Check my logic - yearly cost calcs

Discussion in 'General Discussions' started by Nicholas Bethard, Mar 29, 2021.

  1. Nicholas Bethard

    Nicholas Bethard New Member

    So I'm trying to take at least a rough look at what GSHP might save me vs. ASHP+ 97% propane furnace and my numbers aren't as encouraging as I was expecting. I'm wondering if there is some flaw in my logic. Please critique.

    1) Get heating and cooling loads from Manual J
    2) Divide the heating/cooling loads by the difference from setpoints to design temps. This gives BTUH/degree.
    3) Multiply by 24 to get BTU-day/degree.
    4) Look up degree days on degreedays dot net. Base temp for HDD is 60 (figure decent envelope and internal loads to get to a set point of 70), base temp for CDD is 65.
    5) Also look up HDD to a base of 20 degrees. This is to figure how many degree days a high efficiency ASHP will need the propane furnace to take over.

    For figuring running cost of a high efficiency ASHP+propane furnace,
    Cooling - (BTU-day/degree * CDD) / 18 SEER / 1000 w/kW * $0.105/kWh
    Heating (ASHP) - (BTU-day/degree * (HDD - HDD20degree)) / 10 HSPF / 1000 w/kW * $0.105/kWh
    Heating (Propane) - (BTU-day/degree * HDD20degree) / 88756 BTU/gal * $1.99/gal

    For figuring running cost of a high efficiency GSHP,
    Cooling - (BTU-day/degree * CDD) / ave. 30 EER / 1000 w/kW * $0.105/kWh
    Heating - (BTU-day/degree * HDD) / ave. 4.3 COP * 0.000293071 BTU/kWh * $0.105/kWh

    For a 4097 sq. ft. house I'm only coming up with $570/year in savings for geo. What am I doing wrong?
     
    Last edited: Mar 29, 2021
  2. gsmith22

    gsmith22 Active Member Forum Leader

    use looplinkRLC or get the book I mentioned in your other thread. that is what you are doing wrong :) You have attempted to vastly simplify what is a massive number crunching exercise that is highly dependent on the number of hours your equipment runs and when it is runnning. And all equipment will run more at colder/warmer temperatures then it will at moderate temperatures in comparison to your interior temp. To complicate further, as winter drags on, your entering water temp (EWT) is going to drop, so not only will your equipment run longer because of the exterior to interior temp difference, but it will also have to work harder because the incoming water temperature has fallen too. Same thing happens the opposite way with summer and cooling. The simplification is leading to why you can't estimate it properly. I can't possibly describe the procdure here. Best I can do is say trust LooplinkRLC or get the book and read about how to do it.

    This procedure estimated my pre-geo propane use as $7300/year + my elect use for ac as $350/yr and my geo electric use (heating and cooling) as $1800/yr. All of these numbers were damn near exactly what I experienced the 1 year I owned this house before deciding to switch to geo. I have no idea how the prior owners spent upwards of $7k/year on fuel for 30 years.
     
  3. Nicholas Bethard

    Nicholas Bethard New Member

    Holy hell $7k/yr in LP is insane. I'm nowhere even close to that... Maybe $2-3k/yr AT MOST in LP if I include my very leaky outbuilding with a single stage 80% furnace. That said, my actual elec usage is way higher. Need to sort out why...might not be tied to the HVAC system...

    I'll look at Looplink...hopefully it'll make more sense than the oversimplified thing I tried to do. I understand I'm oversimplifying with my current logic but I'm just trying to land on some level of monetary justification to go GSHP over ASHP+propane furnace. The two stage GSHP+propane system already quoted is roughly the same price as the VS ASHP+propane system (AFTER tax credits and rebates), but I know when I get the VS GSHP quote later this week it's going to be higher so there needs to be justification.
     
  4. gsmith22

    gsmith22 Active Member Forum Leader

    yes it most certainly is. My house came with a buried 500gal propane tank that was leased from Suburban. They end up with a monopoly on your property - you can only buy from them at an absurd $/gallon and they refused to sell the tank to me. Its apparently a common thing around here (I had natural gas in my prior house). I looked around at possibly buying my own tank to bury and having Suburban take theirs but all of that would have been sunk cost to the tune of like $5k that I decided was better put to use in something more efficient. I should have added, my analysis included $3/gallon propane (which I was averaging from Suburban) and my elect rate is $0.13/kWh. I did all this too when the 30% federal tax credit was available - a little less now at 26%. My house is probably typically 90s weather sealed/insulated so not great but not terrible either. I plan to do air sealing which will likely reduce geo usage further in the future. Being variable speed, equipment just adjusts on its own.

