Out with the old, in with the new

The Island Coho houses were designed to have heat and domestic hot water (DHW) supplied by an oil-fired boiler (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, and the two bedroom houses got two heating zones worth of fintube baseboard heat, one per floor level, and the three and four bedroom houses have a third, for the first floor ell. When this boiler was specified, it used the smallest oil burner nozzle available, which was 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.) I've previously noted that these aren't superinsulated houses, but they're relatively small houses - 1,166 ft2 to 1,800 ft2 - in a mild climate. So the design load of our house is about 22,000 BTU/hour (NB: this is primarily an IP blog, sad to say). 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 vent 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 eight 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 59F, 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. 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 datalogger 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 10F, to below 50F. The difference was all the heat not being lost from the boiler and the heat distribution piping. Brrrr.

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 2-4F 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 8-9 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.

The image below is a snapshot of the burner state from 6:00 am one morning to 6:00 am the next morning. The boiler fires pretty often, on for a couple of minutes then off for six or seven. At about 3:00 pm and again about 10 hours later there is a long cycle, about thirteen minutes, for DHW. A little after 11:00 am, the boiler stops firing for heat - most likely a sunny day. It picks up again around 6:00 pm, then it's off a bit after 8:00 PM - 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 (NB: disclaimer - I'm really fond of Marc's Sloppy Math (tm) - 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 sliderule back in the Stone Age.) Now let's compare with the energy used by 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 dumping it into your warmer house. The heat pump delivers warm air from a unit high on the the living wall. Here's the outdoor unit:

Here's the indoor unit, over the stunning Nicky Corrao watercolor painting:

The hunky looking white pipe contains the refrigerant lines and wire that connect the indoor and outdoor units. Normally this would be outside but the gutter interfered...

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!

 

 

 

 

Comments

[Margaret Dillon]

Wonderful descriptions - especially of heat pumps, Marc - and blog, (dare I say Housebook?), in general! I'm wondering how low a temperature that COP3 will hold true? Have you seen the e-monitors at work? I suspect your usage hardly justifies the $/carbon investment, but for those of who love to measure. its minute by minute data logging by circuit matched up with hourly temps could be interesting. Can you tell us why you selected the Fujitsu?

[Marc]

Fujitsu makes the most efficient minisplit heat pumps, and I saw some independent test data that confirmed the performance. I'll say more at some point.

[Christopher Yaun]

Burning Down the House

Hello Marc, Chris of Heather and Chris with the low energy house in Portsmouth NH.

Is it accurate to compare the oil furnace at 290,000btu against the Fujitsu at 43,000btu? Will the electic grid have consumed 120,000 to a 160,000btu to deliver the 43,000btu consumed by the Fujitsu?

290,000(oil burner) vs 140,000 (electric heat)?

It is still a 100% improvement in efficiency.

Chris

[Marc]

Chris, that's one of the next posts - primary vs. site energy!

[Paul Pimentel]

This is a great idea. I predict a limited but engaged audience. Thanks.

NB: Pedantry times 2: When I think of boilers, I visualize steam as the product (pedantry times 3: must be vapor to visualize; one can't actually see steam). When hot water is the product, I think of a heater, as in domestic hot water heater.

[Christopher Yaun]

Burning Down the House

Heather asks me to tell you that her purchase of a heating system for the guest rooms are on hold pending your post on primary vs site. I think she is in a hurry if you don't mind:|

Chris

[David Fay]

Now that's something you don't see every day: three different heating systems all on the same wall.

I have one question about the mini-split: is there an efficiency reason to mount it high on the wall or is it just to get it out of the way of the furniture? Is there a price to be paid for this location?

David

[Marc]

Hi David
Great observation - a heat pump, baseboard fintube forced hot water heated by oil, and a propane stove heater!

We put the wall unit high up to make it more effective during cooling. The louvers on the outlet are adjustable up and down, and the unit on Auto aims them down for heating and up for cooling. If you were doing heating only, perhaps you'd mount it lower with no ill effects. It does protrude from the wall about 10 inches, so you do need to think about what is in front of it.

[Scott Witt]

Mark,

Great Blog.

