After a productive summer filled with deck building, a shade structure, landscaping, and a building a cellar, fall arrived! I really can't recall a summer that went by so quickly. To reiterate, in the previous post I talked a little about food security. I tried to focus some of the landscape in our yard on producing food...and it was a huge success! We had a steady supply of baby root vegetables and greens up until the end of September (Picture 1). It was quite a bit of work but it was amazingly enjoyable watching the garden flourish and picking throughout the summer. I battled with nature and learned a lot from my planting mistakes. I will put those lessons to work next year as I work to add new crops and add resiliency to the garden.
Getting ready for the fall harvest was no easy task. I had never owned a cellar before now, nor had I harvested or stored vegetables. A lot of effort went into researching the old methods of root cellaring vegetables to ensure they will stay fresh for a long time. Storage for most root vegetables is basically about creating underground conditions to keep the vegetable "alive". A low constant temperature (< 5 oC) with high humidity (>95%) is necessary and can be achieved by adjusting dampers to regulate air flow through the ventilation stacks inside the cellar. Luckily, my father and my father in-law have been around a few cellars in their days growing up. They grew up in a time when every family had a garden. It was a necessity. They provided me with much of the knowledge needed to create good storage conditions. Storage is pretty easy for potatoes; a large bin works well with slatted sides to promote air movement around the potatoes. Picture 6 illustrates some bins that I made from left over T&G flooring used in the house. These were to be used for the other root vegetables like carrots, beets, and parsnips.
My fall harvest provided our family with a great bounty. Some of the root vegetables were ready for harvest through the summer so we picked away while it continued to grow. Nature provided us with turnip greens, lettuce, beet greens, and a variety of small vegetables including carrots, rutabagas, potatoes, beets and onions. Picture 4 shows the potato garden around mid-July. By October the potato plants had completely died back and I let their skins "harden off" in the ground to toughen them for storage. By Mid-October we pulled the potato plants (Picture 5.). We left them to dry for a day, and moved them directly to the cellar. We ended up with about 300 lbs! The cellar at this point was cool ie <10 oC but humidity was low. A perfect combination to promote drying for a couple of weeks to further harden-off the skins before closing up the door for winter. I left most the vegetables (carrots, parsnips, cabbage, beets) in the ground until the beginning of November while I waited for the root cellar to cool. Gently pulling them (on a cold day) to ensure minimal damage, cutting off the greens and leaving the dirt on them is all you really need to do for preparation. They were then stacked inside of a burlap lined crate with sawdust between each of the layers. I paid careful attention to ensuring that the roots didn't touch each other since it can promote rot in storage.
One month later, the carrots had little green sprouts (1/4" long) growing out of the top of them: A good sign that the vegetables are still alive and kicking! Exactly what you want! The cold temperature should really slow the growth while the vegetables are still alive and fresh! Picture 7 shows the cellar where the vegetables are spending the long cold winter.
What was once green (Picture 2.) is now gone and the garden is now covered in a blanket of snow (Picture 3.). Although it is a beautiful sight, looking out at an empty garden does lead to some heavy feelings. This being said I get to enjoy the fruits of our labour (or should I say vegetables) for the whole winter and start again in the spring. Creating food security does take work. Some of the work is easy. Some of the work is hard. The end game is all pure enjoyment! Going to your personal grocery store on a cold winter day to pick roots for a warm winter stew is priceless. Next year I plan on adding more perishable varieties of vegetables that lend themselves well to canning practices. Now that the days are short now and the nights are long....I guess ill have plenty of time to research what comes next.
