Hydronic heating in 4-season vacation home?
Hello, I am looking for some guidance related to heating and cooling a 4-season vacation home near Ottawa, Ontario, Canada.
Details:
– We plan to build a 1300 square foot bungalow and insulate it fairly well.
– There will be lots of north-facing windows.
– We will build slab on grade.
– We expect to use it a lot during the summer months, but realistically, only two weekends a month the rest of the year.
– For heating, we are considering installing pex tubing into the slab and using a propane-fueled combi boiler (closed looped system) with glycol for in-floor heating and domestic hot water.
– For cooling, we are considering using a cold climate heat pump, like a Senville, since it can (allegedly) heat and cool down to -30C at 75% efficiency, and pairing that with standard central air ductwork. (I’d rather not use wall mounted units.)
– We plan to install an electric fireplace, but it will be mainly for cosmetic purposes.
– We will install an HRV
– Propane is cheaper than electricity in my area
Questions:
In the winter months, how would you recommend heating this place?
Considering that we will only use the place every second weekend, and that we like the comfort of hydronic floor heating, how low should we set the thermostat ? (I plan to buy a wi-fi thermostat so I could control it from home.)
I suppose my main question is, what would consume the most energy?
1) setting the hydronic to 10C for the majority of the two weeks, but raising it to 20C on the Wednesday before we head there for the weekend.
OR
2) setting the hydronic to 15C for the majority of the two weeks, but raising it to 20C on the Thursday before we head there for the weekend.
OR
3) setting the hydronic to 18C for the majority of the two weeks, but raising it to 20C once we arrive on Friday.
I know that the lower we set the ambient temp, the longer the slab will take to heat up. And if we want to enjoy the hydronic heating when we arrive, we will have to preheat it for some time before that. I am wondering what the ideal thermostat setting is for when we are not there, in terms of propane fuel consumption in a two-week span. As in, at which temperature setting does the “start-up” fuel consumption to heat the slab from 10C to 20C equal he fuel consumption of running a higher temperature, closer to 20C?
Additionally, electricity rates are pretty high, so I do not want to depend on the heat pump as a primary source, but would gladly use it as a secondary source to take the chill out of the air.
Any thoughts or guidance on this would be greatly appreciated.
Thanks in advance!
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Replies
>"setting the hydronic to 10C for the majority of the two weeks, but raising it to 20C on the Wednesday before we head there for the weekend. "
There is no need to give it a TWO DAY head start. Most slab heating can recover from a 10C setback in well under 12 hours.
>"...electricity rates are pretty high, so I do not want to depend on the heat pump as a primary source, but would gladly use it as a secondary source to take the chill out of the air."
In most locations heating with a cold-climate mini-split would be cheaper than condensing propane. Both Fujitsu and Mitsubishi have modes that allow setting them to 50F/10C- I'm not sure if Senville has a comparable feature.
Getting to the optimal solution requires running the heat load numbers at the 99% outside design temperature, at indoor temperatures of both 10C and 20C. Without the load numbers it's not possible to optimize the design of either a radiant OR a ducted/ductless minisplit solution.
Hi Dana, thanks very much for your reply!
I suspected it would not be possible to find the optimal solution without testing the system. Oh well...
You mention in most locations, a cold-climate heat pump would be cheaper to run than propane. Does that hold true in Ottawa, Ontario, where on some days it can get to -30C or even to -40C on rare occasions? I’d hate to rely solely on a heat pump if on those extreme days, it can’t heat my home. Is there a cold-climate heat pump that you know of that would be capable of such extremes? My fear is that my pipes will freeze if we end up with a few consecutive days of cold extremes.
What would you do in my situation?
Thanks very much!
>"What would you do in my situation?"
For starters, I would run an I=B=R type heat load calculation on the house, using +10C as the indoor temperature, and (in Ottawa) -22C as the outdoor design temperature, and design the slab radiant to be able to deliver that with some margin:
https://www.greenbuildingadvisor.com/article/how-to-perform-a-heat-loss-calculation-part-1
https://www.greenbuildingadvisor.com/article/how-to-perform-a-heat-loss-calculation-part-2
Since the house is as-yet unbuilt, you have control over what the actual heating loads will be. If the house is going to be superinsulated it may not even need a heating system for freeze-control if designed and implemented well.