    Edit: I should add that Looplink should do a comparison with alternative heating/cooling sources. I didn't use it for my analysis but the book method I used included it. I'm assuming looplink also does. Typically, if you are on propane and don't have access to natural gas, geo is competitive when looked at over ~10yr time horizon. If you are only concerned with upfront cost, then geo won't compete at all.
     
  5. Nicholas Bethard

    Nicholas Bethard New Member

    I'll poke at Looplink later today and see if it makes more sense of the numbers than my over-simplistic approach.

    I'm still in awe at the amount of propane you were using. Even with an 80% furnace, $7200/yr. @ $3/gal works out to something like 175MBTU/year. A truly prodigious amount.
     
  6. gsmith22

    gsmith22 Active Member Forum Leader

    my house has separately ducted upstairs and downstairs zones. so I had two furnaces (now have two geothermal units). both furnaces were original to the house - 65% efficient equipment which apparently was high tech back in 1991 when they were installed. so very likely they were something less than 65% efficient. and they were both way oversized so lots of short cycling going on. both ac units (R22) were also original. upstairs ac unit broke 2 days after we moved in Aug 2018, blower fan on one unit broke in fall of 2018 (which I fixed with new fan) followed by a $7k winter of propane 2018-2019. Geothermal got planned early 2019, we limped through the 2019 summer using downstairs ac unit, and installed geo late summer/early fall of 2019. It was very uncomfortable in my house 2018-2019 and now it is heaven. Part of the reason my payback is so quick is because I had lots of "house money" to work with - we knew that we were going to be replacing all mechanicals regardless of which way we went so there was going to be like $20k spent (at least) no matter what we decided to do. The existing equipment was so inefficient it wasn't hard to save ongoing operating costs with pretty much anything new. and then there was the 30% federal tax credit added in too if we used geo. I also took the opportunity to put in a new hot water heat pump with buffer tank (which were on the geo install bill so 30% off that too) tied into both geo unit desuperheaters. Propane usage got cut to nothing (save for dryer) and we let that go for awhile until Suburban decided we had to buy at least $120 of propane a year in 2020 and I told them to come get their tank off my property in fall 2020. Obviously everyone's situation is different but I had a lot of tailwinds pushing me toward geo. If your propane usage and/or cost more closely mirrors natural gas cost, then that may be a nail in the coffin for geo. Cheap natural gas distorts everything. I also wanted an all electric house so I could create my own fuel with solar panels so there were lots of positive externalities associated with going geo too. Can't do that with propane or natural gas.
     
  7. Nicholas Bethard

    Nicholas Bethard New Member

    Wow that's really interesting. Luckily I'm not painted into such a corner and have plenty of options/flexibility, although the idea of moving away from propane is alluring to some extent.

    LoopLink is coming back with numbers slightly better than my oversimplistic approach, but not by a ton. Just looking at my downstairs system, once I adjust average EER and COP to match what LoopLink came up with, my approach's result is only ~$70/month higher.

    The picture it paints for a two stage GSHP system is cost to operate being more or less similar to a highly efficient VS ASHP. Given that those systems have similar up front prices AFTER all tax credits and rebates, it kind of seems like the two stage option doesn't make sense. For the VS GSHP, the cost to operate improvement is better, but I'm not sure it'll be enough to offset the additional upfront cost of the VS GSHP system in a realistic amount of time.

    The thing I'm not understanding about LoopLink's comparison is it gives a choice to select ASHP or Propane+AC, but no choice for ASHP+Propane. So I'm not clear on how its making up the difference for the days out of the year that an ASHP can't meet my heating needs...there's no way its just assuming electric strips because as a test I set the HSPF of the ASHP to 6 and the cost to operate the ASHP didn't shoot through the roof. I emailed LoopLink about it, we'll see what they say I guess. In the meantime, maybe I can find another tool to model the ASHP+propane operating costs.
     
