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Best heat for the money

user-6740240 | Posted in Energy Efficiency and Durability on

We just built a new home in new england with a new heating system. House was built tight within the new stretch codes. The heat system has 2 unico I series heat pumps with three unico air handlers. My back up (aux) heat is a 5 section buderus boiler. So there is a hot water coil in all three of the unico AH’s for the aux heat. The data I got from the unico rep is as follows.

EER- 7.45
SEER – 14
HSPF- 8.35

So from info I found on this site to the the COP I divide the HSPF by 3.412 This gives me a approx COP knowing that the HSPF is based on seasonal data.

COP- 2.44

If I Understand this correctly this means the Heat pump is 244% heat conversion efficiency.

So I then found this site. http://nepacrossroads.com/fuel-comparison-calculator.php
that gives me a comparison price per million btu’s for a given fuel.

After plugging in the above info with a oils cost of 2.09/ gal and a electric cost of .20 cents per KWH I get the following results

Oil heat at 70% efficiency ———- $21.53 for 1 million btu

Heat pump at 244% efficiency– $24.02 for 1 million btu

According this I should never run these heat pumps at these fuel prices.

Please Let me know if I have done this wrong or if there is a better way to calculate this. The one variable I have not fully researched is the efficiency of the boiler. The site had a default setting of 70%. Does that number seem reasonable for a brand new oil boiler with a hot water coil in a air handler?

Any info on this would be much appreciated…

Thanks, Rich.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Rich,
    Your conclusions sound roughly correct -- and I don't think you should waste too much time sharpening your pencil. Fuel prices go up and down over time, and this comparison will change. But you're in the right ballpark. The costs are about the same.

    Green builders and environmentally conscious homeowners are moving away from fuel oil toward heat pumps -- not for economic reasons, but because heat pumps are a good match for a roof-mounted PV array. Most green homes these days are all-electric homes. Burning fuel oil contributes to global climate change.

  2. Stephen Sheehy | | #2

    Rich: Most new boilers have efficiency of better than 90%.

  3. Expert Member
    Dana Dorsett | | #3

    No oil boilers sold in the US have an efficiency above 90%. Most are in the 85-87% steady-state efficiency range.

    The-as-used efficiency of a ridiculously oversized boiler (which most 5-plate boilers would be for typical US homes) won't come anywhere near their steady-state efficiency for an AFUE, due to the low duty-cycle and high standby losses. Even at only 3x oversizing for the heat load a pretty-good Buderus 5-plater such as the G115WS/5 would achieve no better than ~80% as-used AFUE, despite the 86% AFUE on the name-plate.

    See the performance details on system #5 in this Brookhaven Nat'l Labs research project report:

    https://www.bnl.gov/isd/documents/41399.pdf

    (No Buderus is better than that boiler!)

    With heat purge controls and a buffer tank to purge boiler heat into at the end of every burn it might get close though (see system # 3).

    To hit the published & tested AFUE numbers:

    a: The boiler must be no more than 1.7x oversized for the heat load

    b: The boiler has to be installed inside the thermal envelope of the house. If the boiler is in an uninsulated basement most of the standby losses go outdoors rather than accruing to serving the heat load. If it's in a garage on the other side of the insulation (or a basement insulated at the ceiling but not the walls) take 10-15% off the AFUE efficiency to account for the standby and distribution losses.

    If you have been heating with the boiler it's fairly straightforward to use a "K-factor" stamped on a wintertime fill-up or exact fill up dates & quantities to determine an upper bound for the heat load (that would include the distribution & standby losses, since they can't be separated out) by establishing the BTU/heating degree day ratio. See this for details on how to do that:

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

    Alternatively, you can use the FSAcalc 2.0 for a a more nuance fuel-use calculated heat load that includes the oversizing inefficiencies, provided they have weather data from a location near you: http://www.fsacalc.com/?page=home

    So, assuming the Buderus is a chimney-drafted G115WS/5, it has a DOE output of 136,000 BTU/hr. If your fuel-use calculated heat load is less than (136,000/1.7 = ) 80,000 BTU/hr (say, an uninsulated 3000-4000' house) it is going to undershoot it's nameplate efficiency. If the fuel-use calculated heat load is under 40,000 BTU/hr (typical for most houses) you're more than 3.4x oversized, and it would be running at least 5% less than the nameplate AFUE. A typical tight "stretch code" 2500' house in US climate zone 5 that isn't over-ventilated (or HRV ventilated) would have a heat load of ~30,000 BTU/hr, often less.