Then I would run the numbers using +20C indoors, -20C outdoors and find heat pump that could support that load, or at least 90% of that load at -20C, using NEEP's cold climate heat pump database search:
https://ashp.neep.org/#!/product_list/
I'm not up on what current code minimums are in Ontario, but a 1300' 2x6/R20 cellulose house with R35+ in the attic and low-E double panes and an R15 foundation could easily come in with a load under 15,000 BTU/hr at an indoors to outdoors temperature difference of only 40C (10C indoors, -30C outdoors / 20C in, -20C out.) At a temperature difference of only 32C (10C in, -22C out) it will be proportionally lower, or under 12,000 BTU/hr. There are several ductless heat pumps out there that can deliver that much at -22C with plenty of margin. But run the numbers to be sure.
I might start looking at only Fujitsu & Mitsubishi, since they can be set to maintain +10C indoors, and would use a wall-thermostat remote with Wi-Fi for monitoring purposes. Others may have the capability for a 10C setpoint too, but most don't.
Operationally, heat with the heat pump whenever possible when unoccupied. If the heat pump is losing ground, also turn on the radiant floor too (with a Wi-Fi thermostat). You should be able to figure out during the first cold snap how cold it has to be before the heat pump doesn't keep up, and be a bit more proactive going forward, turning on the floor when the overnight temps are predicted to drop below that point.
If warm floors are important to you from a comfort point of view, while you're there, run the floors off a floor thermostat set to something comfy-cozy but not foot-roasting, say ~ 23C and let the mini-split maintain the room temperature at 20C. With ~1000 square feet of active 23C floor in a 20C room the floor would be delivering about 10-11,000 BTU/hr, which could be 100% of the load when it's not super cold out. But bumping the room temp to 21C with the mini-split would drop the floor's contribution to only ~7000 BTU/hr, and would probably cost less than heating to 20C. At room temp of 22C the floor's contribution would be about 3500 BTU/hr with the mini-split carrying a bigger share of the load, which is probably the cheaper way to go when it's above 0C.
BTW: A low mass combi-boiler would be ridiculously oversized for the heating load of a code-min 1300' house, and is likely to short cycle even on a radiant slab. A condensing tank type water heater with an external heat exchanger for the heating loop is probably a better choice.
Dana, I can’t thank you enough for taking the time to give me such a detailed response. This is incredibly helpful to me, so thank you for doing that. You’re the man!
One last thing, if you don’t mind...
I would prefer not to use any wall mounted or ceiling cassettes with the outdoor unit. That said, is it possible to use the outdoor Fujitsu/Mitsubishi unit and set it up to work with ductwork in a configuration that is typically used in a conventional HVAC layout? That way I could have both central heating and central air, but with a more efficient outdoor unit. I’m not finding much on this configuration in my web searches. But I’m assuming that if it is indeed possible, I would need an outdoor unit, an indoor unit(?)/blower(?)/device(?) which would reside in the mechanical room, and be connected to the household ductwork. Anyway, if you could point me in the right direction one more time, I’ll mail you a beer!
>"I would prefer not to use any wall mounted or ceiling cassettes with the outdoor unit. That said, is it possible to use the outdoor Fujitsu/Mitsubishi unit and set it up to work with ductwork in a configuration that is typically used in a conventional HVAC layout?"
Both Fujitsu and Mitsubishi have mini-ducted cassettes, and ceiling units, but SFAIK no single zone units for ducted or ceiling cassettes that would have a PAN HEATER in the outdoor unit for managing defrost ice build-up. Defrost ice management would be extremely important to have in your situtaion, since you won't be there to monitor and manually de-ice if the pan drain develops an ice plug, which though somewhat rare, can happen.
Midea has a series of single zone mini-duct cassette units that DO have pan heaters, and might be a better bet eg:
https://ashp.neep.org/#!/product/26537
https://ashp.neep.org/#!/product/26535
In the US Carrier has been re-labeling and supporting some of the ducted Midea units with defrost ice management pan heaters:
https://ashp.neep.org/#!/product/26450
I'm not sure whether the cold climate ducted Midea/Carrier units can be set to 10C while you're away, or whether you'd have to kludge up some other means of achieving that. You may be able to dig up an online manual.
