Heat pumps and defrost mode in winter. Heat pumps and a back-up furnace.

These first four answers are provided by Bob Zogg the head of the Heat Smart Alliance:

Q1. Do all ASHPs’ have a defrost mode?  

A1. Yes, unless installed in a climate where it doesn’t get cold enough for the outdoor coil to frost.

Q2. Is it conventional to use the auxiliary heater to defrost the HP, or is it most common to simply reverse the flow using the AC mode to draw heat from the house to defrost the outdoor HP?  

A2. To the best of my knowledge, all ASHPs reverse cycle to perform defrost.  (I assume that’s the easiest and cheapest way to do it.)  Central, ducted heat pumps typically operate the supplemental heating system (furnace or electric resistance coils) during defrost to prevent cold blows.  Ductless heat pumps do not have a supplemental heat source, so they rely on the thermal mass of the indoor units to provide heat during defrost.  (I’ve never figured out how ductless systems get away with this, while it seems that central, ducted cannot.)

Q3. Is the method of defrost left up the installer, the smart thermostat, or the OEM equipment manufacturer?  

A3. In general, it’s probably a combination of all three.  Some installers may leave default settings as they are, and other installers may tweak the defaults.  For my heat pump (central, ducted with supplemental resistance coils), the defrost is based on compressor runtime (adjustable between 30 and 120 minutes), but it does not attempt to operate if the outdoor temperature is sufficiently high that frost won’t form.  The supplemental resistance heat runs full blast during defrost, and this is NOT adjustable through the t-stat.  I didn’t deal with this serious energy waste for many years.  The installer set the defrost time for 30 minutes (or that was the default and they didn’t change it.)  I finally changed it to 120 minutes.  That leads to greater frost build-up on the coil and sometimes longer defrost periods (perhaps 10 to 12 minutes instead or 5 – 6 minutes), but I think lower energy use overall.  

I then opened up my air-handling unit and disconnected the resistance elements, but that led to cold blows (the blower runs at fairly high speed during defrost—don’t ask me why).  So, I convinced an electrician to put switches on each of the 5 kW resistance elements (2 elements totaling 10 kW).  Normally, I leave one 5 kW element on, and the other off.  We don’t have any cold blows.  If we travel, I switch both elements on in case the heat pump fails while we are gone.

Q4. Do you see an advantage to one form of defrost over another?  

A4. If cold blows can be avoided, there’s probably an energy advantage of using no supplemental heat during defrost.  Ideally, the controls would be smart enough to use just enough supplemental heat during defrost to avoid a comfort problem, but no more than that.  This should be easy to do with electric resistance, but, it doesn’t change the rated HSPF, so I guess manufacturers just don’t bother.

I raise these questions because my Bosch/Ecobee system uses the auxiliary heat (oil furnace) to defrost the HP. The furnace comes on for a short cycle (~5 minutes) in defrost mode which can occur multiple times per day. On average the defrost mode uses 4-5 gallons of oil in the winter months. I mentioned this to David Green (Zero Carbon Home) who also has the same 5-ton Bosch ASHP as me but his unit uses indoor air to defrost not his back-up oil furnace, so he says he doesn’t use any oil for defrost.  

You can reach the Heat Smart Alliance’s website here: https://heatsmartalliance.org

Q5. And this is my answer to the last question about how to get the highest efficiency from a heat pump when installed with either back-up electric resistance heating strips or with a back-up fossil fuel furnace.

A5. On the questions: the algorithms in the thermostats/brains of the heat pump are really not very good and, if there is a back-up furnace, often lead to short cycling of both the HP and the furnace in the spring and fall which kills the efficiency of both. So, I do not operate mine this way. I switch off the oil-burner circuit at the electrical breaker panel and leave it off almost all year round. This forces the heat pump to operate on long cycles when it is more efficient. It is almost like the heat pump is my only heating source. Usually, about mid December each year, I come down in the morning and the house is at 65F with the thermostat at 70F. This happens when it is about 20F out. I turn the thermostat up to 75F or even 78F and this keeps the house at 70F until the outdoor temp is about 10F. Below that I turn on the oil-burner breaker and the furnace comes on. I run it for about an hour or two to get the temp up to 70F then turn it off again. This works unless we have several days of arctic cold (say minus 10F, like this February) and for times like this I leave the furnace on. This strategy has reduced my oil use to between 10-30 gallons a year. Before my Fab Four retrofit I was using 3,000 gallons a year and when I let the thermostats run both the hp and furnace it was about 300 gallons a year. Without the furnace, my house would probably get to 60F in arctic-cold weather. If I closed off some rooms and vents I might be able to keep the kitchen at 65F. In this case a small electric fan heater might actually be a better way to do it rather than the furnace but I have the furnace so that works for me. 

