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Lower-than-expected efficiency of heat pumps.

People, including me, other people I know, and academic researchers, have reported lower-than-expected efficiencies on heat pumps. The efficiency of heat pumps is measured using a confusing number of terms including COP, or coefficient of performance, HSPF or heating season performance factor and SEER or seasonal energy efficiency ratio. Let’s explain each one of them:

COP is the heat energy delivery by the heat pump divided by the electrical energy used by the heat pump. If the heat pump delivers say 4kWh of heat by using 1kWh of electricity, then the COP is 4. People are often familiar with using kWh to measure electricity because that is how their utility company delivers and bills them for electricity. People are less familiar with kWh to measure heat but both heat and electricity are forms of energy and so can be measured using any unit of energy such as kWhs, BTUs, therms or joules. Using these different terms for measuring energy is similar to using centigrade and Fahrenheit to measure temperature or miles and kilometers to measure distance, they are just different units for measuring the same thing. People are more familiar with using BTUs to measure heat because that is how the energy in natural gas is sometimes measured (though it is also measured in therms, one therm is equal to 100,000 BTUs). One kWh of energy is equal to 3,412 BTUs of energy. As long as you use the same units, (either kWh or BTUs) for the electricity used by the heat pump and the heat output of then you will get the right COP. COPs for heat pumps are usually between 2.0 and 4.0. The higher the COP the more efficient and the more money it will save you.

HSPF is similar to COP, except it measures the heat output in BTUs and the electricity used in watt-hours. There are 1,000 watt-hours (Wh) per kilowatt-hour (kWh). Hence, in the above example with 4kWh of heat output for every 1kWh of electricity used, there are 4 x 3,412 BTUs = 13,648 BTUs of heat delivered for every 1,000 Wh of electricity used. So the HSPF is 13,648/1,000 = 13.6. Mathematicians will quickly realize that HSPF can be converted to COP by dividing the HSPF by 3.412. The higher the HSPF the more efficient and the more money it will save you.

SEER measures the efficiency of the heat pump in cooling or air-conditioning mode. Now the output is the amount of energy removed from the room (which cools it down) in BTUs and the input is the amount of electricity used by the heat pump measured (just like HSPF) in watt-hours. 

I believe that consumers would be far less confused by these terms if HSPF was simply called “heating efficiency” and SEER was simply called “cooling efficiency”.

For SEER, HSPF and COP higher is better. Manufacturers are required to state the SEER and HSPF on the units in the energy label which is a bit like EPA’s the miles-per-gallon sticker you see on the windows on new cars.

However, the SEER and HSPF that are required to be stated on the equipment’s label are measured under continuous usage under ideal laboratory conditions. Under real-world conditions, HSPFs can be far lower than those stated on the equipment. It is as though the EPA measured the mpg on cars when they were all going downhill with a following wind.

In the real world, heat pumps often work in conjunction with a fossil-fuel heating systems and have to heat the house in short bursts in the spring and fall whereas the tests are done over long time periods of continuous operation, in other words, the tests simulate winter or summer operating conditions but not the conditions in which the heat pumps operate in spring and fall. Both the integration with the fossil-fuel furnace and the operation in spring and fall reduce the efficiency of the heat pump. Hence, the real-world, year-round COPs, HSPFs and SEERs are often lower, sometimes much lower, than those advertised by the manufacturers.

This comment is in reply to data shared by someone who had installed Mitsubishi Hyper Heat heat pumps and, like me, was not getting the performance he had expected.

Hi, XXX,  

This is great data and very consistent with both my own experience, that of academic researchers and the members of the Heat Smart Alliance. If you did a year-round average COP calculation (weighted by the energy used) then I would guess you would come out to a COP of between 2.5 and 3.0. When I do consulting work for other people, I use 2.5 in my calculations for ductless systems and 3.0 for ducted systems. Both numbers are far below the claimed COPs of manufactures which are in the 4-5 range. COPs, HSPFs and SEERs are all good ways to compare heat pumps from different manufacturers, but they are highly misleading if you use them to predict your energy-bill savings.