>"Anyway, if you could point me in the right direction one more time, I’ll mail you a beer!"
Has the twitter-in-chief slapped a tariff on beer yet? :-( (My friends in NS told me to bring a case or three next time I come up, since as of last summer it was noticeably less expensive to buy it in the States.)
If it gets any weirder here maybe I'll pull up stakes move to Ottawa to collect on that beer. :-)
Hi Dana,
So, I was on the Mitsubishi website and they have an interesting product called the ZUBA. According to them, it is specifically designed to handle Canadian winters, providing heat down to -30C. Additionally, it has a defrost mechanism and can be equipped to run centrally with an Air Handling unit and ductwork. Furthermore, the unit can be equipped with an auxiliary electric resistance heater in instances of extreme temperatures, essentially eliminating the need for a back-up heat source during a cold snap. Lastly, apparently the Zuba-central system can be zone controlled, which is something that I thought only the mini-splits were capable of. I'm guessing the zone control feature may be nothing more than the dampers automatically opening and shutting, but still. This system sounds perfect for me!
Are there any obvious flaws with this? https://www.mitsubishielectric.ca/en/hvac/professionals/zuba-family/Zuba-Central
Brochure: http://cdn.agilitycms.com/mesca/productdownloads/mem-201711-e-zuba-family-brochure-single-page-final.pdf
Finally, in light of the Zuba news, and based on all the comments in this thread, I am leaning toward going with the Zuba and dropping the radiant hydronic floor heating altogether. Although, in consolation, I will probably install electric under-floor heating in both bathrooms. Since we plan to use the home only two weekends a month during the winter, this proposed home-heating/cooling solution has to be cheaper to run overall; plus the fact that I will have only installed one heating system, not two.
Does this seem like a reasonable solution to you?
I'm going to need an address to mail you that beer!
Thanks!
Lots to cover there!
For one, a high quality cold climate heat pump will cost very close or even less (as is the case in my area, Belleville) to run (yes, even with Ontario's ridiculous hydro rates) than propane in our climate. Especially as our carbon tax continues to increase the cost of propane.
It does take a while to bring a house up from 10°C but not days. A temperature setback will always be cheaper than maintaining a steady temperature if your fuel source is not a heat pump. Heat pumps are different. But if you have a boiler heating the place, the larger the temperature setback, the more you will save. Keep in mind that thermal expansion of all the different materials in your house with really large setbacks can play havoc with trim joints, doors, and other stuff.
If your primary reason for choosing hydronic radiant floors is the "warm floor" feeling, you will be disappointed. With the minimum insulation levels required by code in Ontario and a regular sort of bungalow, the heating load is low enough that the floor temperature never reaches that "barefoot friendly" feeling. If it did, the house would overheat! There are other great reasons to choose radiant hydronics but not that one.
I'm not familiar with the Senville brand and their technical specifications are hard to find. But I would triple check the performance at cold temperatures, heat pump manufacturer brochures love to mislead with clever wording about cold weather performance. I saw a brochure for Midea boasting about energy efficient performance down to -30°C, but when I dug into their technical specifications, the COP at -30°C was 0.8! That's worse than electric baseboard!
If you won't be there often in the winter, I wouldn't recommend wasting money on two heating sources. You don't need a heat pump if you have a boiler and vice versa.
Better yet, find a good hvac designer and have that person help you through all these decisions. :)
Thanks very much for your reply! I have asked a follow up question to Dana above, and since you and I have very similar climates, I would be very interested in getting your opinion as well.
Additionally, if I do as you say, and choose one heating source only, I would likely go with a cold-climate heat pump, because as you know, our summers can get pretty hot. That said, is it crazy to go this route with the added consideration of possibly adding solar and/or wind power to the system in the future as a retro fit into the mechanical room? That way, I could use the battery first and the grid when the battery taps out.
Any thoughts on this or links that could point me in the right direction?
Thanks very much!
