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Energy HOME: Calculating a confusing heating situation …

January 4th, 2010 · 2 Comments

Today, a poll popped up:

How do you primarily heat your home / apartment?

This question gave me pause.

Sadly, for a self-proclaimed (and labeled by others) energy ‘geek’ and one who values feedback (systems) to enable better decisions/energy use patterns, I realized that I simply can’t answer this question.  The problem: multiple heating systems, with multiple changes over recent years, with difficulty in trying to figure out the relative share of heating provided by each system.

Within my household:

  1. Natural gas furnace: An 80% efficiency furnace, with a bit better efficiency in impact due to variable fan system. (Couldn’t put in a 90+% gas furnace due to central location and difficulty of doing ducting for higher efficiency system.)
  2. High efficiency (SEER-16+)  heat pump unit which, by the way, didn’t see an hour of cooling this past summer.
  3. Radiant heating (in 1/5th of house) operating off solar hot water system and, if there isn’t enough hot water from the sun, a  natural gas hot water is the heat source.
  4. High efficiency fireplace insert with about half the wood coming from trees / limbs cut down in the neighborhood rescued from being hauled away (and, well, even storm damage limbs, which are great for starting fires).
  5. There is the small (high efficiency) heating element used in the basement office (when the rest of the house is set to go to 55 degrees during the day, unless there is a fire and/or lots of sun to support the radiant system).

My off the top of the head, hopefully in the ballpark of being right, answer as to the relative heating from various systems.  The 4th (the fireplace) probably covers in the range of 40-50% of total heating load; the 3rd perhaps 5-10%; the 5th likely 1% or so; and, the first two roughly split the remainder.

The difficulty in figuring out relative heat load bearing among these elements is complicated even further due to (near constant) change in the household system.

  • The first and second were a combination replacement three years ago of an old (about 50% efficiency) gas furnace and an old (EER-7) air conditioning unit, in a “fossil-fuel system”, which is that the system chooses which is the most energy efficient way to heat based on outside temperature, switching from the HVAC to natural gas generally when the temperature falls below about 40 degrees Fahrenheit. (The controller enables me to change this, which provides an option for varying heating options based on the relative cost of natural gas vs electricity.)
  • About 2 years ago, the radiant system became part of the heating system with a small (350 square foot) addition (that is well insulated compared to the rest of the 1958 house, with R-49 in the ceilings, R-19 in the walls, and R-13 below the radiant system).  It used to operate off an about 50% efficiency natural gas hot water heater.
  • Just about a year ago, in January 2009, we added the high-efficiency fireplace insert.  In addition to really changing the household dynamic (with the living room the ‘favorite’ room when there is a crackling fire) and driving some real exercise (splitting wood into 1-2 lb pieces & assuring enough kindling), this picks up a serious portion of the heating load.  We probably have a fire burning in the range of 70 hours/week (generally not through the night and not every day) in winter, (maintaining the household fan at low level to move this air through the house). If it is above about 40 degrees, this takes up basically the house’s heating requirements and is partial heat otherwise. (In other words, if there is a fire, the HVAC unit rarely turns on.)
  • Last summer, we added solar hot water to the house, replacing the natural gas hot water heater with a smaller and higher efficiency unit at the same time that is the backup to the 120 gallon solar hot water tank. The radiant system now operates first off the solar hot water tank, if there is enough in the tank, and then off the natural gas.  (Estimate: perhaps 50% of radiant load provided either directly or indirectly due to the solar hot water system.  Indirect: solar hot water tank warming water and lowering the natural gas required to heat water to 120 degrees F.)
  • Later this month, the solar PV system that is going up on the roof will start to pick up some of the heating load.

Of course, when it comes to the calculation of what is the house’s heat source(s), lets add more problems.

  • As noted above, the house fan runs with the fireplace but there is also a fan unit at the fireplace itself (to increase the heating from the fire), thus that is likely several kWh/day to support the fireplace.
  • And, there have been multiple rounds/items in the leak sealing, insulation adding, new curtains (really making sitting by windows far more comfortable at night), and other heating-efficiency changes over the years.
  • And, well, there is the nighttime 53 degrees where the heating is anthropogenic (our body heat with good blankets).

