Solar strategy at my house- your thoughts?

[semipro]

I'm looking for advice/comments for how best to start taking advantage of solar at my house.

Here's what our situation is now:
-Two story wood framed house with rock and cedar exterior
- Poured concrete basement - finished but uninsulated
- Geothermal open loop heating/AC system (uses well water)
- Wood stove in 1st floor great room
- Nice southern exposure on house in winter, tree shade in summer
- Nice shed roof aligned for good "southern exposure" (incidentally the name of a strip club near here)
- We don't usually use AC
- We have two hot water heaters but I'm joining the systems so we'll only have one
- We have two full bathes both upstairs, on half downstairs
- We don't heat upstairs in the winter since its all bedrooms and baths
- Heat collected (from wood stove mostly) against the ceiling downstairs is routed to our bathrooms upstairs.
- We have a lot of windows but most on the 1st floor face north. Dumb I know but I didn't do it.
- I think most of our electricity goes to heating DHW year round and to the Geothermal and well pump in winter
- Other uses include lights, electric stoves, clothes dryer, computers, etc.
- We have an 8500 Watt generator we can use for power outages
- I've done a lot of upgrades with respect to infiltration and insulation. Further improvements here would involve pulling off all the cedar siding which I'm avoiding.
- I plan to fully insulate the basement walls and probably floor this winter

My goals
- Harness some bad ass solar power and reduce our electric costs
- Handle power outages relatively well
- Ultimately, generate what electricity we need even if we use the grid for storage. Since we use no oil, gas, or outside water this would get us relatively self sufficient.

My plan
- Install some hot liquid solar panels on the roof (propylene glycol probably), probably not drain back type.
- store that heat in water tanks in my basement
- Use that hot water for DHW with backup from my HP water heater and maybe a tankless electric unit (mainly for the Jacuzzi) all in series.
- Use the solar heated water in a below subfloor (in between joists) PEX hydronic system to heat our 1st floor
- I may install some PV but it would mainly be for backup and powering solar system pumps.
- With some other work we're doing install a sunroom on the south side of the house.

I guess I was initially thinking I'd install PV but since most of my load is heat I thought collecting heat in its native form would be a better use of my resources. Can I even use the solar hot water I've generated with hydronics?

Am I thinking straight?

Thanks in advance for your input.

 

[woodgeek]

I had similar ideas for active thermal heating and DHW, but when I pushed the numbers it had poor ROI (which may not be a concern for you). Basically, solar thermal systems put out a lot more heat in the summer than they do the winter, so you typically size them for spring/fall, dump the extra in the summer and fire a little backup in the winter. If you oversize it from there, you reduce backup (in the winter) and dump more in the summer. Commercial DHW systems I have researched have limited dumping ability (just circulating the glycol at night when the tank is above a high setpoint), so oversizing too much might require extra hardware for dumping the heat. In your case, you could prob rig up a HX to the jacuzzi for this purpose, but it is an added expense/complexity.

You can look at your monthly insolation using the PVWatts webtool. You can get a better winter/summer balance to an extent by using a steeper tilt on the panels (if that is practical), or even mount flush or tillted on a vertical wall. PVWatts will estimate the different scenarios for you.

As for active heating, the situation is tough--if you are like my climate, your system hits a minimum output in Dec/Jan, not a lot of heat to spare. To beat that you essentially need a system that is much larger than the DHW system, both collection and some extra storage, and then only 'use' it a few months a year, which kills the ROI. From either a self sufficiency, or bang for the buck POV, a deep energy retrofit will probably beat active solar heating with storage. That is, upping the sidewall insulation, if all the other low hanging eff fruit has been collected.

Just my amateur musings.

 

[semipro]

WoodGeek,
Thanks for the thorough and thoughtful reply. As usual, my plans are not as well thought out as I thought. I hadn't even considered producing too much hot water but that makes sense. I'm thinking now that I should size the system to carry the heating load in Winter with the assistance of wood. I want to avoid running the HP but may still need to.

ROI is important. One of the reasons I'm doing this is so in 20 or so years when we retire that we won't have to deal with utility bills (much). I have to admit that working on my house is pretty much my main hobby and, as such, installing solar HW is much more appealing to the geek in me that are structural shell upgrades. I do everything myself and would probably build the collectors and supporting system components. However your point about doing the deep energy retrofit is well taken. I stress about the amount of energy I know that our shell is leaking. I have plans for the near future to have an energy audit with blower door test done so I can address the worst issues. I'm afraid ultimately that I'll have to pull all the siding, which is no small task.

I was at least considering elevating our solar HW panels to maximize winter gain. I hadn't thought about vertical installation on outer walls but will now. I'm also toying with the idea of some sort of automated cover for the panels that would reflect sunlight to them when open, shield them from sunlight and insulate them when closed. That needs more research/thought.

I wonder if anyone has installed angled louvers on top of their solar collectors to address the overproduction issue? That would seem like a relatively simple solution.

I've included a photo of the south side of the house. This was some time ago (10 years) so the trees are much larger now.

 

[woodgeek]

Handsome house. is the front facade facing north or south?

I don't live in a passivhaus, and I get that at current (or plausible near future) energy prices, it is 'overkill' from a cost/benefit POV, but I think they are really onto something. That is, passivhaus is 1 BTU/sqft/degreeday, most US houses are ~8-12 BTU/sqft/dday. At 8-12, 100% solar space heating is verging on the ridiculous, huge glass area and thermal mass requirements (compared to conv construction) for passive, and a rather large up front cost and huge storage for active heat. At 1 BTU/sqft/dday, 'normal' glazing area and thermal mass (from framing and drywall) gets you close to 100% passive solar. When you need a little backup, a small wood stove is perfect (and the house thermal mass keeps the temp swings to a minimum with the required very small burns).