    Last edited: Mar 29, 2021
  8. gsmith22

    gsmith22 Active Member Forum Leader

    Looplink may not be that sophisticated with respect to ASHPs. It is probably assuming the ASHP can do the full winter. I looked at my spreadsheets/book and I didn't see a methodology for a comparison to a dual fuel ASHP+propane system and that book is what Looplink is based on. I don't know if it breaks the propane use down by months, but you could probably assume Dec, Jan, Feb to be maybe 50% propane and the rest ASHP+ac. I didn't pursue the ASHP particularly in depth because I wanted to ditch propane and didn't think I could realistically get a ASHP for full winter time use without it resorting to electric heat for relatively long periods. My situation had a lot of room for error because I would have been spending so much on propane. Truth be told, Suburban sealed their fate with us. They showed up in May 2019 (after winter was over) and filled up our tank one last time (without us requesting a fill up). My wife was furious. Selling geo to her after that was very, very easy.
     
  9. Deuce

    Deuce Member

    Keep in mind that your GSHP will have an estimated operating life of approximately 28 +years, while a ASHP is 1/2 of that.
     
  10. gsmith22

    gsmith22 Active Member Forum Leader

    Deuce is totally right that any reasonable long term cost study should assume that you will replace the ASHP at least 1 time (ie double the cost of an ASHP install) in the timeframe that you own a GSHP. I would just point out to anyone reading that if cost is the only factor, GSHP may have already lost. Shopping for anything based purely on cost is likely to result in short term joy and long term disappointment. I can't tell you how much more comfortable nearly constant temperature and humidity at the inside of my home are compared to the up/down of furnaces and ac turning on/off (both at this house in the first year and my last house with natural gas furnace). A real benefit I noticed is that since there is no more combustion air required (we aren't burning anything), air infiltration went way down and my interior humidity was much more stable through the winter barely dropping below 50%. obviously an ASHP will have the same effect but a propane furnace will not. and the propane would be coming on at precisely the time that infiltration would be greatest (stack effect at its worst when interior to exterior temp difference is maximum).
     
  11. Nicholas Bethard

    Nicholas Bethard New Member

    Looplink responded and basically said their ASHP modelling isn't great, and certainly doesn't account for LP/NG backup. So I just used my method for those... It got me in the ballpark for the GSHP stuff as compared to LoopLink so it isn't miles off.

    I decided NOT to factor in equipment lifecycle. Some people say current gen advanced ASHP and GSHP lifecycles would be similar (north of 20 yr), some say otherwise. The answer seems 100% based on who you ask. Nothing concrete, so not banking my decision on that.

    That said, it looks like we might go VS ClimateMaster. It is a HUGE investment, but when hot water is factored in, it does eventually pay for itself compared to VS ASHP/LP, but not until somewhere around yr 18. That said, there is value in consuming less energy and being greener.

    Thanks again you guys for your thoughts and time on this.
     
  12. gsmith22

    gsmith22 Active Member Forum Leader

    I would definitely include desuperheaters with buffer tank. If you can swing it, I have had great success with my heat pump hot water heater tied into the buffer/desuperheater system. Remember, everything on the contractor' bill is applicable for the feeral tax credit. I basically get hot water from the desuperheaters all winter and most of swing seasons and heat pump hot water heater picks up the slack in the summer. History shows that GSHPs last a long time and that ASHP don't. So while the ASHP industry can make claims, until they produce a unit capable of lasting 20 or more years, I will remain a doubting Thomas.
     
  13. ChrisJ

    ChrisJ Active Member Forum Leader

    The Climatemaster Trilogy with I Gate tank does 100% of domestic hot water (DHW) and is a variable speed compressor.
     
  14. Nicholas Bethard

    Nicholas Bethard New Member

    Yup you got it. According to my calcs, even compared to a new HPWH (and replacing it again in 10 yrs.), this feature saves me something like $7600 over 20 yrs. Even more if my girls start taking stupidly long showers when they're teenagers.
     
  15. Noobie

    Noobie Member

    No slight to accountants or accounting intended, but it’s worth noting that there’s a difference between knowing the cost of something and knowing the value of something. I barely understood what we were undertaking with removing our oil burner and outdoor AC units and replacing with GSHP, but I could not be happier.
     

Share This Page