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Help me develop a secondary heating plan for our solar passive house.  RSS feed

 
Sean Rauch
Posts: 136
Location: Winnipeg, Manitoba
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I live in central Canada just north of Grand Forks which is just north of Fargo ND. I posted an earlier thread about this but I think I want to start from scratch as I thought I knew what I want to do and now I'm not sure lol. I'm a commercial sub contractor so I have experience building etc and I know that we want a passive solar house. The real issue of course is that during peak winter months and in peak summer a passive solar design won't be enough to heat our home. We will need a secondary and maybe a third level heat source. I'll try and make this a living type document where I adjust as time goes on and better suggestions or solutions come up that way it can help a lot more then just myself. If you think you can only address one area I've used a numbering system so we can keep things organized.

Firstly I'm pretty sure we want to do a RMH (rocket mass heater) to help heat our home in peak winter months but additionally I think it would be a good idea to have a system that offers function while we travel and are generally away. I don't want to be away during the middle of winter and come home to frozen pipes etc that cost us a fortune to fix. I want the house to really be sustainable.

So my first question and I know it is vague but will passive solar with a high level of insulation plus a high level of thermal mass be enough to keep our home above freezing if we are gone for say 2 weeks at a time when the outside temperatures are -30C? I've looked at the stuff in Alaska and I'm not finding a lot of good specific answers that I can translate to my application.

Here are some of my ideas for the home and I'm hoping to get comments on them as well I'm all good with abandoning some of them as I go to replace with better ones.

1 Water. Solar hot water: I really like the idea of a hot water thermal storage tank in my home, the key benefit I see is consistency the downside is cost and maintenance so I'm willing to let this go if its too much. So how would you guys and gals think this would work. I think a high pressure system isn't in the cards as it can go awry if not constantly monitored:

1a. Modified trickle down solar water heating panels build into the roof system of the house to charge a thermal water storage tank. Do these systems perform ok in extreme cold? Any suggestions?

1b. Tank less hot water heater that is fed from the thermal storage tank to run the domestic hot water and in floor heating. I wonder if it would be worth while to do this, in my mind it makes for a really low load on the water heater which which should reduce energy costs and make it last longer.

1c. In floor heating in the main floor slab that would circulate from the thermal storage tank or tankless water heater and back. I'd really like the whole system to be an open loop system without any heat exchangers that would build up cost and complexity. So the water that goes through the floor could also be the water we use to shower etc. The real benefit I see is that we would never have stale water anywhere in the system, no need to flush etc. Can I find a way to make this in floor heating also in floor cooling in the peak summer months? I really like the idea of manipulating the temperature of our thermal mass to control house temperature as apposed to forced air.



2 Air. I know we will need a high efficiency HRV to handle the air exchanges in our place, its a given. I'm thinking a ground sourced pre heating type is best for overall efficiency.

2a. I've seen this idea used on earth ships where you run your intake air vent 10' or 20'+ under the ground at a depth where there is no freezing
to precool incoming air for the summer, would this also work to preheat air in winter? Would it just cause the ground around it to freeze making it a waste or subject to frost forces that ruin the pipe? I'd probably put the actual intake in an outbuilding like a shed or something to protect the intake from snow, rain, debris etc.

2b. Would it be worthwhile to run the incoming air through a solar air heater (maybe screen type) before the HRV to further preheat the air? If I didn't run the intake underground would there be any value in the really cold times to just drawing fresh air directly through the solar heater or would the frost etc kill the system? I can't find a lot of information on how these air heaters perform in really cold temperatures.

2c. Can I run a non passive solar air heater from the top down so cool air enters at the top of the collector? I'd like to have warm air enter the house low on the south side of the building so that a natural convection loop circulates the air in the house. I just want as simple a complex system as I can get.

2d. where is the best place in a passive solar home to have your air intake/exhaust happen? I really want to depend on a convection loop that draws air up along the large south facing windows to remove the moisture from the glass and keep it dry.


3 rocket mass heater. Originally I came across masonry heaters a couple years ago and really like the concept, then I came across RMH and the cost plus simplicity have me sold.

3a. My wife finds that my parent's wood stove irritates her eyes. They have a pretty simple system with an in house intake, closed burn box, a catalytic converter and a fan that blows air over the cat when it reaches a certain temp. I'm not interested in a comparison of the systems I know theirs isn't a great setup compared to RMH but would the RMH give off irritants like the one my parents have? I think theirs really drys out the air and that might be the major cause. Either way I want to be able to safely run a higher level of humidity in our place then the average home.

3b. Where should we place the RMH in our home? I'd really like to put it against the south wall under the windows with venting under the bench so that as the RMH gives off heat after we're in bed it makes the convection loop of the house continue to operate and keep the glass clear of frost. Plus it will accumulate heat from the sun during the day. I've read however that you want it in the center of the room so heat is radiated away in all directions and summer sun won't heat up the mass. I think I can deal with the summer sun using shades and awnings.

