Calculating Savings for Oil to GSHP
Last Post 05 Sep 2017 03:55 PM by Dana1. 35 Replies.
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RMDUser is Offline
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25 Aug 2017 01:37 PM
I've got a fairly large home built in 1997 that uses about 2500 gallons of oil in the last year. It has a forced air system. I live 25 miles north of Boston.

The home has two oil furnaces which have tested at about 82% efficient, and two A/C condensers whose SEER at this point is probably between 10 and 11. All original equipment circa 1997.

I'm awaiting a quote for a GHSP installation (vertical closed loop / WaterFurnace 7 series), and am trying to determine what my expected savings might be.

So here are my assumptions so far. I'm sure they're wrong, so please let me know how. :)

Expected Average Heating COP: 4
Expected Average Heating Savings: 37%
Expected Average Cooling EER: 27.7
Expected Average Cooling Savings: 61%

I assumed the COP of 4 as a bit of a hedge. I know COP varies by load, but I'm hoping that's my average based on my climate. The cooling savings was originally higher based on WaterFurnace's listed 41 EER, but again, that's peak and not average, so I'm hedging.

I'm not sure what size system I'll get yet. No Manual J yet, but based on oil consumption and some guess work, I likely have a peak heating load around 130K BTU/h during the coldest days, and my worst monthly average BTU load is about 105k BTU/h. (March of 2017.)

I'm sure the fact I've got such high loads will affect my average COP/EER.

In terms of how I calculated savings, I converted the BTUs/h into KwH equivalent, and then used the COP to determine my savings versus that KwH number. For example:

(2500 Gallons) x (31.86 KwH per Gallon) = 79,650 KwH per year.
79,650 KwH / (COP 4) = 19.921 KwH
19,921 KwH @ $0.20 per KwH = $3,984 / year.
1-($3,984 / ($6,288 oil cost)) = ~37% savings.

Electrical savings for A/C is a bit more straight forward, obviously.
So am I doing this wrong? Too conservative? Not conservative enough?
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25 Aug 2017 04:22 PM
Number 2 oil with a nominal source fuel heat content of 138,000 BTU/gallon burned at 82% thermal efficiency delivers 113,160 BTU/gallon into the heating system, which at 3412 BTU/kwh is equivalent to 33.17 kwh/gallon. How did you end up with 31.86 kwh/gallon? Mind you, the power used by the pumps and controls count too, but that is a relatively small error in most hydronic systems.

To calculate the peak load requires the heating degree-day data for the period between fill-ups (or K-factor, which is the same information presented differently) and the 99% outside design temperature. For locations 25 miles north of Beantown the 99% outside design temp is between 0F-7F, depending on exact location. You offer no clue as to how you're coming up with the "...monthly average BTU load is about 105k BTU/h..." Heat loads are instantaneous at particular indoor and outdoor temperatures, not "monthly average" numbers.

To figure out how fuel use load calculations work, see this bit o' bloggery:

http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new

Idling standby and distribution losses can introduce a significant error with a boiler in an un-insulated basement, since those losses would be much lower with a low-temp GSHP system.
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25 Aug 2017 05:19 PM
Great information!

I'm not entirely sure where I got 31.85 Kwh/gallon, to be honest. The intertubes some place. I'll modify accordingly.

The average BTU/h for a month was calculated by averaging the total BTUs consumed (as calculated by oil consumption for that time period) and then averaging that over the entire hours for that month.

I do have the HDD information. March had 1145 HDDs. For that month, we used a total of 521 gallons in March. We used 202.6 gallons between March 2nd and March 15th, during which there were 568.1 HDDs. How would I calculate peak load based on this information?
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25 Aug 2017 06:04 PM
Your load calculation method doesn't work, since it isn't temperature dependent.

