Help selecting a heat pump
Hi Guys,
I’m going to be meeting with my builder this week, and he wants to show off the furnace of his choice, which is a Dettson Chinook. It’s a small modulating natural gas unit, that can be paired with their Alize heat pump.
It’s a pretty neat unit, designed to allow for very long cycle times, often measured in hours. The problem I have is that the heat pump portion kind of sucks in cold weather, it uses proprietary small high velocity ductwork, and a proprietary thermostat to control it all. It also has a short~ish warranty.
I’m really trying to hit Net-Zero. I’m expecting load at 99.6% design temps of -20C to be a little under 27k BTU.
I think I’d like to pitch two different units to the builder, and I’d like to hear what you guys think.
1) Trane XV18, 3 Ton, HSPF of 10. The Trane XV18 series is rated as a cold climate air source heat pump by the NEEP. The unit will put out 20k BTU at design temps, and would modulate to perfectly match the load and carry the house down to -12C, and happily continue running down to -25C. The HOT2000 application I’m using for modelling predicts I’ll use about 50 kWh of aux heating in the average year. I think I can afford the $5. Equipment cost is around $6k CDN + install.
2) Mitsubishi Zuba Central, also a 3 ton. HSPF of 11. Thanks to Mitsubishi’s hyper heat, this thing will happily cover the entire heating load well below design temp, but it doesn’t have as large a turn down ratio, so I think it’ll end up cycling a lot more. Minimum output is 18k BTU, 2/3rds of design temp. Equipment cost is about 8k. The Mitsubishi puts out more heat at low temps, and does it with a higher COP, but is a bit more expensive, and doesn’t modulate as well.
My builders concern is that “with these heat pumps the aux strips will come on, and you’ll spend a lot of money”, but I don’t think that this is true, especially with the Zuba unit. What do you guys think?
Minisplits are out, because the lot is tight, so the house is built up instead of out. There are three floors plus a basement that need heating, and a Mitsubishi miniplit starts at around $4k each, installed, making my HVAC a lot more expensive, really cutting into my heating budget, and that still wouldn’t really get my great coverage on the floor where most of the bedrooms are. I’d rather spend the extra $8k on my solar array.
I’m leaning the Zuba central unit, but I’m concerned that I won’t get the rated performance, as most of the shoulder season, 18K BTU is going to be too much, and during the cooling season, 18K is going to be a lot. 18k is my cooling design load. I will have a heat pump water heater to help a bit with dehumidification. The Trane unit however, will modulate down to 8K BTU in heating or cooling, so the house will probably be more comfortable, and I’m more likely to get the rated performance with nice long cycles.
Thoughts?
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Replies
Stephen,
Assuming that you have accurately described the situation, I would go with the Trane. But the choice is yours, of course.
Many heat loss calculations include errors, resulting in a design heat load that is higher than it should be. If your design heat load is significantly under 27,000 BTU/h, and I'm guessing from experience that it might be, then the Mitsubishi Zuba -- which you say has a minimum output of 18,000 BTU/h -- will be short cycling for most of the winter.
>>>My builders concern is that "with these heat pumps the aux strips will come on, and you'll spend a lot of money", but I don't think that this is true, especially with the Zuba unit. What do you guys think?<<<
With the hyper heat at least, not familiar with the Trane, there are no heat strips, at all.
Hi Martin,
How bad is short cycling on performance? It's not going to be causing the same kind of overshoot that you get with a furnace, where it's blasting 60k BTU in a house needing 20k BTU. I imagine we'll have a lot more scenarios where house needs 12k BTU, and we're delivering 18k BTU. Is there some way to work out how long the cycles will be in a given scenario? Surely if the heat pump is on 2/3rds of the time, it's going to have reasonably long cycles? Almost all of my heat demand is in January and February where the average temps are around -5C, which is most of the way to design temps, thankfully.
Edit: Part of the attraction for the Zuba is the substantially better COP at low temperatures. At 5F, the Zuba has a COP of 2.5, whereas the Trane has fallen to 1.9. That's a fair bit of cold weather performance to give up.
Thanks Jon.
I'm re-running the analysis, and I might actually be better off foregoing the triple pane windows, and spending that money on a bit more solar panels. The window upgrade costs $8k, and saves about 1000 kWh / year, but the $8k buys me another 3 kW of solar that provides 4200 kWh / year. Assuming of course that I can get the panels to fit on the roof.
