Book recommendation for off-grid or passive house HVAC systems
Hi a newbie here looking for a book recommendation on off-grid or passive house HVAC system design.
I would one day like to buy a vacant plot of land in BC (Zone 6) and design and build a self-sufficient year-round off-grid mostly-passive house. I’m currently in research mode. I enjoy diving deep, so the more detail the better. I’d prefer a modern book which is up to date with the latest and greatest technologies.
Some of my thoughts:
– Lots of PV and batteries. I’m an Electrical Engineer and reasonably comfortable with this side of things. I have a great gook on system design and previous experience with PV simulation.
– Envelope. I have read several good books on passive/solar house design, and have been listening to the Green Architects’ Lounge podcast. I feel somewhat literate in this topic and have some ideas/opinions on which way I’d like to go for the envelope.
– HVAC. This is where my knowledge is lacking the most, and why I am looking for a good book to help fill the void. From some web research and ideas from the depths of my imagination, the house would use a hydronic system with large water thermal storage tanks. An air-source heat pump (or, possibly GSHP) would be the primary source of heat for the system. The large thermal mass would mean very little on/off modulation of the heat pump, and a good tolerance to a range of heat pump sizes. A large buried propane tank feeding a condensing boiler would serve as backup heat if the heat pump goes down (and backup power from a propane generator). I would also like to have a third heating loop which is fed by a high-efficiency wood burning rocket mass heater. When the house is occupied and wood is available, it can be run for half the day to ‘charge’ the thermal storage tanks, to be discharged over night when it is needed most. Optionally, pipes in a concrete solar wall or floor can also collect heat and charge the system during the day. I don’t know if radiant floor heating should be used throughout the house (sounds expensive and complicated), or instead recirculation fans over coils in each zone, or forced air heating/cooling. I have read that a VAV blower with actuated ducts in each zone offers accurate, efficient control. In the summer, the tanks can be cooled with a ground loop under the house. Not sure on HRV vs ERV. I’d also like to service a greenhouse zone attached to the house which will be hotter and more humid than the rest of the house. I would like to select and setup a control system to tie it all together and operate it efficiently depending on the zone-based indoor and outdoor conditions (pick a system which allows full customizable control). I’d like to simulate the entire building and HVAC system in DesignBuilder.
I know this system sounds complicated, but I will extract enjoyment from designing, building (with contractor help), operating and tuning the system for comfort and efficiency. Cost is a factor but not the top of the list.
I’d appreciate any book recommendations on the HVAC and controls side of things to better inform my design choices.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Not a book, but perhaps one of the best sources for Passive House design knowledge:
https://www.phius.org/become-a-professional
I'd respectfully suggest that in your enthusiasm, you are overcomplicating your design. Redundancy is needed less in a passive house. A real passive house only needs a very small amount of wintertime heating, and with such a robust building shell, heat will be well distributed in the interior as losses are very low.
You'd have to be more specific about your location, but passive solar space heating only works well in areas with a sunny but cold winter climate. Nevertheless, south-facing windows show superior thermal performance throughout the year, even in cloudy climates ( https://www.greenbuildingadvisor.com/article/a-quantitative-look-at-solar-heat-gain ).
Thanks for the reply. Sage advise to simplify. Your statement about heat distributing itself well inside a well insulated structure makes sense. I therefore may not need to worry about zones so much. What might be a simpler design is a forced air system which can blow hot air, cool air and provide heat-recovery ventilation at the same time. The central unit perhaps could run heating/cooling off an ASHP coil, and have a secondary heating coil from the water thermal storage tank which can be heated with the wood fired rocket mass heater or propane burner. Being off-grid I think its important to have a backup incase there is no power and nobody is home to feed a fire - hence the propane. In this system at least the hydronics stay isolated to the thermal storage tanks and the central heating/cooling unit. Since the heating load is low, a single firing of the stove may heat the house for days thanks to the thermal storage. The passive solar component can stay, well passive, in the form of a 4" concrete sun-lit surface. The ASHP component I like to have in order to provide the cooling - there should be more than enough power from the PV in the summer. Does this sound more reasonable in terms of complexity? Thanks. FYI, I am looking in the Pemberton, BC region.