This is all going to a simple question as to “how do you primarily heat your house?” without getting to the question of what are the primary energy sources to heat the house. In my home, we heat with:

  • Firewood. Difficult to calculate exact energy inputs, as every wood has a different heat content (and we burn a real mix of woods) and we don’t measure (in cords) the wood collected from downed limbs/such in the neighborhood.
  • Electricity for the heat pump, the fans to move fireplace heat, the small office heater, and the pump for the radiant heating. “Primary” energy mix soon to be dominated by (or perhaps totally covered by) solar pv.
  • Natural gas for both air heating and heating water for radiant system.
  • Solar for about half (estimated) the radiant heating load and soon to be in range of 100% of annual electricity load.

While a feedback-valuing, statistics minded  ‘energy geek’, contemplating the confused (complicated) household heating system, we are unlikely to ever have a robust and fully accurate answer to that simple question: “How do you heat your house?”

The poll and results as of mid-morning 4 January.  Note, I am one of 35 “other”.


How do you primarily heat your house/apartment/abode?

58% 1274 votes
9% 215 votes
0% 13 votes
4% 102 votes
20% 442 votes
3% 85 votes
1% 35 votes

| 2166 votes | Results

Note that neither fuel oil nor propane are part of my household’s heating system.

Tags: Energy · Solar Energy · energy efficiency · energy home · energy smart · solar

2 responses so far ↓

  • 1 Energy Home: Composting with a Cone // Jul 30, 2010 at 2:06 pm

    [...] Energy HOME: Calculating a confusing heating situation … [...]

  • 2 Lhommevert // Jan 26, 2013 at 3:24 pm

    Hi there!
    That is one complex system you got there.

    Without a doubt, too complex. Piece by piece rather than an integrated plan. Sigh …

    You must have spend a small fortune on it by now.

    Perhaps — since my utility costs are, writ large, less than 20% of my community’s average, there are some savings that make up for that ’small fortune’.

    Have you given consideration to bringing your different heat sources together to increase efficiency and make it resilient? You have a thermal storage tank connected to the solar as I understand. It’s not rocket science to add electric heating element to the tank to add heat from the PV panels when available (even low voltage).

    There is no reason. My electricity is net metered, why divert electrons?

    Also, your wood fire place is a great candidate for that. I have fabricated custom exterior mounted heat exchangers for wood fire places before. Since the wood fire place is in use anyway, the excess heat can easily be transferred to the thermal storage as well.

    Something that if I were a clean build that I would do however there are several issues, most notably the capital costs for dealing with the 25 feet between the fireplace and the thermal storage tank. Breaking up concrete foundation nor eating up living space or … are all bad paths forward that would be ’small fortune’ additions for relatively small gains.

    So now there are 3 heat sources feeding the tank.
    (You could even double the storage capacity with a 2nd tank, at that point). Solar, wood, and electric. From that tank you can feed the radiant floor heat, add a fan coil in your duct work for the forced air heating, and (pre)heat your domestic hot water. I’ve designed such systems before with very satisfying results. The thermal storage, in that case, would need to have 2 separate exchangers built in, plus connection for electric element. They are available. PV to run the heat pump will be tricky, unless you can put a 5kW array with hybrid inverter/charger and battery bank as buffer.

    Let’s talk small fortune …

    These come with grid inter tie to make up for shortfall in PV output, and net metering to sell excess production back to the grid. Added benefit would be that your whole heating/cooling with all it’s components, as well as your domestic hot water production would be independent from utility inputs if needed.

    So, fireplace insert has already proved itself as ‘emergency heating’ if the grid goes down/power is interrupted. As to hot water, the solar system feeds into a gas ‘back-up’ — have yet to have natural gas crash and the solar tank has always maintained at least ‘tepid’ water. Could imagine a path to a few days with ‘cold’ hot water but this is a far out scenario in my conditions. And, well, can survive a few days w/out hot water for bathing.

    Lots of data needed of course to make accurate calculations, but doable for sure.
    Then the answer to the question up top could be: “what ever is available” Last system I consolidated involved micro hydro instead of PV. I’m hoping to add web based monitoring and control in the next little while. If interested, I’ll keep you in mind and send you a linky when that’s configured.

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