That would increase my average heat loss, and match up with the Zuba a bit better.
Man. Building is hard. Everything is a compromise.
Note that well insulated/sealed houses have high thermal mass in relation to their peak heat load. So even significantly % over sized heat sources can run for the recommended 6+ minute run time without causing excessive temperature variation. Picking numerous assumptions, I calculated that 9K BTU/hr more than the load is OK.
The current house load vs the minimum modulation rate effects the duty cycle. But duty cycle has little to do with the minimum run time. With the wrong thermostat, even a duty cycle of 2/3 could cause horrible short cycling. A 2x design load heat source (an example, not a recommendation) might always have sufficient run time to be efficient, even in the mildest weather.
IMO, "don't oversize" can be taken too far, creating greater risks of under-heating, slow recovery and shortened equipment life due to greater hours/year - while not offering a significant efficiency improvement.
Even when the AUX heating strips engage the heat pump is still providing the lion's share of the heat at a COP >1, and if sized reasonably the impact on average seasonal efficiency is pretty low.
An "extra" 18,000 BTU/hr isn't going to result in rapid temperature overshoots during loany more than sun streaming through the window. The thermal mass of the house and the hysteresis/differential in the thermostat (which can often be programmed) determines the minimum run time.
Hi Dana,
That's Interesting.
That would push me towards the cheaper, higher modulating unit, would it not?
--Steve
Indeed it might!
I looked into the Dettson system but didn't end up choosing it, mostly due to contractor unfamiliarity. It doesn't sound like that would be an issue for you since your contractor is actually pushing it. I am disappointed that their product line doesn't seem to be catching on. They seem to be addressing the problem of design for efficient houses which we criticize the mainstream HVAC companies for ignoring.
Their duct system borrows some elements from the high velocity small diameter systems, but they don't consider it to be one. It is more of a medium velocity, medium diameter system. The trunk lines are 6-8" diameter with 4" branch lines to each supply duct. They design their air handlers to work with higher static pressure, about double that of a traditional furnace when the system is operating at full airflow. However, since it modulates, it doesn't operate at full airflow very often. They claim that their system is better at getting heat to distant rooms under part load than a conventional low-velocity, large diameter duct system. The claim seem credible to me but I don't have personal experience with it.
If you go with one of the heat pumps you mention, would that mean you don't need natural gas at all?
Hi Reid!
Ya, the Chinook looks like a really great unit, as a little furnace. Fully modulating from 40% of 30k BTU, and the idea is at even lower loads, it relies on the heat pump to cover it, so, you can get anything between 3k BTU and 30k BTU, in increments of around 100 BTU.
The downside is that the heat pump component sucks. 1.5 COP at -15C compared to 2.7 COP at -15C for the Zuba. It also comes with this cool little zoning contraption, that lets you create up to 4 zones, each with their own 2 stage thermostat, and the furnace / heat pump modulate themselves to deliver air at a set temperature, at a set pressure, and the air flow will figure itself out.
The Zuba would completely cover all the load, but it's more expensive I think, though the energy performance is a lot better. It also doesn't modulate down as an AC unit, so minimum AC from the Zuba is also 18k BTU, so I'm a bit concerned that it would be a bit high for the cooling load, whereas the Chinook unit would deliver greatly modulating AC.
If the 99.6% temperature bin is -20C, what are the 99% and 97% bins?
When using a heat pump with auxiliary heat strips (the XV18 even stages the heat strips BTW) it's still important to have the load at the 97th percentile temperature bin completely covered by the heat pump to avoid excessive reliance on the heat strips.
Without looking up the specs myself, there may be some apples vs. pears comparison going on. I doubt that the ZUBA delivers a COP of 2.7 at -15C at anything but it's LOWEST modulation level, and I suspect the 1.5 COP for the XV18 is what it delivers at maximum speed. The relevant comparison would be the COPs when modulating (or cycling) at your actual load level at -15C.
Most ducted heat pumps deliver a COP of about 1.5 at maximum speed at -15C, some of the Mitsubishi and Fujitsu ductless are in the ~1.8-2 range running flat-out at that temp (not counting defrost cycles / pan heater power.)
99% bin is -16.7C. The Zuba covers this easily. The Trane definitely does not. The Chinook heat pump won't even run at that temperature, so I'd be running completely on natural gas at that point.