For a 2x6 framed house that's reasonably tight it's usually more accurate to use base 60F than base 65F HDD unless you keep the place warmer than 72F most of the time. Find the nearest weatherstation on degreedays.net and download a daily HDD spreadsheet for the period in question using base 65F to get more accurate load number. The details of how to run the calculation are in the blog piece, but I'll run the 2-15 March numbers here (presumably base-65F HDD), using 0F as the outdoor design temperature (the 99% outside design temperature for Lawrence: https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf ), by means of example.

You used ~203 gallons over ~498 HDD, or 0.40763 gallons/HDD. Assuming #2 oil @ 138,000 BTU/gallon that's source-fuel BTU usage of 0.40763 x 138,000 = 56,253 BTU per degree day, or (/24 hours/day= ) 2344 BTU per degree-hour. Burned at 82% efficiency that's a load-constant of 0.82 x 2344= 1922 BTU/ degree hour (the other BTUs went up the flue.)

Using a base of 65F assumes a load of zero heating or cooling when the outdoor temperature is 65F. So you really have 65F -0 = 65F heating degrees. A linear approximation of the implied load at 0F would then be:

65F x 1922 BTU/degree-hour = 124,930 BTU/hr

That has to be one really big (or really leaky- do you sleep with the windows open?) house! Most reasonably tight 2x6 framed 1990s vintage houses in MA have a heat load to conditioned space area ratio of 12-15 BTU per square foot @ 0F, and bigger houses often come in under 12. That implies a ~10,000' house (not including below-grade basement area), probably not smaller than 8000 square feet. If the house is significantly smaller than 10,000 square feet you probably have GOBS of low-hanging fruit for improving the efficiency of the building to lower the load that would be cheaper to implement than the up-sized GSHP needed to support the pre-improvements load.

Clearly an AGGRESSIVE Manual-J load calculation is in order, performed by a qualified third party such as a certified professional engineer or RESNET rater, and it should include a blower-door test rather than using a loosey goosey default value inserted into the load calculation tool. Air infiltration is likely to be one of the bigger load factors, and it's important to get it at least sort-of right, since it could easily add more than a ton to the system size.

Doing an infra-red imaging survey while the house is pressurized/depressurized can help find the leaks once the outdoor temperatures fall well below indoor temperatures, and it would help find any gaps in the insulation. By far the cheapest load reduction would be to chase down and rectify the major air leaks in the house, followed by finding and fixing missing or degraded insulation. A simple Manaul-J typically costs $500-800, but might be more for a bigger house in a more upscale neighborhood. A blower door & IR imaging session with on-the-spot air sealing remediation could run $ 1000-1500.
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25 Aug 2017 06:23 PM
Dana1 - this is awesome. Thank you.


Our home is 3300sqft plus another 1200sqft in a finished, walk-out basement. The home itself was modular in construction. Built by Acorn homes back in 1996/1997, who had a reputation for energy efficiency, and it has quite a bit of insulation. So based on your feedback, I'm missing *something* that's causing a major loss in heat. I've scheduled an energy audit, and I'll likely hold off on pulling the GHSP trigger until I've ironed out where this energy is going.


One thought I had was that my family has the terrible habit of leaving bathroom vents on almost all the time. It drives me nuts. I wonder if a couple of 100CFM fans blowing for many hours a day would do the trick. I'm already installing timers on these so it stops happening.
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25 Aug 2017 06:23 PM
BTW: If those are hot-air furnaces, and the ducts go outside the thermal & pressure envelope of the house (say, in an attic, above the insulation) the load from duct losses (conducted and leakage) can be a sizable fraction of the total load. Any change over to GSHP should include new ducts, or a serious re-commissioning of the existing ducts, including duct pressurization leakage test & remediation, and upgraded duct insulation, at least for the portion that goes outside the thermal envelope, as well as sealing all duct boots to the walls/floors/ceiling surfaces where they enter (including return duct registers.)
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25 Aug 2017 06:41 PM
Regarding the fans...

The thermal mass of dry air by volume is about 0.018 BTU per cubic foot per degree-F. So 100 cfm x 60 minutes is 6000 cubic feet per hour. At an indoor to outdoor temperature difference of 70F (0F outside, 70F inside) that adds a heat load of 0.018 x 70F x 6000= 7560 BTU/hr. When it's warmer than 0F the difference is smaller, and the additional load smaller.