My concern with installed electric back up heating elements (like Bob has) it just the cost. They are very expensive to run. And I doubt that this electricity is included in the HSPF calculation, hence the economics of the heat pump with back up electric heating elements is likely not as good as advertised. 

Our Bosch hp does occasionally go into cooling mode in winter (I have noticed it only once in 7 years) and this is how it defrosts itself. It is also how it dehumidifies the air in winter if it needs doing. I set the humidity at 40% year round. It does not use supplemental heat from either the furnace (because it is off) or back up electric-resistance heating elements (we don’t have any). 

My experience with a Tesla Model S and fast charging / supercharging stations

In my experience with a Tesla, what is most important about EVs is not the car itself (all of them are good, as this presentation shows) but the availability of level 3 (fast charging) stations. Level 2 is perfect for home-based overnight charging or charging at your office or work site. But I add about 40 miles of range for every hour of level 2 charging. Hence it is not practical for driving long distances – it would take 5 hours to add 200 miles of range. On a level 3 charger (often called fast chargers, Tesla calls them superchargers) I can add 100 miles of range in 15 mins (just time for a coffee break) and 400 miles in an hour (maybe a lunch stop?). This makes long distance travel a pleasure. I regularly drive from Boston to the New York City / New Jersey area (250 miles one way). The door to door time is the same as flying. Flying costs $250 round trip.  Driving costs me $8 round trip if I charge from my solar panels and $29 if I am paying Eversource. Did I mention the zero carbon footprint?

Today, only Tesla has a large network of fast charging stations. EVgo, Chargepoint and others are building their networks of fast chargers but today they are no where near as good as Tesla’s. This reminds me of the early days of cellphones – it’s not the phone, it’s the network!

Direct Air Capture Carbon Sequestration Technologies

Hi Mark another thought that might make its way into the building code initiatives:

This is in response to an email about Mitsubishi’s new modular carbon capture technology. 

Hi Jan, I am skeptical. Not because of the chemistry – there are dozens of chemistries for sequestering CO2, for instance NASA used lithium hydroxide on the Apollo moon rockets – but because of the cost. Current estimates are about $100-$200 per ton captured. You can buy a carbon offset in the regulated market for about $50 a ton and about $15 a ton in the unregulated markets. They have the same effect.

Unlike solar panels and wind farms where there is no consumable cost, all these carbon capture technologies use a consumable (Lithium hydroxide in NASA’s case, some source of amines in this Mitsubishi example) which means there is a cost you cannot get below for chemical-based carbon capture. Hence economies of scale, that have been so successful in solar and wind, cannot reduce the marginal cost to zero. The cost of the consumable chemicals sets a floor on the cost per ton of the carbon capture. 

The cheapest way to sequester carbon is to cut down a mature tree, plant a new one to replace it, saw the tree into planks and use them to build a house. This sequesters the carbon for about 100 years. There is no incremental cost as we do this already and people are perfectly happy paying $4 for a 2×4. What it needs is a building mandate to use wood rather than steel and concrete. Wood can be made into glued/laminated beams that can build up to 4 stories high. 

Q: My insulation contractor recommends adding 1/2″ XPS (expanded polystyrene) insulation to my walls. I already have R13 fiberglass.

A: It is probably a matter of cost. 1/2” XPS is not going to do much, especially if you have R13 already. However, the contractor may be able to add 1/2” XPS without making any structural changes such as extending the walls. If you have to extend the walls it is almost certainly going to be too expensive to make any economic sense. When I had a similar problem I added a 10mm (approx 1/2”) of Aspen Aerogel’s Spaceloft which is R10 per inch or about R5 for the 10mm product. This is probably the best way to add insulation without making any structural changes. 

What is the cheapest renewable source of electricity?

There are several options in MA to going fully renewable with your electricity. It is a matter of cost. From lowest cost to highest cost, these are:

