Heat pump water heaters

[Anonymous]

Hope this helps:

https://pressbooks.bccampus.ca/collegephysics/chapter/applications-of-thermodynamics-heat-pumps-and-refrigerators/#:~:text=An%20artifact%20of%20the%20second,Php%3DQhW.

[Anonymous]

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:Heat pumps start to suck at places just slightly colder than DC. We’re on the margin of suitable winter climate for heat pumps. They work much better in places like the Carolinas and are outstanding in GA and FL.

If you have a ground loop or geothermal well for the heat pump, where the evaporator coil is guaranteed never to get below 50-55 degrees, heat pumps can work great in even cold climates (we’re not “cold” here in DC) But the typical DC area heat pump set up with an outdoor unit with an atmospheric coil is right on the edge of not working here.

Heat pumps have improved over recent years. There are "cold-climate heat pumps" that can operate efficiently well below at extreme low temperatures:

https://www.reddit.com/r/heatpumps/comments/146jg7k/cold_climate_heat_pump_owners_what_model_did_you/

Some people argue that heat pumps can't help with CO2 reduction because they are typically powered by an electrical grid that runs on fossil fuels. And they point out that a lot of the energy and heat goes up the smokestack of the power plant, which is wasteful. It is true that energy is lost at the site of power generation, but, even so, heat pumps can reduce CO2 emissions (relative to gas furnace heating) due to their high efficiency. Suppose that 30% of energy is wasted at the site of power generation. So only 70% of the original energy reaches the customers. But if customers are running heat pumps that are 200% efficient, the waste at the central power plant is more than offset by the efficiency of the heat pump. The result is that heat pumps can lead to significant emissions reductions relative to gas furnaces. And yes, good heat pumps can keep your house warm even in frigid weather.

I realize I’m only a stupid mechanical and electrical engineer who didn’t get a liberal arts degree from an Ivy, but perhaps one of you smart MBA’s or PhD’s can explain the whole “200% efficiency” thing to me?

Because I’m absolutely dying to hear this one….

Jesus Christ you are stupid. The efficiency comes as a result of transferring heat rather than creating heat. It is much more efficient.

Anyway, to op’s point, we have one. Over time it has worked out somewhat cheaper than gas.

You don’t even understand enough about thermodynamics to know what you don’t know.

And you think I’m the stupid one!


You went to an Ivy and have two MA’s, don’t you?

[Anonymous]

I realize I’m only a stupid mechanical and electrical engineer who didn’t get a liberal arts degree from an Ivy, but perhaps one of you smart MBA’s or PhD’s can explain the whole “200% efficiency” thing to me?

A heat pump is moving heat from outdoors to indoors, rather than creating heat by burning fossil fuel. If we put one unit of energy into a heat pump to power it, but it provides us with two units of energy in the form of heat, then its efficiency is 200%.

Even on a cold day, there is plenty of heat in the atmosphere that can be "harvested" by a heat pump and moved indoors. At the molecular level, heat is the movement of molecules -- and air molecules are always moving, even on a frigid winter day.

In contrast, a gas furnace burns fuel to create heat. The max possible efficiency is 100%, assuming we have a perfect gas furnace in which their is no loss of energy. With a perfect furnace, one unit of energy (stored in the gas) is translated into one unit of heat output. So the efficiency is 100%.

Modern heat pumps can achieve efficiencies of over 300%, versus a gas furnace that has, as a theoretical limit, an efficiency of 100%.

Its true that heat pumps are usually powered by electrical grids that depend heavily on fossil fuels, and it is also true that many power plants that use fossil fuel to generate electricity are wasteful and inefficient. So one has to consider the net effect -- the waste at the power plant versus the high efficiency of the heat pump. If a power plant wastes 30% of the potential energy stored in fossil fuel, but a heat pump is 300% efficient, then one still comes out way ahead using a heat pump -- that is, using a heat pump is, overall, a more efficient option for heating our homes.