I don't know the 97% bin, nor how to find it. I'm close enough to Toronto, Ontario that I've been using their bin data, found here:
http://cms.ashrae.biz/weatherdata/STATIONS/716240_s.pdf
But I'm not sure if I see a 97% bin in there?
For performance data:
The Zuba delivers a COP of 2.54 at 5F, at it's minimum modulation of 15k BTU, or a COP of 1.82 at 38k BTU.
The XV18 delivers a COP of 1.92 at 5F at it's minimum modulation of 16k BTU, or 1.95 at 20k BTU.
The Chinook heat pump is the terrible one, that delivers a paltry 7200 BTU Max modulation, at a COP of 1.5, and that's not even at 5F, it's a bit warmer than that.
At low loads and temperatures, the Zuba rocks, and if you need the heat, it'll deliver it, and at a similar COP, that is nearly twice as much heat! It'll also keep working a lot colder.
How big a problem is it to a have the Zuba with a 18k BTU minimum AC delivery, and a house with a design load of 18k BTU. I mean, I guess it's not massively oversized... but it's not going to dehumidify like the Trane would, modulating all the way down to 8k BTU cooling...
For "only" USD$130 you can get all the temperature bin data you ever dreamed of on ASHRAE's Weather Data Viewer DVD, Version 6.0:
https://www.ashrae.org/resources--publications/bookstore/climate-data-center
Competent heat pump contractors would have (and use) that information.
Most Mitsubishi heat pumps can be operated in "DRY" mode if the latent load is getting ahead of the sensible load. The fact that the Zuba's minimum modulated cooling output at test conditions matches your load (at a substantially different set of conditions) doesn't mean it won't be able to keep up with your fairly modest latent cooling load too.
The test conditions for the Zuba are:
Indoor: D.B. 80°F (27°C), W.B. 67°F (19°C); Outdoor: D.B. 95°F (35°C), W.B. 75°F (24°C).
My interiors set point is lower, but so are my outdoor temps.
As I mentioned before, we will have the heat pump water heater to help with the humidity as well, and potentially can run a standalone dehumidifier as well.
I was also planning on zoning off the upstairs, to keep the upstairs cooler in the summer, and the basement cooler in the winter,, at least at night.
Any chance someone wants to share what my 97% temp bin is for Toronto? :)
The 1% outside design temp for Toronto is 84F, 11F cooler than test conditions. Your indoor design temp in summer isn't likely to be 69F (11F cooler than 80F), though 77F (25C) would probably work for most people, maybe 24C/75F. The Zuba's minimum output at the smaller temperature difference (7F to 9F instead of 15F) will be more than 18K. But that's still OK if you're running in DRY mode when outdoor dew points are in the 18C or higher range if the HPWH isn't handling the latent load sufficiently .
I'd share the 97% bin-temp info for Toronto if I had it (but alas... :-( )
I don't know how the data is organized, but there is 10 years of weather data for many locations in Ontario in this file, and you may be able to dig it out of here (Caution- it's a 296 MB file- could take some time to download over slower connections.):
ftp://[email protected]/Pub/Engineering_Climate_Dataset/Canadian_Weather_Energy_Engineering_Dataset_CWEEDS_2005/ZIPPED%20FILES/ENGLISH/CWEEDS_v_Historical/ONTARIO.zip
The ASCII text file explaining it is small enough though:
ftp://ftp.tor.ec.gc.ca/Pub/Engineering_Climate_Dataset/Canadian_Weather_Energy_Engineering_Dataset_CWEEDS_2005/ZIPPED%20FILES/ENGLISH/CWEEDS_v_Historical/CWEEDS%20documentation_Release9.txt
I can find reference to a "dry" mode in the thermostat linked to the Zuba, but I don't see any mention of it directly related to the Zuba, as a mode it has.
I'll comb through the data tonight.
Talked with the builder today.
He's OK with the Mitsubishi units, and is going to look into them further. My wife doesn't want a split unit on the wall, but we can use the air handlers and concealed duct units, to create two zones.
The nice thing about the Zuba-Multi system is that it modulates much better! It's not *quite* as efficient, but it still runs down to -30C, and I get nice zoning.
The other plus side, apparently the building envelope upgrades are significantly less expensive then I thought. Going for R20 walls to R35 will only cost about $3k, Another $6k gets me triple pane windows! It's all coming together!
--Steve