120-140F depending on brand and age)...

So the customer is MAD.

So..

Install a heat strip and what's called an "outdoor thermostat".

The ODTSTAT does this:

You install it in the condenser. You typically set the ODTSTAT for 40F.
If you do the math for your region you will find the correct temperature where the comfort level will be greater with a second source of heat than the heat pump, or that if you installed a heat pump and a gas furnace, where the balance point of COST will make the second source of heat cheaper.

In any case, you set the ODTSTAT to the correct temp. (Let's say 40F).

Below 40F it will automatically close W2.

Which means your secondary source of heat kicks in 100% of the time, regardless of the house's thermostat call, if the ODT is below the setting of the ODTSTAT.

This is great! So you can bring on heat strips automatically any time the ODT is below the setting (Say 40F).

So, your BTUs on the heat pump drop off, then the extra 5KW heat strips (15,000 BTUs) kick in automatically.

Nice.

That's something we've come across a lot in Phoenix this winter. Unusually cold this year. Normally your heat pumps here don't even need heat strips due to the heat island effect. (Phoenix tends to drop temperature only a fraction of the desert outside of the city during the night). This effect has raised the temperature of Phoenix 1 degree every decade since 1940. Night time temperatures only drop 5-8 degrees where outside of the city in the desert they can drop 20-30 degrees depending on time of year.

This "heat island" effect caused by asphalt and cement and buildings etc, means that heat pumps here are the primary and ONLY source of heat in many homes-- something not possible in any other place in the country.

[rifter1]

The one last point I will raise:

According to the local electric company, the average life of a heat pump in Phoenix is 14 years.

The average life of a gas furnace is 25+ years.

So. YES you are operating at a far less operating cost per BTU, BUT ---
What does it cost to replace the compressor after 14 years? (I know the cost now, and I know it from 10 years ago, and it has doubled...) These compressors run like crazy all summer long in cooling mode so they have a short lifespan here. To give you an idea, your typical electric bill in an 1,800 sq ft home jumps from $70 a month in winter to near $280 a month in July/August. That means A/C is about 75% of your electric bill in summer.

Gas furnaces have a far less REPLACEMENT/maintenance cost.

If you factor in the cost of repairing/replacing a heat pump every 14 years (per the local power company, APS) then you see gas is still far cheaper.

Nobody around here brings that factor into play with their math, per my experience.

There are websites out there that will answer your question regarding the cost per Therm - between a Heat Pump and a Gas Furnace --- but they do NOT include maintenance and replacement costs, and the life cycle of a heat pump in your market. (14 years here). Look at the price of replacing a heat pump. Double it. That's what you will likely pay in 14 years if you are in Phoenix, then think of your gas furnace working another decade or so beyond that before parts become unavailable.

Tell me which you think is cheaper.

[ShannonRawls]

awesome reply

[mark beiser]

Rifter1, what you describe is not the typical way that an outdoor thermostat is used on a heat pump, and certainly is not very economical.

If you look at the wiring diagrams with just about any heat pump system, you will see that the optional outdoor thermostat is wired to keep part of the electric heat from working if the outdoor temperature is above the set point.
This only affects how much electric heat is used during defrost cycles and when the thermostat calls for 2nd stage heat. It doesn't bring on electric heat with the heat pump when the thermostat isn't calling for it.

You could wire an outdoor thermostat to do what you describe, but you run the risk of having failure modes that result in runaway heat, or the heat strips running with the AC.
I have never seen documentation from an equipment manufacturer that shows or supports wiring an outdoor thermostat to do what you describe.

If you want tighter temperature control, use a Honeywell Vision Pro or Focus Pro thermostat that has P+I control instead of the fixed differential between stages that dumber thermostats have.
If the customer is getting drafts from air blowing on them, use better registers that put the air where it needs to go anyway, instead of blowing down into the rooms and on the occupants like the cheep stamped face registers do.

[dash]

Rifter1, what you describe is not the typical way that an outdoor thermostat is used on a heat pump, and certainly is not very economical.

If you look at the wiring diagrams with just about any heat pump system, you will see that the optional outdoor thermostat is wired to keep part of the electric heat from working if the outdoor temperature is above the set point.
This only affects how much electric heat is used during defrost cycles and when the thermostat calls for 2nd stage heat. It doesn't bring on electric heat with the heat pump when the thermostat isn't calling for it.

You could wire an outdoor thermostat to do what you describe, but you run the risk of having failure modes that result in runaway heat, or the heat strips running with the AC.
I have never seen documentation from an equipment manufacturer that shows or supports wiring an outdoor thermostat to do what you describe.

If you want tighter temperature control, use a Honeywell Vision Pro or Focus Pro thermostat that has P+I control instead of the fixed differential between stages that dumber thermostats have.
If the customer is getting drafts from air blowing on them, use better registers that put the air where it needs to go anyway, instead of blowing down into the rooms and on the occupants like the cheep stamped face registers do.



Exactly!

Plus what is the average life of a stright cool system in that area?

[jrbenny]

I'd blow a gasket if I found someone wiring a stage of heat to always run below a certain temperature. Talk about wasting money. :eek:

[Airmechanical]

The one last point I will raise: if you factor in the cost of repairing/replacing a heat pump every 14 years (per the local power company, APS) then you see gas is still far cheaper.

so by (using furnace in winter) does this mean you think

the condenser will last for 28 years? :eek:



.

[rifter1]

Probably not.

But it should last longer, given you're only using it half the year instead of all year?