How can I calculate payback period of different HSPF/COP heat pumps?
Hillside_Ian
| Posted in Mechanicals on
First post. I did use the the search function but couldn’t find what I am looking for (even though it seems like it would be a very common question, so I probably didn’t search properly)
I got to GBA by accident! Just wanted to determine somehow what size central a/c unit I needed in my 70’s house and somehow ended up here! Now I am trying to grasp HSPF, COP and less important SEER for a heat pump! (I knew it was going to be a rabbit hole when I got here, I just didn’t know how deep!)
I live just outside Ottawa, Ontario but in Quebec in the country. Slightly colder. Approx. 5200 heating degree days I think. (Zone 6?) I have a 10 year old electric furnace (vari-speed fan) which uses 45 year old ductwork from an old (monster) oil furnace in a 1140 sq.ft. cathedral ceiling bungalow. Also a little used and minimally heated 900 sq. ft. poorly insulated walk-out basement. All windows double-glazed thermopane except 2 basement sliders. Newer ones argon filled. Average air-infiltration for this vintage house(went through door blower test/sealing and insulating/retest 10 yrs ago but can’t find results) No natural gas here.
Surrounded by deciduous trees. Will be shocked if I use a/c more than 25 days/yr but will now require it. More to reduce relatively high humidity here in this region. Bedroom and home office on West with little glazing; living space with most glazing on East. Currently have to dehumidify the basement in summer.
One contractor suggested 1.5 ton a/c, 2 ton heat pump. Seemed reasonable for a/c to me from the little I read here and lower than another contractor, by 0.5 ton on each and lower still than by another contractor. But should I worry about the best a/c sizing to avoid short-cycling in summer when I may not use it in cooling mode very often anyway?
I have electricity bills for the past 2 years only with annual kwh consumption and cost and can search and compare for those years down to an individual day I think.
So,,, how can I determine pay-back periods for different advertised HSPF values(which I now know don’t accurately reflect the actual HSPF for this region) for 2 ton? ducted heat-pumps?(Seems like a big difference in cost in some cases for a small increase in HSPF even within the same manufacturer).
Also, I see so many posts about Mitsubishi and Fujitsu and wonder if I should be considering anything other than them given my cold location and then what size I would need.
Don’t know if this is important but my supply air plenum starts at 15×8 at the furnace and at some point changes to12x8 before the 36′ run ends and all of the offshoot ducts to registers are all 4″dia.(all ducts are in basement)
(I have an old fireplace insert in the living room that I use on some of the coldest days and for ambiance some evenings. Don’t think it is very efficient but it does heat things up. I normally burn around 2 face cords of wood. I am too lazy to use wood more often.)
Lost in the weeds, deep down the rabbit hole!
Ian
Ian
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Replies
If you're willing to do some learning, you can try using BEopt (https://beopt.nrel.gov) which will let you model various aspects of your house and then run simulations using different equipment configurations.
Thanks Patrick! Never heard of it. Will check it out.
In your area, the heat load is what will drive the sizing of the heat pump. With the cheap electricity there, it might make sense to undersize the unit a bit with a strip heater backup for the coldest of days.
Since the unit will be sized for heat, it will generally be over sized for cooling, important to get a unit that has decent modulation. Once you figure out your sizing, I would dig through here for options:
https://ashp.neep.org/#!/product_list/
There are many mid static/furnace heat pump units out there, not just Fujitsu or Mitsubishi. Important is to select something that has service support in your area.
Since you were heating with electricity, it should be pretty straight forward to figure out your actual heat load and size the unit accordingly. I would not trust an installer's recommendation on this front. Follow the steps here:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
With cheap electricity, I doubt there is much return for a super high HSPF, but they are generally better for comfort (more modulation, hotter supply air).
If you decide to build a BEopt model make sure you find and watch the training videos and learn to input the cost and HSPF from your bids.
Walta
Akos and Walta thanks for the replies. Sorry for the late response, I have been moving my girlfriend in ;-) I do think I will slightly undersize the heat pump(there are existing baseboard heaters in the basement) and add insulation.