Humidity and Woodstove Woes
It was a fairly cold fall this year. There was a noticeable drop in temperature around mid-September. This being said, it was somewhat humid as expected for our climate. As during the summer, the humidity inside the house was high. After a purge with open windows on a dry day, humidty would drop dramatically in a very short period of time ie <1 hour dropping by 15%. After closing the windows humidity would continue to climb over several days until we would have to purge again. Although the temperature inside the house was fine, humidity did make it feel stuffy at times and running a dehumidifier wasn't much of an option because dehumidifiers just add sensible heat to the living space. Our heat pump water heater (which has some benefit of dehumidification) helped a little with dehumidification but our hot water usage was just too low for it to be effective. I really should have listened to my passive house designer from the beginning and installed a mini-split. Lesson learned!: Always listen to your passive house designer! From what I recall his primary concern wasn't actually humidity. Instead he expected the interior temperature to be an issue during the summer months but the house has performed fine in that respect. In any case, we opted to install a mini-split in order to address dehumidification. The minisplit made a huge difference up until the end of September and was able to keep the humidity around 50% while running dry mode (Fujitsu 9RLS3H). As the exterior temperature decreased I expected the interior humidity to fall also, albeit slower than an HRV. It was much slower than expected (the opposite was true during the summer). It would take days to drop a couple of percentage points. Depending on the moisture load in the house some days it would increase, reach a new equilibrium humidity, then start falling again. There was some fear that as the temperature decreased lower, I would see a significant amount of condensation on the windows and doors. In response, we ran a dehumidifier continuously.
High humidity levels have come up time and time again for many new air tight homes. Where does all the moisture come from? Construction materials are filled with moisture. Drying those materials takes a long time. A concrete slab could take years to dry. However, the house had been closed in for almost a year now so I suspected that most of that construction moisture would have dissipated by now. Given that we could quickly purge the house of humidity (it could drop 10-15 percentage points) in 10-15 minutes just by opening the windows and letting the wind blow through indicated to me that there was some sort of ventilation issue.
Some of the questions I asked myself were: "Is it the ERV? Is it the air-tightness? Is it construction moisture? Is the current moisture load to high for ventilation to eliminate?" I think the answer is a result of a combination of things going on. Most houses are leaky (3-5 ACH50). Even the ones that people call energy efficient (<2 ACH50) are still leaky when compared to the air tightness of this house. At 0.36 ACH50, the house is about 4 times tighter than the air tightness requirement for NetZero. At this leakage rate the natural air infiltration is about 2% of the volume of the thermal boundary. At 3 ACH50 the natural infiltration is about 20%. If 20% of the air in a house (under average weather conditions) is being exchanged from outside to inside I would think that the effect it has on interior humidity must be astoundingly greater than the same effect in an air-tight house. When the effect is compounded with ventilation humidity should drop rapidly, especially if operating an HRV.
It is pretty well know that gaining energy efficiency through air tightness can lead to indoor air quality issues if ventilation is not addressed properly. I started investigating every aspect of the ventilation system from materials to installation methods, and balancing. Then I started thinking...and here's where it gets complicated. Back in April when we moved in I lit our Walltherm stove. Smoke started leaking from the flue pipe joins. ICC UltraBlack slip lengths are typically not sealed since the natural draft of the chimney would prevent smoke from entering the living space. I immediately opened the windows and the smoke infiltration stopped. The next day, I decided to do some testing. I placed my hands around the stove pipe joins and I could feel cold air leaking through them. My first intuition was that the ERV was unbalanced leading to depressurization of the house. I checked the airflow balance (using a digital differential manometer) according to the manufacturers test method. It checked out fine. I made a flow hood so I could check the air flow volumes at each of the diffusers on the supply and return duct runs...It all added up to stale and fresh air measured at the unit. To further verify I did 50 point averages across the hoods outside for both the stale and fresh air and it too was good. On a still day, I slightly opened one of the windows, taped it around in order to make an air tight diaphram and poked a hole through with the tip of the manometer tube. The tube fell out of the hole and when I went to poke it through i could feel cold air quickly moving in through the hole against my hand. It became evident pretty quickly that the house was under negative pressure even though the ERV was balanced. The pressure reading was small. With the manometer tube placed through the hole the pressure read negative. When I turned off the ERV the differential pressure with the exterior would become neutral so the depressurization was definitley being caused by ventilation. I increased the amount of fresh air supply to the building until the manometer read slightly positive. I tested this setup by burning a match near the stove pipe and it was immediately sucked into the stove pipe joins. I had to unbalance the machine by almost 30 cfm in order to achieve this. I left the ERV in this unbalanced state so I could use the wood stove. I decided that I would revisit this in the fall when I had more time.