Yes that's certainly simpler. Even simpler and cheaper might be a separate HRV/ERV and a propane heater. Attached are charts of average outdoor temperature, cloud cover and solar energy over the year (from WeatherSpark.com). If you have sufficient thermal mass and meet passive house specs like you plan, you indoor temperatures would be quite stable. Your interior might lose very little heat 2-3F/1-2C during winter when using no heating system other than the sun, even with no appliances adding to indoor heat gains (e.g., during an electrical outage). Note that your wintertime solar gains would be minimal, however, heavy cloud cover. Having propane and/or wood to carry you through extended outages would be good but probably rarely used. The Passive House Planning Package (PHPP) or similar software could model your home's performance more precisely and help size heating and AC options. The original Passive House concept suggested a simple resistance heating element in the HRV/ERV as the only winter space heating necessary in a certified Passive House.
One of the problems combining various systems is that if performance doesn't match expectations, it may be difficult to diagnose and tune to your satisfaction. But you might enjoy that engineering aspect.
Well, you've certainly given me a lot to think about. Resistance heating, though simple, feels like a waste of precious off-grid electricity when an ASHP could operate at a COP of 2-3. Especially when I look a that plot you attached showing the pitiful amount of solar incident energy available over the winter. Thank you for reminding me of WeatherSpark by the way - what a treasure trove of data if you are lucky enough to have it for your site of interest. I like your advice of putting together a model to get an idea of the heating and cooling requirements. I have heard of PHPP (and WUFI), I may give that a try since its free as opposed to DesignBuilder which is thousands of dollars. I may end up finding that with good passive house design, the solar gains and plug loads alone are all the heat I will need 90% of the time. When you said separate HRV/ERV system and propane heater, what kind of propane heating system is this exactly? I would still probably throw in the wood-heated water tanks as a fun side-project. If I do end up passing on the ASHP, I could instead get cheap cooling from blowing air through a bank of hoses buried 8' under the foundation.
Edit: The more I think about resistance heating, the more I am coming around to it. The hardware is inexpensive, zero-maintenance, reliable, and silent. If it is needed only very little then big picture, it may be superior to ASHP
I can't speak much about off grid but I can give some feedback on solar power. I have 10kW grid tied array in a forested area in northern Ontario. The amount of power in the winter is significantly less than in the summer, much less than even what PVwatts predicts for the location. Typically I see about 1/5 of summer production for Jan/Feb (~8kwh/day).
The challenge for these types of location is shading and snow. With the low winter sun, you need either a steep south facing slope or clearing (and maintaining clear) a large area of forest. The low winter sun also means that it takes very little tree cover to kill your production because of shading. No matter what you do there is always that little spindly pine shading the array.
Snow storms can also last for many days, there have been 1kwh days for close to a week.
Having enough solar to cover you house convenience loads on a bad day is already pretty hard, trying to heat with it is asking for trouble.
The standard for off-grid living seems to be wood heat with through the wall propane heater backup. Anything that needs to run all the time such as fans and pumps are a challenge and should be avoided. You should NEVER have to rely on electricity to keep your building from freezing.
You can probably get away with a mini split for summer time air conditioning and maybe shoulder season heat, but that is about it. Even that will need large batteries for overnight power, so may be only possible during the day.
Even the energy to run a ventilation system might be too much. You don't want to end up with having to get a very large battery ($$$) or running off propane generator.
The engineering challenge is to come up with systems that require no power. Things like thermosyphon water heater off wood stove.
It would be really interesting to design an HRV that can use stack effect for air flow.
Very good advice, thank you.
For your solar, did you tilt it to optimize for winter sun? Conveniently, the high tilt also sheds snow more easily. I'd think about designing a mounting system that can be tilt-adjusted depending on the season. I hear you on shading...any amount can really kill output.
What I like about the wood-fired rocket mass heater is that it would be build with a large thermal mass of cob. That way, if there is no power for the pumps to carry the heat to the thermal storage, it can still 'charge' its local mass and radiate overnight.
I have been reading that for batteries, the PowerWall is actually the best bang-for-buck these days.
I have simplified my design concept a bit based on another reader's suggestion, so I invite you to read my revised idea in my reply to that comment. Interested to hear feedback. Is your house insulated to the levels of a passive house? E.g. R-60 roof, R-40 walls, R-20 slab?
Panels are at 45deg angle and still hold snow. They do shed it quite well once the sun comes out, but until then, you are producing no power.