  1. The first option is rooftop solar with a reasonably sunny roof (like my house), about 5-8c/kWh, by far the cheapest option for 100% renewable electricity. Even roofs in half shade (like my garage) can produce electricity at 12c/kWh.
  2. Even with a no-cash-down solar lease (or power purchase agreement) the cost is around 13c/kWh. A huge discount to the Eversource or National Grid basic rate at 29c/kWh.
  3. Move to one of the towns (Wellesley, Littleton, Boxborough etc.) that have a Municipal Light and Power (MLP) source of electricity. Often around 18c/kWh and many offer 100% renewable options, usually for a slight premium.
  4. NexAmp offers a 12% discount to the Eversource basic rate (the R1 rate, now 29c/kWh, so the NexAmp price is around 26c/kWh) for 100% solar, 100% generated in MA. There is currently a waiting list. There are other suppliers than NexAmp but the others offer a 10% discount. This is accomplished via the z-metering provision of the net-metering law that allows the generator of renewable energy to sell the credits to other people with electricity meters. You end up getting two bills each month, one from Eversource (net of the credits from NexAmp) and one from NexAmp for the credits. The two net to a 12% discount to the ES basic rate. 
  5. If you switch to heating with primarily heat pumps (you can still have a backup fossil fuel furnace) you can ask ES for the R4 rate which is for electric heating. R4 is about 1.5c below R1. This is regardless of whether you have solar or not.
  6. Community choice aggregation offered through a town usually offers several options with a basic rate (ISO NE grid carbon emissions), a 50% renewable mix and a 100% renewable mix. The 100% renewable mix is usually more than basic rate but this is highly dependent of rates at the time the town bids out the contract.
  7. ES offers greener options through various generators for the supply portion of your current bill. I do not know current pricing but it used to be about 5-6c/kWh  more expensive than basic rate for 100% solar, so say 35-36c/kWh

Case Study: 1,300sf, 1934, colonial-style home with limited solar potential

Despite being a vintage home that did not allow for adding wall or basement insulation, and without changing any windows, this house’s bills and carbon emissions were both cut in half.

The house is in Arlington, MA with 5,600 heating degree days.

The program consisted of:

  1. Insulation was added in the attic, some air sealing was done in the attic (but neither insulation nor air sealing was done in the basement),
  2. Air sourced heat pumps were added
  3. A fairly small solar array was added to the roof (which has considerable shade from nearby trees)
  4. The natural gas furnace was retained
  5. A wood-burning stove was added for both ambience and heating

Here are the details:

Congratulations are in order! You have cut your carbon emissions by 56% and your bills by 32%. The cut in the bills is despite the price of natural gas being 32% higher than it was when I measured your house back in 2019 and electricity having gone up 13% since 2019. Natural gas will soon have doubled in price from 2019 and electricity will soon be 50% more expensive so you have insulated yourself from those increases as well. You are also not funding Putin’s war in Ukraine.

In the year before I worked on your house your total bills were:
$2,632 for gas, $830 for electricity
$3,462 total
In the last 12 months they were:
$1,033 for gas, $722 for electricity net of solar, $600 for wood
$2,355 total

Your total carbon emissions before the “fab four” were 11.4 tons per year and after they were 5.8 tons, for a cut of 56%.

So I saved you $1,107 per year so that is what you owe me. This is 32% cut in your bills. And this is with the prices for both natural gas and electricity having gone up considerably since I started work on your house. If I recalculate your “before” costs at today’s prices for gas and electricity your “before” bills were $4,207, so you can think of your savings as having been more like $1,851 or a 44% cut compared to the recalculated costs before the Fab Four. So roughly roughly you can say you cut your carbon emissions in half and cut your bills in half too. I assumed you were using electricity from the grid at the average ISO NE emissions per kWh rather than the zero emissions from the “green” electricity you buy because you pay extra for that.

With the limited amount of sun available on your roof I think this is a very good outcome. You are absolutely right that it would be better for everyone if the utilities supplied more green electricity. In addition to the money savings and carbon emissions savings you have almost certainly increased the price of your adorable little house with both the heat pumps and solar panels.

Did you burn the entire cord of wood in the last 12 months? If not let me know how much you burned as I suspect I have overestimated your carbon emissions.

Any thoughts on honeycomb shades for windows?

Q: I have seen these claiming R4.7. Any thoughts?

A: I think they are a very good idea. They are cheap, look good and are very effective. Also, thick, pleated, lined curtains that go to the floor (or window sill) can add R3-R5 to any window. Window inserts can add R1-R2 to any window, but they also block drafts which neither shades nor curtains can do. Many older windows, especially sash windows, leak like sieves, so for these the window inserts are probably the best idea, but if your windows are not drafty then I think either curtains or shades work great and are pretty inexpensive.

Heat Pumps for Hydronic (forced-hot water) Radiator Heating Systems

Q:  For homes with existing hydronic (hot water) radiators, what do you recommend?  Is it possible to use air-source heat pumps to heat the water on those systems?  What about increasing the radiator sizes to operate with Lower water temperatures?

A: Yes you can do this. Daikin, Jaga and SpacePak all make air-sourced heat pumps designed especially for FHW systems. I think it will be a lot cheaper to use the existing radiators and leave the old furnace in place in case the heat pumps can’t keep the place warm in the middle of winter.

Questions to ask a solar-panel installer:

  • 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?