    If the heat pump air handler and ducts are outside of conditioned space there are similar issues.

    A heat pump sized small enough that it can run long cycles at a high duty cycle even during the shoulder seasons, the average efficiency during the shoulder seasons will be a bit higher than the simple HSPF model. If oversized to where runs only very low duty cycle during the shoulder seasons it's efficiency could be even lower than the name plate HSPF.

  4. user-6740240 | | #4

    @ Martin-- Thanks for the reply and information. We did have the green team in mind when designing the house. I made sure to have our large standing seam garage roof facing due south for potential solar install. That is one reson we went with the heat pumps. But at this current point in time I would like to use the most cost effective heat source. Taking a uneducated wild guess I would think the electricity coming off the grid is above 90% fossle fuel so my guess is it would have the same impact on the environment to use the heat pump or oil heat.

    Thoughts?

  5. GBA Editor
    Martin Holladay | | #5

    Rich,
    The carbon intensity of grid electricity varies widely from state to state. West Virginia has dirty electricity; Vermont has relatively clean electricity (from a carbon emissions standpoint). Study the two graphs below for more information.

    .

  6. user-6740240 | | #6

    @Dana
    Wow.. thanks for that..

    The boiler is in a basement insulated at the ceiling but not the walls

    We have a 4000 sqft home with 3 vemar K-7 44163. (erv's I think) set to run at defualt program times.

    So... I will check out your provided info to get a closer number for the boiler but it does sound like 70% is in the ball park...

    Thanks again.. Rich

  7. user-6740240 | | #7

    @Martin, Thanks for the education. So in mass It would seem to better to use grid electric as apposed to burning oil. I am presuming natural gas is less offensive then oil..

  8. user-6740240 | | #8

    @Dana, I forgot to ask, I assume I loose some efficiency with the heat transfer in the AH as apposed to a straight forward radiant heat from the boiler as well?

  9. user-6740240 | | #9

    One more quetion if you don't mind..

    I can get a chart on haow many btu/hr the heat pump will deliver at ginen temps but I can find no data on how the defrost mode affects the efficiency. I assume

    The lower the temp = more defrost = less efficient.

    I am sure there are variables involved but what is the average outside temp that the defrost kicks in and how much does it affect the efficiency?

    Thanks again.. Rich..

  10. Expert Member
    Dana Dorsett | | #10

    "I assume I loose some efficiency with the heat transfer in the AH as apposed to a straight forward radiant heat from the boiler as well?"

    The boiler probably loses heat by convection, and conduction into the attached plumbing than from direct radiation. If the air handler and some of the ducts are outside the thermal envelope too, it only piles on to the losses.

    Whether you remove the insulation from the basement ceiling or not, insulating the basement walls is likely to be a net win, since it retains most of the distribution and standby losses to the basement. If after insulating the basement walls the basement ends up being warmer than the house, removing ceiling insulation would be a net win.

    Colder outdoor temperatures do not mean more defrost cycles. Colder air has less moisture to condense, and thus slower frost formation. The heaviest defrost cycles in most areas are when the outdoor temperatures are in the high 30s or low 40s, since the coil is then running in the 20sF, and the outdoor air still has a lot of moisture in it. Depending on the particular manufacturer's defrost methods defrost cycles can affect capacity, efficiency or both. Those that use resistance heaters operated with an unsophisticated timer tend to have the biggest negative impact on efficiency. Modulating mini-splits that defrost by reversing the cycle, taking indoor heat and applying it to the outdoor coil when low air flow is detected have the least impact on efficiency.

