The Island Cohousing houses were designed to have heat and domestic hot water (DHW) supplied by an oil-fired boiler. (Time for a pedantic distinction: a furnace heats air and blows it around a house, and a boiler heats water which is pumped around the house).
They chose a pretty good boiler: a German Buderus G115. The two-bedroom houses got two heating zones’ worth of fin-tube baseboard heat, one zone per floor level. The three- and four-bedroom houses have a third zone, for the first floor ell.
When this boiler was specified, it used the smallest oil burner nozzle available, rated at 0.5 gallon/hour, and therefore its heating output was about 70,000 BTU/hour. (For geeks: this is higher than you’d think, because the nozzle is rated for an oil pressure of 100 psi, and the burner on this boiler is running at 150 psi, which means it bumps the nozzle flow up to about 0.6 gallon/hour.)
A small house in a mild climate
I’ve previously noted that these aren’t superinsulated houses, but they’re relatively small houses — 1,166 square feet to 1,800 square feet — and are located in a mild climate. So the design load of our house is about 22,000 BTU/hour. So on the coldest night of the year, the boiler only needs to run about 1/3 of the time. On an average winter day, it may run about half of that, and in the shoulder seasons, even less.
When the boiler is not firing, it loses heat up the chimney flue and off the boiler itself as it cools down. Our boilers here are sidewall-vented (don’t do this with oil, it’s a black mess on your exterior wall around the vent) and so may not be losing as much as if they might if vented vertically up a chimney — but here I speculate.
The boiler, DHW tank, and the uninsulated heating piping all reside in the basement.
On Martha’s Vineyard it is common to put fiberglass batts in the basement ceiling (the first-floor framing) and leave the basement walls themselves uninsulated. A basement like this can take all the heat loss you can throw at it. Heat goes into the ground and into the outdoors above grade. An 8-inch-thick concrete wall with an air film on the inside has a thermal resistance (R-value) of about 1.5, which is halfway between a single-glazed and a double-glazed window.
Right around Christmas, the basement was 59°F, which feels fairly temperate. Just before Christmas, we installed a Fujitsu 12RLS single-zone minisplit heat pump in our living area. (More on that in a bit.)
The boiler is keeping our basement warm
On the morning of December 26th, we turned the thermostats controlling the oil boiler down to the minimum setting so the heat pump could do all the heating work, and then the boiler only fired to make DHW.
I put a Hobo state data-logger on the oil burner. This nifty instrument shows when the burner is on and when it is off — it logs its “state.”
When the boiler was operating to make heat and DHW, I saw it run between 3 and 6+ hours daily. When the boiler operated only to make DHW, its operating time dropped to about one hour per day.
In what I recall to be little over a week, the basement temperature dropped about 10°F, to below 50°F. The difference was all the heat not being lost from the boiler and the heat distribution piping. Brrrr.
Monitoring the oil boiler run-time
We operated the boiler without using the second-floor heating zone and relied on the heat from the first floor rising by natural convection up the stairs to the bedrooms and bathroom upstairs. These rooms ran 2-4°F cooler than the main level.
For a week in early February, I turned the heat pump off and used the oil boiler again, so I could get some good data on burner run-time and oil usage. During this week it averaged about freezing outdoors, and from my burner run-time I figured we used just over 2.5 gallons/day of fuel oil. Some of this was for DHW, but less than would be indicated by the roughly 0.6 gallon/day we measured when the boiler only ran for DHW.
The reason is that when making only DHW, the boiler would run for about 20+ minutes every 8 to 9 hours, because it had cooled off in the interval and first had to heat its own water storage and cast-iron mass up to temperature before it could effectively heat the DHW.
Once the DHW tank thermostat was satisfied, the oil burner turned off, stranding all that heat in the boiler, there to cool off to the flue and the basement. When the boiler was making heat also, it never cooled off for very long, so it didn’t use as much energy to make the DHW.
Interpreting the data
The image below is a snapshot of the burner state from 6:00 a.m. one morning to 6:00 a.m. the next morning. The boiler fires pretty often, on for a couple of minutes then off for 6 or 7 minutes.
At about 3:00 p.m. and again about 10 hours later, there is a long cycle, about 13 minutes, for DHW. A little after 11:00 a.m., the boiler stops firing for heat — most likely a sunny day.
It picks up again around 6:00 p.m., then it’s off a bit after 8:00 p.m. It looks like we turned the thermostat down, because the boiler doesn’t fire again for heat until the wee hours.
During this week, we used somewhat over 2 gallons of oil daily for heat. That is perhaps 290,000 BTU per day input energy. (N.B.: Disclaimer — I’m really fond of Marc’s Sloppy Mathâ„¢. I don’t care much about 2% accuracy. Most useful conclusions and design directions can be made with 10% accuracy. I am good about getting the decimal point in the correct location, which comes from using a slide-rule back in the Stone Age.)
A new ductless minisplit
Now let’s compare with the energy used by the minisplit heat pump.
I’ll say more about the technology in another post, but the short story is that the Fujitsu is an air-source heat pump, which uses electrical energy to operate a compressor to extract heat from the outdoor air and deliver it to the inside of the house.
It takes no special genius to get heat to go from hot to cold, but the other way around is harder. Your refrigerator is a heat pump, using electricity to take heat from inside the cold refrigerator compartment and dump it into your warmer house.
The Fujitsu heat pump delivers warm air from a unit high on the the living wall. The hunky looking white pipe in the photo below contains the refrigerant lines and wire that connect the indoor and outdoor units. (Normally this would be outside but the gutter interfered…)
Ductless minisplits are energy misers
We’ve heated the house (with the exception of the ell, which we keep closed off from the house, and which seems to run at basement-like temperatures without direct heating) for a bit over three months now with the heat pump.
When the heat pump was installed, I had the electrician wire an old-fashioned electromechanical electric meter in line with the heat pump so I could see how much energy it was using. I’ve been reading the meter at least once daily.
On the coldest day, we used almost 21 kWh. On mild days more recently, we use as little as 3 or 4 kWh. During similar temperatures as occurred during the week we ran the oil boiler, the minisplit has used 12-13 kWh/day. Converting that to BTU gives a daily input of around 43,000 BTU.
The oil boiler was using 290,000 BTU, remember? How can the heat pump be over 6 1/2 times better?
Well, for one thing, the reason we use heat pumps is that they use one unit of electrical energy to move more than one unit of “free” energy in the outdoor air. This ratio is the Coefficient of Performance (COP), and higher is better. The Fujitsu is probably operating at a COP of 3 or more. So the energy input into the house might be 130,000 BTU or even more per day. Plus, the heat pump is not heating the basement or losing energy up a flue.
So we’ve gone from a cost of over $8 per day for heat, to $2.25 per day. And we’re heating to slightly higher temperatures, for reasons that need to be left to another post!
Marc Rosenbaum is director of engineering at South Mountain Company on the island of Martha’s Vineyard in Massachusetts. He writes a blog called .