3c. Is there a good way to use a RMH to add heat to a water heat storage system? Something that doesn't look like a dogs breakfast in our living room? I've seen radiators placed above the barrel and I'm not sure how I would make that look good. Keep in mind my wife is an art director and things that are visually unappealing will make her unhappy! Thus I will be unhappy... I want to avoid heat exchangers wherever possible but why waste all that heat coming off the drum if I can store it overnight or even over days...


 
Brian Knight
Posts: 554
Location: Asheville NC
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1. I think solar thermal is great for domestic use (pre-heating hot water tank). Using it to heat radiant floors on a passive solar home is a much tougher situation to make cost-effective but certainly possible. With both systems I prefer PV powered DC circulation systems. You will need a backup. Not a fan of tankless.

2. Earthtube supply for the HRV is possible but risky. The inside walls of the ground tube will get moist, debris in the air will accumulate and nasties will grow there. I like the idea of a solar air heater for the HRV supply much better; More control.

3. Combustion appliances are bad and risky for Indoor air quality. RMH in particular, seem to contribute smoke and dust more than conventional biomass heaters in my opinion. If it were me, I would explore having the "firebox" or feed outside of the house and then run the mass or stove pipe through the conditioned space.
 
Sean Rauch
Posts: 136
Location: Winnipeg, Manitoba
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Yea I'm sold on the ability to store heat in water but I'm starting to think some basic base board heaters might be the better.

I thought RMH was one of the better options for air quality?
 
Al Turner
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Sean, because of the extreme cold weather and the unpredictability of what the weather is going to be I would opt for something more failsafe. They make ventless propane heaters with thermostats that will come on at a preset temperature that could be used to insure no pipes will freeze. Trying to outguess Mother Nature can be tuff and costly if you guess wrong. A heavy snow and ice storm may make your solar products useless and cause the inside temp to drop quickly. You don't want to come home to major damage from frozen pipes and water leakage in the middle of winter. While propane may not be what you were thinking about it is a more reliable backup should all else fail. It can also be used if for some reason you need the extra heat while you are there. It is recommended that you have a carbon dioxide monitor inside the house so you can monitor the loss of oxygen that is consumed by the ventless propane heaters. Propane will be there when you need it and if you buy it in the off season the price is not too bad.

I am from Michigan and currently live in the mountains of North Eastern Arizona at 5000 feet. A couple of years ago the temp here dropped to around -18C for several nights causing many homes to have frozen water pipes that resulted in much water damage. Our normal nightly lows can get down around -12C. One must learn to expect the unexpected when it comes to weather.

Al
 
Brian Knight
Posts: 554
Location: Asheville NC
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Ventless combustion appliances should only be used in emergency situations. You would never want to design a primary or secondary system around one. Besides the CO concerns, they introduce huge amounts of water vapor.
 
Al Turner
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Brian, these have been around for years and are designed for everyday home use. These are certified for full time home use. There should always be concerns about safety when using any kind fire in a home. This includes gas stoves, wood stoves, gas or oil lamps, rocket mass heaters, fire places, pellet stove, and any other fire breathing stove or furnace. I have a CO2 monitor for safety because I have a fire place and a pellet stove. They are recommended along with a smoke detector even if you don't have a fire breathing device for safety.

The bottom line is you want something that is safe and dependable. Something that will protect your pipes from freezing while you are gone.

Al
 
Brian Knight
Posts: 554
Location: Asheville NC
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I agree that it might be a good idea to include one for that situation. However, ventless fireplaces are not safe for indoor air quality. They are also not safe for the structure as exfiltrating humid air can more quickly cause durability and M word problems. They will probably be illegal to install in new construction in the future for good reason.

Its possible that a properly built passive solar design built to the german passive house certification level may not need freeze protection measures even at that frigid location. However, Iam with you on providing one for a just in case/long vacation/emergency situation.
 
C. Letellier
Posts: 228
Location: Greybull WY north central WY zone 4 bordering on 3
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I am from northern WY and have lived in a passive solar home for 29 years. It is an earth berm on 3 sides and clearstory windows design with a full basement under it. This house would survive those temperatures without breaking pipes with no heat. But we are in one of the sunnier areas of the world for sun days. I only started the aux heat 2 days ago having had nearly a month of freezing at night temperatures. I had gotten down to 56 degrees because I forgot to close the dryer vent. What happens without heat. On sunny days the heat takes a bounce up and it takes about 2 to 3 days overcast to completely lose that bounce. When unheated it settles to somewhere between 50 and 55 degrees. At that point the passive geothermal from the basement begins to help carry the house through gray days. During the construction the house was unheated for nearly 2 months of nearly steady 20 F below days. It sometimes dropped to the upper 40's inside but mostly sat in the 50 to 55 range. The only morning it was seriously colder was while the water line was being put in from the cistern. That was an open on both ends 1 1/2 poly line. No one gave it a thought but the house was down in the upper 30s the next morning and that line was blowing into the house hard with cold air.(so hard you couldn't stuff a paper towel in the pipe because it would blow it out) So that small an air leak is enough to cause problems. That means the house needs to be really tight to work. If you build correctly though you should be able to manage it. Mass, insulation, how tight the house is, amount of sunlight, amount of exposure to the sun all come into play.