    While most simple models of heat pumps have a linear decline in capacity with outdoor temperatures, there is actually a bit of a squiggle in the capacity curve around the freezing point. Compared to their mid-30s F performance, most ducted heat pumps actually GAIN capacity as it drops below freezing, peaking in the mid-20s F, then falling off increasingly steeply once it drops below ~+15F (despite the fewer defrost cycles.)

  11. user-6740240 | | #11

    Thanks again for all the detailed information. I have been searching for such answers for a while now without much luck and you seem to be very well educated in the these areas.

    Perhaps you could help with one other issue that I am having a lot trouble with. This pertains to when there is a unscheduled recovery of 3-6 deg of house temp. These unico air handlers (2430 I series) will not allow the thermostat to run the boiler heat when the outside air temp is above the boiler changover set point. The units were put together so the hot water coil is before​ the condenser coil so I can not run both at once. I truly think I have researched all options after several calls to unico they say it can not be done and I just need to rely on the heat pumps for heat when outdoor temp is above the boiler changover set point. This seems absurd to me. I feel the T-stat should be able to run the aux heat whenever I think the heat pumps just are not giving enough heat. The one option I do have is to run the Air handler fan on low by powering the 'fan' terminal on the AH. I can now start the boiler heat without calling for heat from the AH. Even with the fan on low using the boiler the house temp rises a lot faster as apposed to using the heat pump.

    My a question is... How much efficiency do I lose when running these air handlers on a low fan setting as apposed to med . I do not use the High setting on these units because the of the noise level.

    These are the high velocity flex ducts (3" or 4" ?) with a ecm fan. The slow speed gives about 300 cfm and the med is around 625 cfm.

    I realize there are most likely several variables depending on my exact system. But I was just looking to see if you thought It was a bad idea to use this method for unscheduled recovery's.

    Thanks Again, Rich..

  12. D Dorsett | | #12

    The answers are all system and equipment specific. As a general rule, high velocity air is quite a bit less efficient than low-velocity air, and the heat pump efficiency varies with cfm over the coil as well as the indoor to outdoor delta-T, humidity levels, etc. There are no simple generic models, and very few well-designed more complex models. At some point it's going to be a judgement call.

  13. user-6740240 | | #13

    ""As a general rule, high velocity air is quite a bit less efficient than low-velocity air, ""

    I did not know that one... :)

    I think I might be getting a grasp on it. low velocity air will pick up more heat from the coil wich is good for this situation but these small flex ducts are not efficient at moving low velocity air to where I need it which is bad for this situation...

    Thanks, Rich..

    Just a note for the green side of things. These discussions prompted me to revisit the solar idea. My biggest problem with the solar was In the event of power loss (from the grid) wich happens often around here in winter. If I spent the money on solar I would expect it to work when the grid goes down.. We are considering a whole house generator to the tune of 10k plus. It appears that money could be better spent. I did find that Telsa has recently purchased Solar City and introduced some interesting battery alternatives. So we have the rep coming out. I realize we will not get "whole house" power during the power outage but if we can get the essentials I would be happy with that.. I think 10k will buy me a few batteries.... :)

  14. Expert Member
    Dana Dorsett | | #14

    The hit in efficiency with higher velocity is all about the air-handler power. At a higher velocity it takes more blower power to overcome the friction of the ducts. At the first-order most dumbed-down approximation impedance of the duct increases as a function of the cube of the velocity, just like aerodynamic drag does. Twice the velocity takes more than twice the power and more than 4x, it's more like 8x the power. The real numbers are actually quite a bit different than that when all duct factors are included, but that's the gist of it.

  15. GBA Editor
    Martin Holladay | | #15

    For use in an emergency power outage, it's better to spend $2,000 on a 5-kW gas-fueled generator than $5,000 or $10,000 on batteries and an inverter. The main reason is that the generator can produce 5,000 watts indefinitely (as long as you have gasoline). The battery will run down quickly when loads are high.

    For more information on this issue, see Batteries for Off-Grid Homes.

    -- Martin Holladay

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