The depressurization and smoke issue seemed quite complicated and eluded me for many months. I figured it was related to the X24ERV ducting so I started an investigation in the fall. My initial thoughts were that the depressurization was caused somehow by the duct configuration on the supply and return so I switched them just to test the hypothesis. The good news was that the ERV basically remained balanced meaning that the supply and return duct runs had nearly equivalent lengths. I rebalanced the unit and once again could feel cold air leaking from the stove pipe joints. I decided to change around the ducting at the outside hoods. I switched them and made a few other configuration changes and rebalanced the machine...with much disappointment after rebalancing, the cold air kept seeping out the joints of the stove pipe. Each test lead to the same result and further dispair! Then I thought...If the results are always the same it obviously has nothing to do with the unit and instead there must be some underlying fundamental physics going on. Then it came to me: Even a tight house is still a "leaky" plenum. Both fans in the ERV operate independently. Fresh air is being fed into the house by the supply. The exhaust has no way of differentiating where air is being drawn from. It obviously has no way of just pulling air from the fresh air being supplied to the house, it just pulls air from the space. It will pull some air from the fresh air sources but air will naturally start infiltrating through holes in the envelope since they offer a path of least resistance....unless they are plugged. Plugging the holes can be done artificially by unbalancing the ERV so that the pressure difference inside and outside the envelope is neutral. There are obviously more holes in the envelope then just the stove pipe so when pressurizing by using supply air, you have no control over which holes the supply air exits through or the rate at which air exits through each hole and there would be a distribution of airflows through those holes based on their size and the interior pressure...so a large amount of air (in my case 30 CFM) could potentially be needed to effective "plug" all the holes and create neutral pressure. I am guessing that with air tightness at 3-5 ACH50, most homes are so leaky that there is little to no depressurization. I can almost bet that anybody in a passive house who has balanced their ventilator would see depressurization if a manometer us used to measure the difference between interior and exterior pressure. I am yet to find anybody willing to test this...if you do please let me know!. This effect is simply not local to Flatrock! Physics is the same everywhere....in this universe anyway!
It seemed like a good idea to just continue operating the ERV in an unbalanced state to deal with the stove issue. What could possibly go wrong?!?! The problem is that a solution to one problem can often lead to issues in some other way. Needless to say, unbalancing the ERV had unexpected consequences. After a lot of research I determined that operating in an unbalanced state (supply greater than exhaust) will collect more of the moisture from the exhaust air stream than when it is operating in a balanced state. When it is dry outside, interior humidity will drop much slower in this unbalanced state. In the summer, this unbalanced state will lower the moisture removing capacity of the unit and the interior humidity will climb much quicker than if the machine were balanced. The assumption during planning is that our ventilation would take care of exhausting some humidity during the winter but leave enough to be comfortable. The assumption is fine as long as the machine is balanced. My solution for the wood stove was based on a lack of understanding about the mechanism of moisture migration from one air stream to another in the ERV core. I balanced the ERV airflows and interior humidity would drop dramatically quicker than in the unbalanced state. So operating in an unbalanced state fixes the stove issue but leads to humidity problems! This also explained why humdity climbed so quickly during the summer when it was humid outside. So another solution had to be investigated.