I agree with you that the short cycling reduces COP. Short cycling is not only a problem in the shoulder seasons but is also a problem if there is a back-up furnace that comes on with an outdoor temp of say 40°F. This mean that both the furnace and the heat pumps are short cycling which kills the efficiency of both. From the heat pump’s point of view, it does not care if the outdoor temperature is mild or if the back-up furnace is coming on to help, it leads to short cycling either way.

I believe that this effect is why people who have only a heat pump (and no back-up furnace) in cold locations (and have enough insulation to enable the heat pump to maintain 70°F year round) actually get better year-round COP than those with back up furnaces and / or milder climates. I have had such people on my webinars, and they cannot believe that other people (like you and I) aren’t overjoyed with our heat pumps. 

These short-cycling issues lead to lower COPs in practice than the manufacturers advertise. Academic research shows that the year-round actual COP of Mitsubishi Hyper Heat units in MA is about 2.5. If you are paying 23c/kWh (current Eversource or National Grid rates) then the 2.5 COP translates to a cost per kWh of heat in the house of 9.2 cents. Oil heat costs about 8c/kWh of heat in the house. This is paying $2.75 a gallon with an 85% efficient furnace. This is a 15% increase in your heating bill. If you are heating with natural gas, which costs about 5.2c per kWh of heat in the house (paying $1.45 per therm with a 95% efficient furnace), then your heating bill will almost double. This is why I disagree with the common conventional wisdom in MA that heat pumps are for everyone. The only way (in MA) to make heat pumps pay is to install solar at the same time (in which case, if the solar generates electricity at 8c/kWh of electricity – which is easy to do) then with a COP of 2.5 the cost of heat in the house is 3.2 cents or a 40% cut in your heating bill if you are heating with natural gas and a 60% cut if you are heating with heating oil. This cost saving can pay back the cost of the new heat pump in around 10 years for a return on investment of about 7% per year. It is only in these far more limited circumstances that I recommend heat pumps to people. My advice often comes as a nasty shock to the proponents of “electrify everything”. This is why I think it is essential to take “whole house” approach that looks at the financial returns on these investments. The momentum to cut carbon will be stopped dead in its tracks as soon as stories of doubled heating bills with heat pumps start to spread.

So, if you are suffering from high bills after installing a mini-split heat pump, here is my advice on how to improve the performance:

  1. Do not have upstairs heads and downstairs heads on the same outdoor unit. The upstairs heads will sometimes be trying to cool while the downstairs heads are trying to heat. This gets very confusing for a heat pump with very little brain.
  2. If the heat pumps have enough heat output to heat the house at a very low temperature, like 0°F, (this should have been determined in a Manual J calculation before the system was installed) then it will likely be more efficient to set the temperature at which the back-up furnace comes on to a very low temperature like 10°F rather than the common practice of setting it to 40°F which leads to short cycling (and inefficiency) in both the heat pump and the furnace. 
  3. If it gets too cold in the winter with the heat pump thermostat set at say 70°F, turn up the thermostat rather than turn on the furnace or turn on an electric fan heater. Keep turning it up until your feel comfortable. Only when this fails to keep the house warm, turn on the furnace.

And let me know if you have found any tips for getting better performance out of heat pumps!

Q: about Passive House design?

A: I have a lot of concerns about Passive House designs including: the cost, mold, the use of big south facing windows for passive gain, the lack of a standard for renovations and the lack of adjusting the standards for either where the house is located or how big it is. Please see this for more details:

and this:

Q: Already have a 6.3 KW photovoltaic array from Sunpower.  Any recommendations for adding batteries?

A: The economics of batteries is highly dependent on the local subsidies. In MA with Eversource as your electricity supply company the subsidies are very generous. I have just ordered a Sonnen battery to go with an existing array of SunPower panels. I have also just ordered a Generac battery to go with a new array of Solaria panels. Generac is not good as a retrofit to an existing array because you need to use a Generac inverter with a Generac battery. I have not bought a Tesla battery, despite it being the cheapest per kWh of storage, because Tesla is not currently able to deliver batteries. 