Now the problem with adding a rocket mass heater to the mix is most of them draw interior air to run on. Well guess what, you just screwed up the tight house if you are going to have an air leak big enough to do it. Here running the closes dryer will back draft the water heater if a window isn't opened to provide make up air. The solution of course is to design an air intake system into the heater. I only encountered the information on rocket mass heaters a month ago so my knowledge there is way less. The listed problems with external air intake are quenching the fire to much with cold air to burn cleanly and pressure differentials causing the stove to back draft. One of the suggested solutions to the back draft problem was putting the chimney and the air intake on the same side of the building so they were at the same relative pressure from the air flowing over building. This apparently helped greatly. As for cold air quenching the fire the applied answer to this was to run the air intake tube through the mass for the rocket stove to warm the air before the burner. There again this apparently solved the problem. This one though I think I would do differently and I would run the intake line right down the middle of the chimney. This would allow far greater preheat of the air before it got to the fire box and should improve the burn I would think. There will be condensation problems to solve leading to pipe reliability problems to solve but it should work by simply increasing the diameter of the outer pipe to make up for the surface are lost to the inner pipe. Another problem you will run into is how little heat the house takes. Likely the rocket heater will get the house to warm before burns out of fuel. Mass will help some and if you can find a way to store that heat under the basement floor that will help more. I burn just over 300 to just over 500 gallons a year of propane.(depending on how many people are living in the house) That runs the water heater, the cook stove, the clothes dryer as well as the aux heat. On a sunny day the heat turns off about 9 in the morning and doesn't come on till nearly 9 at night. On a sunny day in Dec or Jan(peak sun) with company and the the oven running it is fairly common to be opening windows even if the outdoor temperature is 40 below to cool the house off. The heat from the sun + heat from people + oven can easily have the temperature in the low 90's without some cooling. So I would expect a RMH to produce to much heat probably unless a good way was is found to push it into the ground or into really large mass.

This house the basement is 10 inch concrete walls with 2 inches of the waterproof foam insulation on the outside. The up stairs is the same 10 in concrete walls with 2 inches of foam for 3 walls.(in earth berm) The concrete is the mass the house runs on. The walls are the concrete with the high spots ground off and the pockets and pores filled with dry wall compound and painted. The south wall is a standard 2x4 stud wall with 1 inch of foam on the outside.(should probably be a 2 in foam on a super insulated construction stud 6 in wall.) The roof/ceiling is 12 in rafters filled with blow in insulation. When it was built it radically exceeded the code for insulation. It still nearly meets that code 30 years later. If you feel a draft anywhere in the house it means you have a leak or something open. Usually there are no drafts if everything is closed up properly. There are no real cold or warm places in the house.

So for your goal I would aim for electric baseboard heat with a really low set thermostat in the areas where the pluming ran. If the house got down near freezing that would give a boost but mostly wouldn't do anything to the electric bill. The only danger would be a bunch of gray days combined with a power outedge. You could likely beat that by simply setting the baseboard thermostat for 40 degrees or so. Then your mass should carry you through a few days without power even with a problem.
 
Peter Mckinlay
Posts: 182
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Hot water will only keep pipes unfrozen if the hot water is flowing through them. Hot water cylinder will remain unfrozen while all piping freezes.

One way to prevent pipe freeze is by draining. Both hot and cold water piping commence from a central pipe, in this pipe the drain plug is included.
 
Brian Knight
Posts: 554
Location: Asheville NC
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Peters suggestion is the most foolproof as long as you remember to winterize. All plumbing in frigid climates should be done that way. Dont forget to add antifreeze to the toilet tank and bowl. I had to learn that one the hard way..

C. Letellier, thank you for the detail on your home. It sounds amazing. The power of that 1.5" airleak is particularly interesting. You must have done a great job on the airsealing but if it was coming in that much it had to have another place to get out right?

The 2012 IECC actually has some fairly demanding insulation level minimums. In climate zones 6, 7 and 8 you need R-20+R-5 insulation or R-13+R-10 insulation for cavity walls. So your cavity wall suggestion meets code minimums there but barely. The roof should be R49 but I think this may be increased soon in one of the upcoming versions. Your described roof would be about R43 without accounting for the thermal bridging. Iam sure it still meets local minimums but those concerned with energy use should be shooting for International minimums at least. Great job on your home though, would love to see pics and hear more detail so dont be a stranger.

 
C. Letellier
Posts: 228
Location: Greybull WY north central WY zone 4 bordering on 3
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Yes if there was that big a leak in there had to be an equal and probably larger surface area leak headed out.

I realize that it is pushing it to say it meets code now. But it is close. I will bet there were almost no house built 30 years ago that come close to meeting code. My main point is to build to exceed code because it is worth it. And I will be that applies to modern codes looked at in another 30 years.
 
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