If it ran 24 hours per day during weather that averaged 25F outside over the 720 hours in a month, that's only a 45F temperature difference, for an average load adder of 4860 BTU/hr that adds 3,499,200 BTU for the monthly use, which is about 30 gallons of oil. If it's only on half the time it's ~15 gallons. Putting bath fans under occupancy/vacancy switch control with an adjustable time preset for turn-off after occupancy is no longer detected is "worth it" in a house where they seem to get left on a lot. A Leviton IPV15 (which can handle fan motor loads) infra-red sensor vacancy switch runs about $30 at box stores. http://www.leviton.com/en/products/ipv15-1lz Lutron has similar models.

Regarding the heat load, assuming generous (= leaky) 15 BTU/ft for the 3300 ' of fully above grade portion comes to ~50K, and assuming 10 BTU/ft for the 1200' of walk out adding another 12K you're still at only ~72KBTU/hr. Your fuel use load is on the order of TWICE that(?).

If the foundation walls are finished but aren't insulated it could add another 25K to the load, but not much more than that.

For a reference point- my 2400' of 2x4 framed antique wit ~1800' of insulated conditioned basement(mostly below-grade, not a walk out) comes in a bit under 40 KBTU/hr @ 0F.

Do any of the oil bills have a "K-factor" stamped on them? (That's common, for regular fill-up services.) If yes, what are the three HIGHEST K-factors in the past year, and what periods did they cover?
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25 Aug 2017 06:49 PM
Well, I certainly can't wait to figure out why I'm using 2x the energy to heat that I should be for a house my size, age, and location. Thanks so much for the feedback. You probably just saved me tens of thousands of dollars. I owe you a beer.
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25 Aug 2017 06:51 PM
One thing I just realized is that these numbers for oil usage might be off because I also have an oil hot water heater. It's a 60 gallon unit. We have a family of 3 (2 adults and 1 kid) and we definitely use an above average amount of hot water. (Very, very long showers.

Still, that can't account for me using DOUBLE what I should be using. THANK YOU again.
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25 Aug 2017 06:52 PM
Have any good local IPAs in the fridge? :-)
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25 Aug 2017 06:57 PM
The hot water load error (that raises the implied load) is usually offset significantly by the solar gain error (that lowers the implied load), but has to be some ridiculously long showers to add up to another 50-70K of implied load.

A vacancy sensor switch on the bathroom lights set to 10 minutes can sometimes curb the "endless shower" experience!

Also, a decent sized drainwater heat recovery heat exchanger downstream of the showers can cut the fuel use of an endless shower by half. (I have a 4" x 48" version installed in my house.)
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26 Aug 2017 12:29 AM
One more quick question. How did you determine the 12-15 BTUs/sqft for my age home and location?

Thanks again!
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26 Aug 2017 03:40 AM
I offer a layman's experience to offset Dana's extraordinary exhibition...

My 1600 sf 1954 Levitt split on Long Island NY was heated by a 14 year young oil fired hot water boiler and had a 50 gallon oil fired HWH. Cooling was accomplished by 4 window units, in part of the living space only. My annual electric bill was approx $2200 and my $3 per gallon oil cost was about $2700-$3000 per year.

This was replaced by a 4 ton closed loop split GSHP that many may think is oversized, at first glance, plus an electric HWH with desuperheater/buffer tank setup. The manual J that was done confirmed the sizing. The air handler is in my unconditioned attic. Yes, envelope improvements might have been prudent, but were not done initially. I did get an electric company sponsored home energy audit performed, and have done some air sealing, etc... on my own.

3 years into this arrangement and my total annual electric bill averages approx $3600. We also enjoy whole house cooling and do not miss seeing the baseboard heating units. Hot water heat was comfortable though. All in all I am very pleased with my decision.