Q: Would adding acrylic panels to your existing windows have similar 3

pane performance and mucb less life cycle ghgs?

A: Adding a single pane of glass or plastic (these are called window inserts) adds a similar amount of insulation benefit (R value) to going from a double-glazed window to a triple-glazed window. You will not save enough money on the heating bill to payback the full cost of taking out existing windows and replacing them with triple-glazed windows in under several decades. This is why I recommend installing triple-glazed windows only when you need to replace your windows for other reasons like they are rotting or falling apart as ours were. In this case the additional cost of triple-glazed over the cost of double-glazed is a good investment. If your windows are in good shape I suggest adding windows inserts. 

Should I insulate the floor of the attic of the sloped sides of the roof? I also have dampness issues. The house is under insulated and is also drafty.

A: It sounds like you have two different problems: insulation and moisture. You need to deal with both. My fundamental question is where is the moisture coming from? The usual culprit is the basement. If the basement is damp (in summer when the humidity is high dampness in the basement is usually caused by warm moist air condensing on the cooler surfaces (especially cold-water pipes) and in the winter it is caused by water wicking up from the ground. This, in turn is often caused by leaking guttering and downspouts that dump the water right by the basement wall. Leaking pipes cause dampness year round. 
First, fix the source of the dampness. So get downspout extenders and clean out the clogged leaves in the gutters and maybe fit perforated plates on top of the gutter, and fill any cracks in the basement wall with exterior grade spray foam. I did this on my rental property and it greatly helped. Then I would seriously consider replacing your hot water tank (I am assuming it is heated from your furnace) with a heat pump hot-water tank. A Rheem 55 gallon hybrid heat pump hot water tank at Home Depot costs about $1200, you can get a $600 rebate on this. It will take an hour or two with an electrician and plumber to install it. These heat your water at less than the cost of heating oil and about the same as heating with natural gas. However they also dehumidify your basement and that is the big plus. Ideally you would also insulate the ceiling of your basement (just push fiberglass in between the rafters or ask MassSaver to do it for you) to prevent the heat from your house just leaking down into the basement and becoming the source of heat for your heat pump hot water tank. This is a good fix for you but if you also get solar panels you would be saving a ton of money on heating your hot water, even if you are on natural gas today. 
With the moisture dealt with, I think it would be fine to insulate the attic  either on the floor or the sloped sides of the roof. Which you do depends mostly on whether you use the attic as storage space. If you do, the sloped side is better. If ever you intend in the future to install a heat pump for heating the house with an air-handler unit in the attic you MUST insulate the sloped part of the roof or you will get icicles and ice dams in winter. You could either wait a season to see if the moisture fixes work or insulate the slopes with fiberglass or Rockwool because they are permeable to air and so will allow a small amount of condensation to evaporate. The best solution is to add a one-way vapor permeable membrane (but 100% air tight) on the inside of the rafters/insulation such as Intello which you can get from Building 475 in Connecticut. You need a pro to install this and I have consulted with Dolphin but I have not hired them yet because I need to deal with the water issues in my basement first! This membrane prevents moisture traveling from the loft to the roof surface but still allows any moisture from behind the membrane to evaporate to the inside. This insulation-plus-Intello-approach is about the gold standard in roof design and I am about to use it on my other rental property. This is going to be more expensive than just blowing in cellulose to the floor of the attic, but it will still save you loads of money on the bills and give you a completely air tight roof and get rid of the condensation problems in the attic. However, still fix the source of moisture first, because that moisture is causing mold elsewhere in the house, condensation on windows and probably even making the towels stay damp on the rack too.

Net Metering and the Cost of Heat Pumps and Solar Panels in New Hampshire.

Q1. Sustainability of Net Metering

I asked a question about the longevity of the net metering program and Lori framed it as somewhat of a political question. I was trying to ask a question more about energy markets and energy storage. If enough people get solar, the supply of solar electricity at the time of production may exceed the demand. As far as I know, there is no cost effective way to store this excess energy on a large scale. Without storage, the excess energy becomes worthless and as you said, the financials of the net metering program will fall apart. 