[Anonymous]

Anonymous wrote:

I realize I’m only a stupid mechanical and electrical engineer who didn’t get a liberal arts degree from an Ivy, but perhaps one of you smart MBA’s or PhD’s can explain the whole “200% efficiency” thing to me?

A heat pump is moving heat from outdoors to indoors, rather than creating heat by burning fossil fuel. If we put one unit of energy into a heat pump to power it, but it provides us with two units of energy in the form of heat, then its efficiency is 200%.

Even on a cold day, there is plenty of heat in the atmosphere that can be "harvested" by a heat pump and moved indoors. At the molecular level, heat is the movement of molecules -- and air molecules are always moving, even on a frigid winter day.

In contrast, a gas furnace burns fuel to create heat. The max possible efficiency is 100%, assuming we have a perfect gas furnace in which their is no loss of energy. With a perfect furnace, one unit of energy (stored in the gas) is translated into one unit of heat output. So the efficiency is 100%.

Modern heat pumps can achieve efficiencies of over 300%, versus a gas furnace that has, as a theoretical limit, an efficiency of 100%.

Its true that heat pumps are usually powered by electrical grids that depend heavily on fossil fuels, and it is also true that many power plants that use fossil fuel to generate electricity are wasteful and inefficient. So one has to consider the net effect -- the waste at the power plant versus the high efficiency of the heat pump. If a power plant wastes 30% of the potential energy stored in fossil fuel, but a heat pump is 300% efficient, then one still comes out way ahead using a heat pump -- that is, using a heat pump is, overall, a more efficient option for heating our homes.

DP, however natural gas is much cheaper than electricity so you don't really save money by switching and are probably paying more.

[Anonymous]

You don’t even understand enough about thermodynamics to know what you don’t know.

And you think I’m the stupid one!

You went to an Ivy and have two MA’s, don’t you?

The confusing point for many people is the idea that an efficiency metric can exceed 100%. Intuitively, it seems like 100% should be the limit. In the case of a heat pump, that is not the case. The key point is that a heat pump isn't creating heat; rather, it is moving heat from outdoors to indoors. If we can use one unit of energy to move two units of heat into our home, then we have achieved 200% efficiency.

[Anonymous]

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:Heat pumps start to suck at places just slightly colder than DC. We’re on the margin of suitable winter climate for heat pumps. They work much better in places like the Carolinas and are outstanding in GA and FL.

If you have a ground loop or geothermal well for the heat pump, where the evaporator coil is guaranteed never to get below 50-55 degrees, heat pumps can work great in even cold climates (we’re not “cold” here in DC) But the typical DC area heat pump set up with an outdoor unit with an atmospheric coil is right on the edge of not working here.

Heat pumps have improved over recent years. There are "cold-climate heat pumps" that can operate efficiently well below at extreme low temperatures:

https://www.reddit.com/r/heatpumps/comments/146jg7k/cold_climate_heat_pump_owners_what_model_did_you/

Some people argue that heat pumps can't help with CO2 reduction because they are typically powered by an electrical grid that runs on fossil fuels. And they point out that a lot of the energy and heat goes up the smokestack of the power plant, which is wasteful. It is true that energy is lost at the site of power generation, but, even so, heat pumps can reduce CO2 emissions (relative to gas furnace heating) due to their high efficiency. Suppose that 30% of energy is wasted at the site of power generation. So only 70% of the original energy reaches the customers. But if customers are running heat pumps that are 200% efficient, the waste at the central power plant is more than offset by the efficiency of the heat pump. The result is that heat pumps can lead to significant emissions reductions relative to gas furnaces. And yes, good heat pumps can keep your house warm even in frigid weather.

They are "efficient" because they are not providing as much warmth, so that is not efficient because they are not providing the same warmth using less energy. That is like calling HE washer efficient because they use less water. Not they would actually be efficient if they worked the exact same way as the old machines by using less water but they don't. Taking away or providing inferior features to appliances just to create an appliance that uses less energy is not efficient. We are just getting a different kind of appliance.