Hope this helps!
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28 Aug 2017 01:33 PM
Dana1 - I did some more digging and did my best to normalize my oil usage according to HDD information. The results are interesting. If I understand HDD correctly, if I take my oil consumption and divide it by the HDD for a given period, I should get a "gallons per HDD" which, assuming no major changes in how I'm "living" in my home (like being away or changing the thermostats substantially), should remain relatively constant.


What I see, however, is that for the period of Dec-Mar, I had an average Gallons per HDD (GPHDD) of .37. Not only that, but there was very little variability here. Any given period was always between .35 and .38. So far, so good.


If, however, I look at the period of April - May, this number drops all the way down to an average GPHDD of .23. Now, there were far fewer HDD in that time period - so is this likely just an artifact of a change in usage? Like we kept the heat off more frequently because the outside temperatures were more mild? (We have old-style thermostats that aren't programmable.) I ask because I discovered that all the vents in my finished, walk out basement were mostly shut for a good portion of the winter. (I don't recall exactly when I discovered this.) There is a zone thermostat down there, and so I think the furnace was running a LOT to try and heat that space because it wasn't getting any real air flow down there.
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28 Aug 2017 07:33 PM
Consistency is good- it means that the error relative to water use and solar gains isn't moving around much. assuming 0.37 gallons/HDD65 and 82% efficiency your house is using 0.37 x 0.82 x 138,000= 41,869 BTU/HDD65, or 1745 BTU per degree-hour.

So at 0F (65 heating degrees below the presumptive balance point) the implied heat load is about 1745 x 65F= 113,425 BTU/hr. That's still very high for a house your size.

During the April/May spring season the solar gain error comes on pretty strong, reducing the implied load.

Closing vents will unbalance the duct system, which 9 times out of 10 paradoxically INCREASES fuel use, (even at lower average indoor room temperatures) due to the increased air handler driven infiltration. In very air tight houses with ducts completely inside of conditioned space that effect is very small, but in leaky houses the effect can be quite large.
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29 Aug 2017 03:17 AM
Well, I just got back my quote. That's probably the end of that. Quote was for 2x 7 Series (NVV048) with the bells and whistles... and oh boy... $85k before incentives. After incentives, we get down to $52k. But at that cost, the best case scenario would be a 10 year break even. Probably more like 15 years. Now, this assumes no huge spikes in oil costs like we've seen in the past, but it's really, really hard to justify that spend when I can realize a lot of those savings with ASHP + oil backup for perhaps a bit more than half the upfront costs.

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29 Aug 2017 03:00 PM
The other reality is that you likely won’t be in this house in 10 or 15 years no matter what you think now and even if you are you will have spent at least 50% more than that cost for maintenance and repair keeping the system operating. And when you sell this house you will never recover close to what you spent. And ASHPs are only slightly better when it comes to maintenance and repairs. But at least you did the ROI and figured out how dumb it is to do GSHP before you got scammed. Most folks figure this out too late.
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29 Aug 2017 03:54 PM
Posted By RMD on 29 Aug 2017 03:17 AM
Well, I just got back my quote. That's probably the end of that. Quote was for 2x 7 Series (NVV048) with the bells and whistles... and oh boy... $85k before incentives. After incentives, we get down to $52k. But at that cost, the best case scenario would be a 10 year break even. Probably more like 15 years. Now, this assumes no huge spikes in oil costs like we've seen in the past, but it's really, really hard to justify that spend when I can realize a lot of those savings with ASHP + oil backup for perhaps a bit more than half the upfront costs.



It was silly to get a quote for a ridiculously oversized GSHP system prior to a serious analysis of the building efficiency with recommended building envelope upgrades and an aggressive room by room Manual-J on the "after" picture. A pair of 7 Series (NVV048) is literally twice the system that it would take to heat a reasonably tight not even IRC 2015 code minimum house that size.

Assuming that the bid system is what it actually takes to heat & cool the place (a dubious assumption at best), for half the quoted price I'm pretty confident the load could be reduced by more than half.