Here’s a link to one article: https://pv-magazine-usa.com/2016/12/08/report-finds-net-metering-is-not-sustainable-over-the-long-term/
I suppose my questions would be: do you know of any data that suggest a time line for the decrease in value of solar energy? I guess most importantly would be whether the net metering program will change before the pay back period of the system (say 10 years for talking purposes). Or maybe there’s data that say even if there are solar panels on every roof, the production will still not exceed the daytime energy demand, in which case the net metering program can be considered more or less safe. 

A1: I think there are two questions here: the first is for how long is net metering (as currently practiced in MA) sustainable and the second is can excess solar power be cost-effectively stored at grid scale?


The economics of net metering are very unfavorable to the utility company. By law they are forced to buy solar electricity from homeowners at the full retail price (in MA about 23c/kWh) when they can buy it from a power station at about 8c/kWh. This is a subsidy from all ratepayers to those ratepayers who have solar panels. This can only work as long as the amount of solar power subject to net metering is small compared to the total amount of electricity being consumed. This is true today but is becoming less so as more solar power is installed on rooftops and more commercial-scale solar panel farms are being constructed to supply community-sourced solar power as an option to ratepayers. Eventually the utilities will seek to negotiate less favorable terms for net metering. This is a negotiation with the government and hence things will move slowly. I do not expect net-metering to go away, probably ever, but I do expect to see its generosity to the homeowner decrease over time. The net-metering credit rate is already cut back to 60% of the retail value for arrays over 10kW in size. It also already excludes some taxes and fees and  it excludes the monthly fixed charge of $7 per month. In other states net metering is already far less generous than it is in MA. Of all the subsides for solar panels in MA, the federal tax credit, the state SMART subsidy and the net-metering subsidy, net metering is the biggest of all three. 
On the second question about can excess solar electricity be cost-effectively stored at grid scale the answer is yes. Both California and Australia have installed very large batteries to store excess electricity. In MA, Eversource (our utility company) is doing something similar by paying homeowners to have access to their home-based back-up batteries during times of peak demand on the grid. The program is called Connected Solutions and it seems to be quite generous. I have recently placed orders for two new solar panel arrays each with batteries to replace broken propane back-up generators. When a battery is installed with fairly large solar panel arrays (over about 10kW) the combination now pays for itself in seven or eight years. 

Q2. Heat Pump Efficiency


I asked a question about estimating the required electricity to run heat pumps and your answer was to calculate the heat energy contained in the oil I burned this winter (in kWh) and divide by 2.5. This makes sense to me since a 2.5 COP corresponds to a heat pump HSPF of 8.5, which I believe is fairly standard. I’m a little confused by the section of your book where you talk about heat pumps having 400% efficiency, or COP=4. Is this a theoretical value that maybe does not account for the electricity required to run the fans? Or is this based on a very efficient heat pump with HSPF=13.6?


A: In the book I do refer to heat pumps being 400% or 4x as efficient as a furnace. This is true for a heat-pump hot-water heater and a heat pump for heating a swimming pool. When I wrote the book I thought it was also true for heat pumps for heating the air in a house. However, since writing the book, research has been published by well-regarded scientists that shows that the year-round average COP in New England is about 2.5. In the webinar I now use 2.5, not 4.0. When I measured the real, year-round COP on my two houses the one with the Bosch heat pumps has a COP of about 3.0 and the one with the Mitsubishi heat pumps has a COP of about 2.5. This is very similar to what was found by the academic researchers i.e., that ducted systems were more efficient than ductless systems and that Bosch was more efficient that Mitsubishi. These numbers include the electricity to run the fans as well as the compressor.

Q: Along the same lines, you talk about how it only makes sense to heat using heat pumps if you have cheap solar electricity. However, if utility electricity in MA is 23c/kWh, doesn’t that mean you’re making heat with heat pumps for 23/2.5=9.2c/kWh, making it only slightly more expensive than heating oil (6-7c/kWh)? 