We have a cheaper version of the Fujitsu mini slit, a Turbo Air installed in our studio building here in Austin. It is used irregularly - we use it only on the weekends. I have found that at certain times of the day the unit will drip water out of the bottom and down the wall. Dust accumulates on the coil and the blower pulls the water off the coil before it reaches the drain. It has been a real pain. Just a heads up with your painting beneath your indoor unit. I hope you don't have the same problem.

[Marc Rosenbaum]

Scott, that sounds like a very undesirable "feature"! Have you shown it to your installer or a Fujitsu rep?

[Jacksonville Electrical]

Thank you for the sensible critique. Me & my neighbor were preparing to do some research about that. We got a good book on that matter from our local library and most books where not as influential as your information.

[crowdSPRING]

Its minute by minute data logging by circuit matched up with hourly temps could be interesting.

[WFG Canada]

The louvers on the outlet are adjustable up and down, and the unit on Auto aims them down for heating and up for cooling. If you were doing heating only, perhaps you'd mount it lower with no ill effects. It does protrude from the wall about 10 inches, so you do need to think about what is in front of it...

[Seth Downs]

How's this logic? A Fujitsu 9 or 12RLS mini split heat pump has a HSPF(Heating Seasonal Performance Factor) of 12. Input that number into a HEATING FUEL COMPARISON CALCULATOR ( 205.254.135.24/neic/experts/heatcalc.xls ) I found online, that considers your local climate to give a more accurate HSPF. Take that "corrected" HSPF and multiply it by .293 to get your local climate adjusted COP. (Avg COP = Heat transferred / electrical energy supplied = HSPF * 1055 J/BTU / 3600 J/watt-hour = 0.293 HSPF.) I'm no engineer so please correct me if I'm wrong.

[Orlando electrical contractor]

Excellent idea and concept. Absolutely a noble post. Looking forward to see more of your post soon. Keep it up!

[Marc Rosenbaum]

Seth, I can't comment as to the accuracy of that calculator. I put 12 into it and it says 8.2 for Boston. You're correct that multiplying by 0.293 gets you COP.

[Seth Downs]

Marc, did you do computer simulations to determine the size of your Ductless Mini Split? I just had a HERS rater over today to help me with my road to NZE living. He is not familiar with Fujitsu heat pump performance numbers, but will be locating those and modeling our heating needs. On a related subject I will be insulating the exterior of our cement block bungalow with either 2" to 3" of Polyiso rigid board or Polyurethane spray foam, either of which with a stucco or EIFS exterior. Do you see any benefit if burying the refrigerant lines of a mini split (the outdoor compressor and indoor wall unit will be minimal distance apart)?

[Marc Rosenbaum]

No calcs, because I couldn't buy a larger Fujitsu and I wanted to try it. I knew peak heat loss when i'm finished would be under 20,000 BTU/hour and that the Fujitsu was likely good for 15,000, so I wasn't going to be far off. I wouldn't bury linesets if the condesner is adjacent to the house

[Samantha L.]

@Orlando electrical contractor

I couldn't agree more on your premise.

[Heating and Plumbing]

It was great to read th whole stuff, it seems that you are quite expertize in this field. Waiting for new such technological post to read up.

[Jagadguru Kripaluji Maharaj]

This nifty instrument shows when the burner is on and when it goes off - it logs it's "state".

[Air to Air Heat Pumps]

That air source pump looks really good above that painting! It almost looks like a light or something above it. Just goes to show how much some systems can be integrated into a home nowadays.

[Tad Montgomery]

Marc - aside from the propane stove, passive solar and the minisplit do you have any other space heating in the house? If not, are there uncomfortable cold zones in the winter? How about upstairs or in the basement? Or perhaps I've missed something, and the minisplit has ducts?

[Marc Rosenbaum]

No other heating. Basement gets quite cold, because even though I insulated the walls, I left the batts in the floor. It hit about 50F last winter. The 1st floor ell bedroom we didn't try to heat - we kept the door shut - and it floated in temp, getting heat through uninsulated interior walls, and potentially from the basement below if it got colder than the basement. The upstairs ran 2F cooler in normal winter temps (30F) and 4F cooler in severe weather. Remember this is not a superinsulated house! We've put the house on the market, and I've just finished installing a small (4.5 kW) electric boiler so that a future occupant could add heat to the two small zones that the heat pump only addresses by natural convection. I'd left all the forced hot water piping in place, so this was essentially a drop-in replacement for the oil boiler.