Seasonal adjustment only makes sense if you need more power in the summer. With off grid, that is not likely the case. Angle the panels for max production in the winter and a bit towards west. You always need more power towards the end of the day.
In snow country make sure the panels are at least 2x expected snow cover off the ground. The snow that the panels shed builds up into big burms and end up covering the bottom row.
Wood stove+water storage is not a bad idea, but it should be on thermosyphon. You can't have a fire with a water coil unless water is circulating, might not be good to rely on a pump. There should also be a way of dumping heat from the tank with thermosyphon if needed (maybe a high mounted panel rad with TRV).
Some good ideas here, but not necessarly good for off grid:
https://wolfeislandpassivehouse.wordpress.com/2015/07/03/heating-a-passive-house-the-other-last-minute-design-change/
Your heat solution still uses fans. Fans use a lot of power to move heat, for example my ERV is around 60W, something that can move some heat, you are closer to 100W to 200W. So that is about 1/4 the energy in a powerwall overnight. Add in your convenience loads and it quickly increases the size of your battery especially if you want a couple of days of backup.
I work with much larger batteries, so I can't comment on cost. For something that will get a lot of cycles, lithium ion will last much longer with less maintenance. Just make sure you keep the batteries somewhere warmish (you don't want them too hot or too cold). Unconditioned basements are perfect.
Your plan still needs electricity for heat. You need a way to have the place heated while away with no power.
Keeping a water well/lines from freezing with is also pretty energy intensive. I would plan on having it drilled into a bump-out of the basement, that way it is always protected from frost.
Wow, that is great advice about mounting the panels 2x higher than the highest the snow pack - I would have never thought of the berm until I was standing there scratching my head thinking 'oh, damn'.
You're right about just thinking about winter production and loads. Another poster above attached a plot of the available solar resource in the area I am looking at, and it looks quite dismal in the winter.
For the rocket mass heater, I think I would try to build it up with a large thermal mass of cob/concrete, and embed the hoses at a distance from the house source where they do not get hotter than say 80 deg C. That way, things won't start boiling away if the pump is not functioning, and the tanks won't exceed 80 deg C. That said, I will still look into thermosyphons since I do not know much about them, or how effective they are. If it's simple, effective and reliable then it may be the way to go anyway.
As for your advice against forced air heating/cooling, I was not aware that it would take more fan energy than the ERV system uses itself. I figured that since the ERV is blowing air around anyway, why not put a coil in the air stream to heat it up or cool it. If I pass on the ASHP and rely solely on passive solar, plug loads, wood and propane, then for the wood-heated water tanks, a simple alternative is to pump that tank water back through the rocket mass stove over the next several days and allow it to radiate the heat. The nice thing is that water pumps are much more efficient at moving heat around than fans. The propane heater can be something that does not require electricity, and that can be the backup system which kicks in when the indoor temps drop too low due to loss of power. Do you know what kind of propane heating system would fit this bill (a link, or a name)?
Good advice on protecting water/well lines from frost, I will make a note of that. Thanks.
What are you going to do about hot water for bathing, washing, etc?
If you are going to be off grid (and probably in a rural location surrounded by forest) and you will use wood for water heating why don't you use an indoor boiler (that would actually sit in a small outbuilding near the house)? That way the mess of firing up a wood burner is out of the house. You could put all the storage tanks, solar equipment, etc in a utility room in the same small outbuilding. You'd have wood fired hot water for heating and DHW in the winter and solar heated water in the summer.
Also, a lot depends on the size of your house. Why not just use baseboard radiators or wall radiators instead of radiant heating?
I need to do a little more thinking on the DHW. One thought is on-demand propane. The other thought is a DHW tank which is heated with a coil from the large thermal storage tanks and the ability to bump up the temperature with its propane burner if needed. I like the idea of having the rocket mass heater inside the house because I don't like the idea of having to put on my boots and coat to go out and tend to the fire. It should be clean and can be made to look nice indoors, and will provide a large thermal mass of warm cob/concrete to sit on/near. If you are curious about how they work, it is fascinating: http://batchrocket.eu/en/
When you say baseboard radiators or wall radiators, are you referring to electric resistance, or pumping the thermal fluid through them from the storage tanks?
Centmo,
Your options for providing space heating to an off-grid house are more limited than you may realize. There's lots of specific advice in this article: "How to Design an Off-Grid House."