I agree with you that the cost of heating the house is 23c/2.5 = 9.2c/kWh of heat in the house. Heating with heating oil (which contains about 40kWh of heat per gallon) at $2.59 a gallon (what I am currently paying) with an 85% efficient furnace (better than my dinosaur furnace) means that heat in the house costs about 8c/kWh. So paying 9.2c/kWh of heat in the house by using heat pump on utility electricity is about 15% more expensive than heating with oil. If you can achieve a COP of 3.0 (as I do with my Bosch ducted heat pumps) then the cost of heating the house is almost exactly the same as heating with heating oil. For me it was actually a saving because my dinosaur furnace is only about 75% efficient. However, even if the running cost is the same, you have to buy a new heat pump whereas you already have an existing furnace. This is why I recommend you install heat pumps when your AC units fail not when your furnace fails. Replacing a broken AC unit with a heat pump is about 40% more expensive than replacing it with a new AC unit. But now you get heating and cooling. However, leave the old furnace in place so you have a back-up heating system should the heat pump not be able to heat the house in very cold weather and so you have a heating system during grid outages. Note that you will still need either a back-up generator or a battery so that you can run the pumps and fans on electricity during the grid outage. If you don’t do this you will have a hot furnace and a cold house. 

3. Solar Financial Feasibility
I am having some trouble getting the financials of solar to make sense. I am using the numbers you  provided in your book as a general measuring stick (ie the array should produce at under 10c/kWh, and you have seen quotes down around 5c/kWh). I am in NH, and it seems one of the huge differences between MA and NH is in how SRECs are handled. I spoke with an installer today who said the current value of an SREC is NH is $5. Over 25 years, this adds up to about $1200, which stands in stark contrast to the $29000 in your book. (I have read a few articles about how the program is broken in NH because utilities are allowed to collect unclaimed SRECs for free, which depresses their value). Without this benefit, it looks impossible to me to get the solar electricity cost down to 1/3 of the utility cost, which is around what you quoted in the book. 

The value of an SREC in MA is currently about $250/MWh so it is very different to NH!

Q: Here are some numbers, which are roughly accurate. I just received a quote for a 11.1kW system for $27000 (after the federal income tax credit). Production over 25 years is around 238,000 kWh, which works out to be 11c/kWh. 

Current utility rates from NHEC are around 15c/kWh and the net meter rate is around 10c/kWh. Assuming the system produces enough power that there are always net meter credits to work with, the cost of power when the system is overproducing is just the system cost (11c/kWh) and the cost of power in the winter when the system is underproducing is the system cost plus the difference between the utility rate and the net meter rate (11+(15-10) = 16c/kWh). When using heat pumps, a large % of annual power consumption happens in the winter. As far as I understand it, this power actually costs 1c/kWh more than the standard utility rate. But maybe this really doesn’t matter since, as discussed above, if I’m using a heat pump with COP=2.5, then the cost of heat is 16/2.5=6.4c/kWh, which is basically the same as oil. One of the points you make in the book is that the cost of solar electricity is fixed. However, if you are using a lot of electricity through the net metering process, you still have to buy this electricity at the utility rate. 

A: NH has much lower utility rates than MA. This is in part driven by all the subsidy schemes like full-retail-price net metering, SMART and Connected Solutions which drive up the price of electricity for all the MA ratepayers who are not taking advantage of the subsidies. So you are getting a net-metering credit of 10c/kWh on the excess solar electricity you produce in summer but are paying 15c/kWh in winter when the heat pumps are used the most. This suggests a slightly different version of the Fab Four recipe for you. I would invest much more in lowering the energy use of your house such as great insulation, air-sealing and upgrading you windows. Then, when your AC units fail, replace them with heat pumps and add solar panels if it is cost-effective, which it may not be. With such cheap electricity, you may be better off financially using a heat pump (after insulation and air sealing) and replacing your gasoline vehicles with EVs rather than getting solar panels.