Heat pumps provide just as much warmth as gas-fired heating appliances. In our home, we have electric heat pumps in the attic and basement, and radiators in the rest of the house. The attic (which is well-insulated and was a good 10 degrees colder than the rest of the house in the winters before we installed the heat pump) gets nice and toasty now when the heat pump is running. There's nothing inferior about heat pumps at all.

[Anonymous]

We have solar panels and switched to a heat pump hot water heater when our standard one was getting old. It does use less electricity (we tracked and saw our monthly electric usage go down). The bonus is that its in the basement and helps dehumidify -- so much so that we could stop using a basement dehumidifier.

Downside is that it takes longer to heat up when the hot water runs low, so if you have a house full of people who like long, hot showers every day around the same time, it may not be best for you or you may need to size up.

FYI: I live on Cape Cod.

If you live in a very cold climate (northern states, Canada?) and its located in an unheated basement, it may not work well for you as it will take longer to heat up.

Overall, we're happy with it.

[Anonymous]

DP, however natural gas is much cheaper than electricity so you don't really save money by switching and are probably paying more.

Dominion Energy currently charges about $1.5 per "therm" of natural gas.

1.0 therms = 29.3 kilowatt hours

So the price of natural gas can be expressed as 150 cents / 29.3 khw = 5.1 cents per KWH.

For residential electricity, the price is about 12 cents per KWH.

This means that if a heat pump is 12 / 5.1 = 235% efficient, then that would be about the break-even point.

I think most modern heat pumps have efficiencies that exceed 235%, which suggests that heating one's home with a heat pump could be cheaper than using natural gas.

Note that I've assume 100% efficiency for the natural gas furnace, when in fact the actual efficiency would be lower. So I've stacked the deck in favor of natural gas.

[Anonymous]

Anonymous wrote:

DP, however natural gas is much cheaper than electricity so you don't really save money by switching and are probably paying more.

Dominion Energy currently charges about $1.5 per "therm" of natural gas.

1.0 therms = 29.3 kilowatt hours

So the price of natural gas can be expressed as 150 cents / 29.3 khw = 5.1 cents per KWH.

For residential electricity, the price is about 12 cents per KWH.

This means that if a heat pump is 12 / 5.1 = 235% efficient, then that would be about the break-even point.

I think most modern heat pumps have efficiencies that exceed 235%, which suggests that heating one's home with a heat pump could be cheaper than using natural gas.

Note that I've assume 100% efficiency for the natural gas furnace, when in fact the actual efficiency would be lower. So I've stacked the deck in favor of natural gas.

We pay 21 cents total per kwh for electric in Maryland. Not sure what our natural gas costs are. Heat pumps don't always operate at max efficiency.

[Anonymous]

We pay 21 cents total per kwh for electric in Maryland. Not sure what our natural gas costs are. Heat pumps don't always operate at max efficiency.

On the website below the current price for MD is listed as 17 cents:

https://www.eia.gov/state/data.php?sid=MD

[Anonymous]

Anonymous wrote:

We pay 21 cents total per kwh for electric in Maryland. Not sure what our natural gas costs are. Heat pumps don't always operate at max efficiency.

On the website below the current price for MD is listed as 17 cents:

https://www.eia.gov/state/data.php?sid=MD

Tell that to my pepco bill lol

[Anonymous]

Anonymous wrote:

Anonymous wrote:

We pay 21 cents total per kwh for electric in Maryland. Not sure what our natural gas costs are. Heat pumps don't always operate at max efficiency.

On the website below the current price for MD is listed as 17 cents:

https://www.eia.gov/state/data.php?sid=MD

Tell that to my pepco bill lol

There might be a variety of charges on the bill above and beyond the hourly rate. To compare electricity vs gas unit prices, we need to treat those charges symmetrically, so that we don't inadvertently bias the results. Probably, 17 cents is indeed the unit rate for electricity if you strip away the various extra charges.

