Zero Carbon ® Blog

• For how many years does the manufacturer guarantee the panel-power output?
  • Can I make a claim using only the data on the monitoring app, i.e., without having to get the panel independently tested?
• For how many years does the installer guarantee the array-energy production per year (in kWh / year)
• What is the cost per kWh of the electricity guaranteed to be produced over the warranty period of the panels?
  • Cost should be after the federal and state subsidies
  • 5c/kWh is very good but even 10c/kWh is still a 55% cut and it is fixed
• What inflation rate did you use in the financial forecasts? If it is more than 3% ask them to redo the calculations. I used 0%.
• Did you allow for taxes on the SMART subsidy? SMART is taxable income.
• For how many years is my roof guaranteed against leaks?

• Does the Manual J show my house will be at 70℉ when it is 5℉ outside?
• If you have no ductwork ask, “Can you quote this 1) with ductless units and 2) with ductwork in the basement for the ground floor plus ductwork in the attic for the upper floor?” Also ask for a quote for radiant-floor heating, this was surprisingly cheap in our rental house (but this will not do AC for you). Make sure the attic work is well insulated or you will get ice dams.
• I have not tried AC using cold water in radiators. Some manufacturers say it can be done, but I doubt it without getting pools of condensation on the carpets.
• For forced-hot water radiators, make sure the Manual J calculation is done with a water temperature of 110℉ not 140℉
• How much money will I save on heating if I am paying 23c/kWh for electricity? How much if I am paying solar rates (5-8c/kWh)
• How much for a heat-pump hot water tank? How much will it save me per year?
• How long is the warranty, does it cover parts and labor?

I had a blower-door test done at my house and the result was 4.6 ACH50. ACH50 is a common standard for air infiltration and stands for Air Changes per Hour at 50 Pascals. Pascals are, like pounds per square inch, a measure of air pressure. 50 Pascals is about the pressure caused by a 20 mph wind. 4.5 ACH50 is equivalent to 1,035 CFM50 (cubic feet per minute at 50 Pascals). This means that the natural air exchange on my house (i.e., at 0 Pascals) is about 0.23 ACH (sometimes called ACH0). This means that the entire air volume of my house is replaced every four hours due to drafts around doors, windows, walls and chimneys. The natural air infiltration rate in my house is 238 CFM0.

This proves what I have long suspected, which is that it is absolutely not necessary to seal your house to the level of air tightness required by the Passive House (PassivHaus) Institute in order to cut your carbon emissions to zero.

The Passive House standard is often held up as the ideal standard for low-energy consumption houses. But I have never seen any financial analysis accompanying this conclusion. This data proves that you can cut both your carbon emissions and bills to zero (and I am making a 15% return on investment too) without the expense of creating a very tight building envelope.

Very few builders can build to a the Passive House standard of 0.6ACH50 and doing so often requires many hours of skilled labor plus the addition of an ERV (energy recovery ventilator) which, alone, can add $5,000 to the cost of the house. I know one contractor who recently did the air sealing on a Passive House project. He gets paid about 3x what a typical laborer on a construction site gets paid. Labor dollars add up real fast at those rates! Hence, the Passive House standard for air infiltration can only be achieved at considerable expense – an investment that will never earn a return.

Much like geothermal, solar hot-water panels and thickening your walls with insulation, a super-tight building envelope makes energy sense but does not make financial sense.

The highly-acclaimed book “Zero Carbon Home” by “Green Guru” David Green.

How we cut our home’s carbon emissions to zero.

How we cut our home’s utility bills to zero.

Solar panels on the roof of our house.

Our heat pumps.

Fiberglass and ISO board insulation on the roof of our house.

Triple-pane windows in our living room.

Useful links:

The Easiest Way to Calculate Your Home’s Carbon Footprint:https://greenzerocarbonhome.com/2018/07/what-is-the-carbon-footprint-of-natural-gas-heating-oil-propane-and-coal/

Written Answers to over 300 Questions about Zero Carbon Homes:https://greenzerocarbonhome.com/webinar-questions-and-answers/

Finance and Net Zero Energy Terms explained: https://greenzerocarbonhome.com/energy-and-finance-terms-explained/

Zero Carbon Home Website Home Page:                https://greenzerocarbonhome.com

Database of State Incentive for Renewable Energy (DSIRE) https://www.dsireusa.org

Published articles on Zero Carbon and Net Zero Energy:

Department of Energy, EnergySage, “Zero Carbon Home”https://www.energysage.com/project/7230/zero-carbon-home/

Zero Energy Project, “My Zero Energy Retrofit beats my 401k”https://zeroenergyproject.org/2018/09/23/my-zero-energy-retrofit-beats-my-401k/

Cool Effect, “Mr. Green’s Zero Carbon Home”         https://www.cooleffect.org/content/news/green-testimonial?utm_source=Cool_Effect_CRM&utm_medium=EMAIL&utm_campaign=20180814Engage_Testimonial&utm_content=green_CTA

Green Energy Times, “My Zero Energy Pool is a Great Investment!” http://www.greenenergytimes.org/wp-content/uploads/2019/05/GET_May-2019.pdf

Published Presentations on Zero Carbon Zero Bills:

To see a 3-minute interview of David Green by NBC Boston anchor Joy Lim Nakrin, please click here:TV interview

To see a 20-minute TED talk at Harvard Business School, please click here: TED Talk

You can watch a recorded version of my Zero Carbon, Zero Bills webinar here: Zero Carbon, Zero Bills Webinar

Reviews of my “Zero Carbon, Zero Bills” webinar included, “Minute-for-minute, point-for-point, one of the best webinars I have ever attended. Thank you!”, “David is a true inspiration and asset to our community. He inspired us to get solar panels and a new front door”, “This is like a masterclass in ZeroCarbon”, “Thank you soooo much for the sensible advice and your fabulous resources!!” and, although this next comment is a little flowery, it does express the sentiment of many other comments, “Your contribution to Greening America is great and will probably help our survival as a species a bit longer than expected”.

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!

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:

https://greenzerocarbonhome.com/2021/04/q-what-would-be-the-difference-between-passive-house-and-green-zero-carbon-houses/

and this:

https://greenzerocarbonhome.com/2020/07/passive-house-passivhaus-vs-green-zero-carbon-houses/

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. 

A: Plain glass double glazing is about R2. Low-E double glazing is about R3. Plain glass triple-glazing is about R3.3 and low-E triple glazing is about R4-R6.

A: Correct. Academic studies have found that ducted heat pumps are about 20% more efficient that ductless installations. Personally, I prefer the air circulation from a ducted system and I prefer the look of traditional floor registers to having units on the walls or floors.

A: The sill (the area above the concrete foundation and the floor joists) is a major source of drafts and heat loss in a house so sealing gaps and insulating it makes a lot of sense. However, don’t stop there. The ceiling of the basement losses a lot of heat from the house to the outside and insulating that will greatly reduce your heating bills and carbon emissions.

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.