A Fix for the Backdraft Issue also Fixes the IAQ (indoor air quality) Issue
I had read that wood burning appliances in tight homes can be problematic....and they can be! Good quality assurance is absolutely necessary. This being said, I feel like this ongoing depressurization issue is beyond the understanding and scope of most ventilation installers. I do think that other people whom are building super air tight houses should measure to see if the house is depressurized even though the airflows are equalized. Equality of the supply and exhaust exchange rates does not imply that the house pressure is neutral and therefore could lead to dangerous backdrafting, flooding the living space with carbon monoxide.
I read some time ago that the Zehnder brand of ventilators have a function that helps prevent depressurization. I believe its called "ChimneySweep". When activated it unbalances the unit in order to provide 10% more make up air and therefore pressurize the house to prevent backdrafting of smoke into the living space. My experience is that 10% was not enough and infact I required almost +30 CFM differential between the supply and exhaust (on a ventilation rate of 95CFM) in order provide enough to balance to neutral pressure. I decided that I would have to implement a similar strategy before I started using the stove again. By December I had concocted a strategy that would work. I decided that an adjustable normally open motorized damper on the supply duct would do the job. I would just have to set it up on a switch and turn it on or off manually. I balanced the ERV unit (with the damper open) to pressurize the house ie checking pressure between the inside and outside using a differential manometer), then adjusted the damper closed position so that the supply and exhaust were equal. Picture 8 illustrates the damper in the closed/balanced position. When I want to use the wood stove I would just flip a switch on the wall (picture 9) situated by the ERV control that would force the damper to open which would pressurize the house. I installed and set it up and it worked like a charm. If the machine is set to turbo it also works. If the damper motor fails, the damper which is spring loaded opens and the wood stove can still be used. I feel like this is a much safer solution and has redundency built in. Unfortunately it is another mechanical part that adds complication but the safety factor makes it worth the complication. The added benefit of pressurizing the house is that when I add wood to the fire there are now no issues with smoke spilling into the living space. The stove now works like a charm (Pictures 10, 11, 12,13,14) and once the damper switch is reactivated the ERV goes back to its balanced state and maintains IAQ under it's design conditions.
So how about energy use? I currently heat about 3376 square feet of interior living space including both the house and the garage (almost 1200 sq feet on two storeys). This month (December) we depended purely on electricity since I wasn't ready to use the stove until I fixed the ventilation issue. The temperature in the house and the garage (woodworking shop) are both set to 20 oC. We used a total of 1576 kWh. This billing period (Nov 10 - Dec 10) has been cold i.e. 0.2 oC, and I am heating two buildings. I am pleased with usage. From degreedays.net I determined that this billing period had 1042 oF.days (579 oC.days). I have found that the heat loads of the house and garage are definitely lower than the lowest heat output of the fujitsu heat pumps. How do I know this? Simple, the heat pumps short cycle. This can be easily confirmed by measuring the amperage at the breaker panel and tracking how long the machine is drawing power. At 0 C, the heat load for the garage is below the lowest modulated heat output for the fujitsu unit at 3100 BTU/hr. I calculated the load to be somewhere around 2800 BTU/hour including air infiltration and heat loss through the envelope. The heat pump short cycles every 10 minutes or so. Unfortunately this does affect efficiency. How much I have no idea but operating in a steady state would be more desirable. The heat pump in the house has short cycled less. It is the same size as the model in the garage and there is more heat loss due to a larger envelope so it makes sense. As the nights get even colder I have witnessed longer heating times and less short cycling which is great for efficiency. I think that the usage could have been less if the heat pumps short cycled less. I am still investigating what is going on but it appears to be a common thing with heat pumps operating under low load conditions in air tight houses. So far our bills for this house have been about 38% of that used in our previous home which used electricity and propane.
Picture 2. The house and garden in August. The backyard is dedicated to a variety of vegetables that are well suited to long term storage.
Picture 11. Warming up the flue before activating the Secondary combustion.
Picture 12. Secondary combustion activated.
Picture 13. Close up of the secondary combustion chamber.
Picture 14. Pump ground showing wattage required to move 3 gal/minute.