Q: My conclusion here is that without significant SREC value in NH, there is no way for me to push the cost of solar low enough to be a great investment when compared to other investment opportunities (granted, I haven’t done a detailed financial evaluation yet). On the plus side, I don’t think I will lose money here, and it obviously still makes tremendous sense in terms of cutting carbon. Would you agree with this conclusion? Or would you consider cheaper solar panels (ie not LG, Sunpower, or Panasonic) here to try to improve the economics (I have received a quote using REC panels that is closer to 8c/kWh)?

A: REC makes a great panel – I almost bought it until my installer decided to quit installing batteries, since I wanted a battery I had to get a new installer. The new installer gave me a better deal on Solaria panels at about 6c/kWh including a battery. At 8c/kWh and COP 3.0 you would be heating the house at 2.7c/kWh of heat in the house which is 1/3 of the cost of heating with heating oil and about half the cost of heating with natural gas. This is roughly the situation at my house. I heat with a heat pump, cook on induction, run a heat-pump hot-water tank and drive my Tesla, all charged with the cheap electricity from my solar panels. All with a zero carbon footprint.

Designing the pool to be efficient from the start

Q: We currently have a Hayward H250 natural gas heater on our 16’ X 28’ In ground pool and have thought about replacing it with a heat pump. Several years ago I installed a 2 speed pump on the pool and we now save a lot of kilowatts by using the lower speed. However when we want to heat the water we must run the pump on high speed to meet the gas heaters minimum pressure requirement. It’s not really much of a problem because we only use the pool from mid May till mid September and with the solar cover on it holds the heat in pretty good unless we have some unusually cool weather. We also have what I call an indirect solar heating system on the pool that adds a little extra heat as well so we don’t have to run the gas heater very much once we get the water up to temperature. As such I was wondering if the heat pump pool water heater that you use requires you to run your pool pump on high speed to keep your water warm? 

My indirect solar pool heating system consists of 400’ of polybutylene tubing that I put in the concrete that surrounds the pool. I feed it with pool water coming off a tee fitting that I installed after the filter but before the heater. Of course I have a ball valve installed so that I can shut it off at night. Once the water exits the 400’ of tubing that is buried in the concrete it just gravity flows back into the pool. I can send you som photos of it if you’re interested in learning more about it. Another benefit of using this indirect solar system or maybe we should call it a reverse radiant system is that it keeps the concrete a little cooler on those very hot sunny days. My grandkids really appreciate that.

A: This sounds like a well thought out installation – I like the passive pool heater which heats the water and cools the deck, a very nice two-for! I think this would work on most pool decks, but obviously you need to do this from the start. If I were designing a pool from the start I would also connect the pool heat-pump heater to the house AC system, but that is a different subject, you can read more about it here if you like: https://greenzerocarbonhome.com/2020/07/free-ac-in-my-house-from-heat-pump-heaters-for-swimming-pools-and-heat-pump-hot-water-tanks/
Our pool heat pump is made by AquaCal and it has proven to be both reliable and efficient. When we installed it, we left the old propane heater in place but we no longer use it at all. If it did not cost me money to take it out, I would have taken it out by now. 
The heat-pump pool heater does have a minimum water-flow requirement to work and on our pool and it is at about 1,400 rpm on the variable-speed pump motor for the water-circulating pump. I usually set the water pump at 2,000 rpm so that it circulates the entire pool volume once per 24 hours, which is necessary to keep the pool water filtered. So it is usually enough flow to allow the heater to come on. However, if the skimmers and filter are clogged, the heat pump will shut down because the water flow is insufficient even at 2,000 rpm.  
The pool-water circulating pump now runs 24/7 at 2,000 rpm compared to the old fixed-speed motor that ran at 3,450 rpm about 12 hours a day. This alone is saving me about 75% of the electricity used to run the pool. If you already have a 2-speed motor it is probably not going to save you a lot more money to go with a variable-speed motor. But when you need to replace the pump, I would recommend a variable-speed one as then you can set it to the lowest flow rate needed to circulate the entire pool volume in 24 hours. There is a lot more information on how we got to a zero-carbon footprint on our pool in the book Zero Carbon Pool which you can order here: https://greenzerocarbonhome.com/shop/