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Old 06-07-12, 03:04 PM   #1
Helen
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Default back up heat for mini split system

Hope to install an air to air mini split system in a 840 sq ft house, and need to also install a back up system for when it gets too cold to extract heat. This is an old, small house which is being renovated with new insulation for our New Brunswick, Canada weather.
Any suggestions?

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Old 07-12-12, 11:31 AM   #2
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Hi Helen,

You are on the right track to insulate and weatherize first. This is where you will see the greatest benefit and get the most out of a more efficient HVAC system.

Most mini split systems come with resistance heat as a backup, but I am sure this varies between manufacturers. If your unit doesn't come with resistance heat backup, you do have options. The lowest upfront and highest operating cost will be space heaters or baseboard electric heat. Baseboard heaters will run from $50 to $100 per room. Also keep in mind that you may want a new circuit put in just for these guys.

I imagine it gets and stays pretty cold up there, so it may make more sense to add a wood stove or pellet stove. I had a wood stove for several years and heated about 750 sq ft. A friend got an automatic pellet stove when fuel prices went up in 2008 and has liked it. The problem with a wood stove is that the area near the stove is toasty and the rest of the house is cool. This is particularly the case if the den is connected to the rest of the house with narrow hallways. Personally, I like this since I can keep the activity areas of the house warm and the bedrooms cool.

As with any combustion device, be sure that it has adequate fresh air and an adequate chimney so that you don't get carbon monoxide buildup. You can probably get a good wood stove for around $1000 and installation for maybe $1500.

And, of course, there is propane/natural gas. You can get wall mount heaters, but I would only use vented appliances. Un-vented propane or natural gas creates problems with carbon dioxide and water vapo(u)r and can still produce carbon monoxide. It looks like vented heaters run about $600 to $1500 each, so it gets expensive to put several in the house.

Try playing around with the calculators on builditsolar.com. In particular, look at the home heat loss calculator to figure out your heating needs for the coldest days and the fuel cost comparison.

Those are the "traditional" options, but you could look at other things such as radiant solar heat, or AC Hacker's small geothermal heat pump. Another idea that folks have been toying with is using solar to heat up the air around the air source heat pump. Others who have more experience with those may be able to comment.
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Old 07-12-12, 08:20 PM   #3
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How cold does it get where you live? What do the good builders and HVAC folk say is the winter design temperature for your area? a mini split may function perfectly well without any additional heating.
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Old 07-13-12, 02:06 PM   #4
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Insulation in old houses is the best thing you can do to make it easier to heat (and cool).
But don't forget the windows. Windows have such bad losses, I don't know why so many
houses have so many large windows.?.
Up north, large south facing windows are fine for the winter months..
But, just remember that a having a lot of glass in a room will counteract a lot of wall or ceiling insulation.
Windows are one of the big weak points in our house, and we have fairly modern double pane Harvey & Andersen windows.

We use mini-split heat/AC here and electric space heaters for back-up.
And have oil-heat for emergency back-up. (Which we will use if all else fails).
Last year was mild in the Boston area, so we never needed back-up at all.
Gas or propane would be my back-up choice, if we were to move..

Our Sanyo systems will still heat the house down to 8 deg F.
When it drops down below 10F, the power use increases dramatically.
But, around here it rarely stays below 10F for very many hours..
If it just dips down from 10 or 15F to 5F between 4 AM & 6 AM, we might not even notice.
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Old 07-18-12, 03:31 PM   #5
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Thanks for your ideas! This is a very small house: 18' x 24' footprint so only 420 sq ft on each of 2 floors. My guess is that it won't take much heat once it is insulated.
The past 2 winters have not been very cold, however 'usual' temps can get to <5F.
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Old 07-18-12, 03:35 PM   #6
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Thanks for your ideas! This is a very small house: 18' x 24' footprint so only 420 sq ft on each of 2 floors. My guess is that it won't take much heat once it is insulated.
The past 2 winters have not been very cold, however 'usual' temps can get to <5F.
The electrician said to put in what seems like excess auxiliary heaters, including in the bedroom. Perhaps that is because it is business!
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Old 07-18-12, 05:04 PM   #7
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Depending on other factors in the building's envelope and which mini split you choose you probably don't need any backup heat. What is the zip code where you live? We can use that to find temp averages to determine the winter design temp. Don't trust an electrician for HVAC advice. In fact you probably don't want to trust most HVAC contractors for HVAC advice. You need to find a very well informed HERS rater or something like that to get the straight dope on all of this. I can tell you this right now though, you are trying to heat a very small space and if it's insulated and air sealed marginally well just about any minisplit on the market will give you heat on cold days.
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Last edited by S-F; 07-18-12 at 05:07 PM..
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Old 07-18-12, 06:02 PM   #8
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At Caribou Municipal (KCAR) airport, which might be nearby, it does get cold..

Average High/Low Temperatures for KCAR | Weather Underground

But, the average daily hi/lo temperatures during the coldest time of the year looks like it's around 10 to 12 degrees F.
On nights when it's very cold, the next day your ASHP will be working fine at 15 to 20F.

If you had the right data base, you could figure out the (approximate) average number
of hours each year when the temperature was going to be below 8 degs F.

That number would be useful in deciding how often back-up heat would be needed,
and give you a basis for calculating back-up fuel cost.

My goal is to use small electric space heaters for back-up.
Since the number of hours each winter, that my Sanyos won't work, are very few.
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Old 07-18-12, 06:40 PM   #9
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If you choose the right unit you won't need backup heat until you go below -5. Assuming that your unit is properly sized.
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Old 07-18-12, 08:42 PM   #10
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Helen,

It will help if we knew how much heating is usually used in your locale...

There is a website that will calculate heating degree days, which will assist us in giving you appropriate advice.

The website link is HERE.

Just put in your zip code, press station search, find the closest weather station to where you live, click on it, and then click the "average" button on the line labeled 'Breakdown'.

Finally, press the button labeled, "Generate Degree Days"

...there will be a whirling thingie at the top of the page, to let you know that it is working...

...when it is through, press the "download" button and then "Open".

...your average degree days will be on the bottom line.

For instance my heating degree days number Total is 4816... which is considered pretty moderate.

That number provides a big clue as to what someone's heating needs are.

-AC_Hacker

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