[Anonymous]

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:

We pay 21 cents total per kwh for electric in Maryland. Not sure what our natural gas costs are. Heat pumps don't always operate at max efficiency.

On the website below the current price for MD is listed as 17 cents:

https://www.eia.gov/state/data.php?sid=MD

Tell that to my pepco bill lol

There might be a variety of charges on the bill above and beyond the hourly rate. To compare electricity vs gas unit prices, we need to treat those charges symmetrically, so that we don't inadvertently bias the results. Probably, 17 cents is indeed the unit rate for electricity if you strip away the various extra charges.

All the "extra charges" except an $8 customer charge are based on the amount of electricity we used so yes, they are relevant for comparing costs. One of those for example is the Montgomery County energy tax which is a little over 1 cent per kwh. The rates also seem to change - there are summer rates and winter rates for example. If you take out the $8 customer charge, 20 cents is our average over the last 6 months.

The main reason we won't be getting a heat pump water heater is because it would cool our basement in the winter. We do have an air source heat pump as part of our HVAC system.

[Anonymous]

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:

We pay 21 cents total per kwh for electric in Maryland. Not sure what our natural gas costs are. Heat pumps don't always operate at max efficiency.

On the website below the current price for MD is listed as 17 cents:

https://www.eia.gov/state/data.php?sid=MD

Tell that to my pepco bill lol

There might be a variety of charges on the bill above and beyond the hourly rate. To compare electricity vs gas unit prices, we need to treat those charges symmetrically, so that we don't inadvertently bias the results. Probably, 17 cents is indeed the unit rate for electricity if you strip away the various extra charges.

All the "extra charges" except an $8 customer charge are based on the amount of electricity we used so yes, they are relevant for comparing costs. One of those for example is the Montgomery County energy tax which is a little over 1 cent per kwh. The rates also seem to change - there are summer rates and winter rates for example. If you take out the $8 customer charge, 20 cents is our average over the last 6 months.

The main reason we won't be getting a heat pump water heater is because it would cool our basement in the winter. We do have an air source heat pump as part of our HVAC system.

My point is that there are also extra charges on one's gas bill. So we shouldn't ignore the extra charges on a gas bill and include them on an electrical bill. We should use a symmetric approach so that the resulting unit costs are apples to apples.

[Anonymous]

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:

We pay 21 cents total per kwh for electric in Maryland. Not sure what our natural gas costs are. Heat pumps don't always operate at max efficiency.

On the website below the current price for MD is listed as 17 cents:

https://www.eia.gov/state/data.php?sid=MD

Tell that to my pepco bill lol

There might be a variety of charges on the bill above and beyond the hourly rate. To compare electricity vs gas unit prices, we need to treat those charges symmetrically, so that we don't inadvertently bias the results. Probably, 17 cents is indeed the unit rate for electricity if you strip away the various extra charges.

All the "extra charges" except an $8 customer charge are based on the amount of electricity we used so yes, they are relevant for comparing costs. One of those for example is the Montgomery County energy tax which is a little over 1 cent per kwh. The rates also seem to change - there are summer rates and winter rates for example. If you take out the $8 customer charge, 20 cents is our average over the last 6 months.

The main reason we won't be getting a heat pump water heater is because it would cool our basement in the winter. We do have an air source heat pump as part of our HVAC system.

My point is that there are also extra charges on one's gas bill. So we shouldn't ignore the extra charges on a gas bill and include them on an electrical bill. We should use a symmetric approach so that the resulting unit costs are apples to apples.

It doesn't seem to me that we should ignore any of the "extra charges" that are based on the amount of electricity/gas used. Ignoring them means you're not accounting for the full costs of either and therefore you aren't doing a useful comparison at all.