Seriously, hire some competent home energy nerds to fully assess the place, and use THEIR Manual-J on the "after recommended upgrades" picture for sizing the replacement equipment, not the HVAC contractor's best guess.

When the sizing and room by room loads is known you can then assess whether cold climate ductless air source heat pumps (mini-split or multi-split) could provide an appropriate solution (sometimes yes, sometimes not), and how much roof area is available for solar. In MA ductess heat pumps qualify for subsidy, and they're a heluva lot less expensive up front than GSHP. While subsidies always distort these markets, it's becoming fairly common for ductless + solar to pencil out more favorably than ground source heat pumps in this area.

As a point of reference, about a year ago I was involved in a project in eastern MA on a house roughly your size. Half the house was older 2x4 stuff, but it had been doubled in size at IRC 2012 code minimum construction. They oversized it a bit with 4-tons of ductless multi-split (6 zones), and it came to about $15K all-in, prior to subsidy, which after subsidy came in under $12K. They put off doing solar (until they cut a deal with the neighbor to remove some trees), but they have sufficient room for about 8000 Watts of panel, which is currently running ~$3-3.50/watt (for higher efficiency panels), so assume $25-30K for solar, which is enough to more than offset the difference in electrical power use between ductless at a seasonal COP average of 2.5 and a better class GSHP system delivering a true 4.5.

An 8kw PV system produces ~9000 kwh per year in eastern MA, leveraged at COP of ~3 (do-able, if right-sized) that delivers about 92 million BTU/year, or about ~800 gallon's worth of oil (burned at 80% efficiency) which is what your place SHOULD be using, if the system & house were efficient.) So for about half your quoted price ($15K ductless plus $30K PV, prior to subsidies) their annual operational costs will go to near zero.

It's not clear exactly why your house is using so much oil (probably a combination of inefficient duct design/implementation and issues with the house).
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29 Aug 2017 05:01 PM
So fix the crappy and leaky duct work. Also do a door and smoke blower test to see how bad outdoor air infiltration is and try to fix that too. Way cheaper than a expensive new heating system that you will never recover cost when you sell house. House prices based on location and floor area. Only get 10 cents on dollar for expensive heating and PV systems. Especially true for a old house...it's like throwing money down a hole.
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29 Aug 2017 05:32 PM
Posted By icfbound on 29 Aug 2017 05:01 PM
So fix the crappy and leaky duct work. Also do a door and smoke blower test to see how bad outdoor air infiltration is and try to fix that too. Way cheaper than a expensive new heating system that you will never recover cost when you sell house. House prices based on location and floor area. Only get 10 cents on dollar for expensive heating and PV systems. Especially true for a old house...it's like throwing money down a hole.


While a duct-blaster leakage test may help explain the high fuel use, it's unlikely that the design & implementation is appropriate for re-use with a GSHP system, and probably not good enough to get the most out of an old school ducted ASHP either. It's a prime suspect as to why the house is using 1.5-2x more heating energy than it should.

Getting off oil and onto heat pumps is protection against commodity price volatility, and in New England is a heluva lot greener to boot. A substantial amount of the up-front capital cost can be at 0% interest on a 7 year loan through MassSave...

https://www.masssave.com/en/saving/residential-rebates/heat-loan-program

...as well as a modest amount in direct rebates, if the heat pump efficiency clears the hurdles:

https://www.masssave.com/saving/residential-rebates/electric-heating-and-cooling

It's possible that spending $10K on recommissioned or new duct design & implementation and another $10K on building upgrades could bring the oil use down to something more normal, but while the building upgrades are clearly "worth it" (up to a point) and will improve year-round comfort, it's not clear that fully recommissioning or replacing a bad duct design makes economic sense. It's worth exploring that if the home efficiency audits tags that as a primary cause of the high energy use.

[edited to add...]

There is other state and utility rebate money to be had for converting to heat pumps as well, but the details vary, eg:

http://www.masscec.com/air-source-heat-pumps
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