As far as cold climate heat pumps go, you don't really do need a backup heat source in your climate. The design temperature is -17°C and a good heat pump like a Mitsubishi FH is rated to -25°C with a listed cut out temperature of -28°C. It very rarely drops below -25°C (Obviously not counting the wind chill), just take a look at the historical data online. If you were still worried about things freezing if you had a very unlikely sub-30°C storm that lasted days, you could install some electric baseboard to keep the house above freezing in the event of the heat pump turning off (or breaking down, good insurance if you don't live there).
If you intend on connecting renewable power to your house, electric heat makes sense for sure and a ducted heat pump is probably your best way of doing it.
It's great to be working on all this early in the design stage, while you can still adjust system and building parameters easily.
Hey, I really appreciate the comment. Thanks for taking the time to write it. I found the Mitsubishi Zuba on their website and it sounds like the perfect solution, so much so that I may drop the hydronic heating altogether and just install under-floor heating in both bathrooms.
I wanted to avoid a multi-zone mini-split situation but still use the heat pump, configured to work with ductwork instead. This system can do that:
https://www.mitsubishielectric.ca/en/hvac/professionals/zuba-family/Zuba-Central
Consider a crawlspace design. This will heat up faster, has less embodied carbon, and IMO, looks and feels better. It makes it easier to run ducts if you want AC. It can allow a warm floor without hydronics.
I'd also consider what it would take to safely leave the place unheated when you aren't there. Less energy use and heating systems fail - without freeze proofing this will cause big problems.
Edit: my furnace failed last night. Luckily I'm here to notice and fix before pipes freeze.
I appreciate the recommendation, however, a crawlspace is not an option due to flooding in the area. Without a crawlspace, we are safely 5-6 feet above it. So, slab on grade it is!
To determine whether propane is cheaper than electricity via a cold climate heat pump, you need to know:
Cost of propane per gallon or liter.
Cost of electricity per kwh
Efficiency of propane heating source
Average COP of heat pump.
A kwh is 3412 btus
A gallon of propane is about 91,000 btus
Here in Maine, propane is about $2.50 per gallon
Electricity is about 18¢ per kwh
A 90% efficient boiler means a gallon of propane puts out about 82000 btus.
A heat pump with COP of 3, puts out about 10,000 btus per kwh.
So it takes about 8 kwh ($1.44)to match the output of a gallon ($2.50) of propane. Propane might be a bit more expensive, since we haven't considered the cost of electricity that's powering the propane system.
> A heat pump with COP of 3
I'm curious - do you have good data that indicates that this is a realistic heating season COP in Maine?
A COP of 3 would be consistent with in-situ measurement done by the NEEA folks in zone 6A parts of Idaho and Western Montana almost a decade ago. See Table 9 and Figure 4:
https://neea.org/img/uploads/e14-274-dhp-final-summary-report-final.pdf
Given the incremental efficiency improvements that has taken place since 2010 (the test year) I would expect a seasonal COP of 3.0 or slightly better for a 6A Maine location.
I have to ask why select Hydronic heating system for a building that will be mostly unoccupied in the heating season?
Yes hydronic system has its advantages but has a few disadvantages.
1 If you later want to have AC it will require its own duct work hard to do and a huge additional expense.
2 Hydronic systems are much slower at raising the temp of a cold house something it seems you are likely to do often.
3 Pipes leak sooner or later. Leaks damage things. Why take that risk for so little reward?
Walta
Thanks for the comment. And I agree with you. I am leaning toward a heat-pump solution and no hydronic at all. As a consolation, I may install electric under-floor heating in both bathrooms to enjoy while we are there.
If your reason for heating the the floor is simply cozy feet for the 2 weekends a month I would think about just doing electric cables like Nuheat. Granted its electric resistance and not the cheapest to run I would think the limited amount you would be using it, paired with the much quicker response since it can be installed directly under the flooring materials, would end up pushing the break even point way out to the point where it wouldn't matter.
Have you priced out the hydronic option yet? All the equipment, controls, pumps, labor, etc can cost a lot upfront just to keep your feet "not cold".
The nu heat is more or less plug and play and could be set up in only the rooms you frequent. Plus, in a well insulated house it will offer some emergency space heating.
Just my thoughts...I have both electric cable and hydronic floor heat in my house and if I had a time machine I would ditch the hydronic without a second though.
Jon