So the questions in an existing house is whether you
--beat your house losses down to 4-5 units (pretty doable for DIY with some auditing help) and then build a big solar system, with a large active component (b/c it is impractical to retrofit enough thermal mass for a passive approach)
OR
--if you beat down to 2-3 units (requiring residing and perhaps some window work) and then go for a more passive approach, augmented with a more modest active solar system if desired. At 2-3 units you are not a passive house, but you could probably heat a good sized house for 1-2 cords of wood per year.

 

[dougstove]

If you have a family, and they are around in the summer, in an existing house your best solar ROI is usually domestic hotwater.
I thought about adding capacity for solar heated hydronics, but was advised against for the reasons cited above.

 

[semipro]

That's the southern exposure. The front is to the right.

I agree with you both. I also like the PassiveHaus concept. But, as you can see, its going to take a lot of work to pull all the siding and insulate and seal. The windows aren't bad but are getting old. They're Andersen dual glazed casement. The original owner builder did some things right like 2x6 framing with added fiberglass insulation but the details are pretty poor and it leaks air like crazy.

I'm gong to do solar DHW but was wondering whether to combine DWH with space heating via hydronics at the same time.

I"m thinking now of doing a combination of solar DHW and some shell improvements.

Thanks for the comments.

 

[dougstove]

I have the storage tanks for the domestic hot water in my (insulated) basement.
The heat leakage from the tanks keeps the basement warm, which in my climate is fine for all year.
The feeder tubes from the panels to the storage tanks run down the corner of a cool closet, so any small heat leakage there is a small useful contribution.

 

[woodgeek]

A BTU saved is a BTU earned.

Don't underestimate your ability to minimize your util bills without residing. DIY airsealing of the attic, rim joists and windows can make a huge difference, as well as attic and basement insulation. I dropped my 60s house from ~12 BTU/sqft/dday to 6 in three years of tinkering, without replacing a window or touching the siding and using less than $500 on materials. I think I will bottom out at 4-5, better than that will require $$ work on the shell. Compared to me, you can do better--I have 2x4 walls with lousy 60s FG in them, you have 2x6. I am heating with a single speed ASHP, you've got a geo. I've got 50 yo windows, yours sound better than that.

In other words, I think you can DIY down to 4 BTU/sqft/dday no problem w.o touching the shell (esp with the help a good audit). At 3000 sq ft and 4000 ddays, that would be 48 M BTU. Appliances and body heat are ~8 MBTU/yr, so you would need to supply 40 MBTU. With a properly sized and maintained Geo, that should be ~4000 kWh. At $0.15/kWh, that is only $600/yr for heating. OR 2.5 cords of wood. Lots of folks are spending thousands on oil and freezing their butts off.

 

[semipro]

A BTU saved is a BTU earned.

Don't underestimate your ability to minimize your util bills without residing. DIY airsealing of the attic, rim joists and windows can make a huge difference, as well as attic and basement insulation. I dropped my 60s house from ~12 BTU/sqft/dday to 6 in three years of tinkering, without replacing a window or touching the siding and using less than $500 on materials. I think I will bottom out at 4-5, better than that will require $$ work on the shell. Compared to me, you can do better--I have 2x4 walls with lousy 60s FG in them, you have 2x6. I am heating with a single speed ASHP, you've got a geo. I've got 50 yo windows, yours sound better than that.

In other words, I think you can DIY down to 4 BTU/sqft/dday no problem w.o touching the shell (esp with the help a good audit). At 3000 sq ft and 4000 ddays, that would be 48 M BTU. Appliances and body heat are ~8 MBTU/yr, so you would need to supply 40 MBTU. With a properly sized and maintained Geo, that should be ~4000 kWh. At $0.15/kWh, that is only $600/yr for heating. OR 2.5 cords of wood. Lots of folks are spending thousands on oil and freezing their butts off.

Other than the basement I've done about as much envelope sealing and insulating as I can without pulling siding. There is little attic space as most ceilings are cathedral type. I'm working on the basement walls and skirt area this winter. All doors and windows have been checked or reworked and foam placed between them and framing. The floor joists are trusses and there's a large skirt area associated with each that I can't really access without tearing down ceilings or floors.

As an example of "detail" problems: Last winter I pulled up some subfloor in our master bath to do some work and stuck my head down into the truss space between 1st and 2nd floors. I felt a cool breeze and when I looked towards the front of the house I could see daylight. I found a large hole in the foam board sheathing where they had tied in the front porch roof to the outer wall in the skirt area. I ended up pulling up the subfloor along that wall and using spray foam to seal the leaks I found there. And that's only the places I could see. That's why I feel I'll need to pull all the siding to reseal.

Its disappointing. It would have been so easy to do it all right the first time. As I said, a blower door test and IR survey is planned. I should be able to use that info to make some better choices.

 

[GaryGary]

Hi,
Agree with others that first thing to do is work on insulation and sealing, but it sounds like you have done that.

Solar thermal systems that you build can be very cost effective -- this is the system I use on my house:
http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/Main.htm

Its a very simple system that is not difficult to build (but its not a one weekend build). Its a drain back system with a large, non-pressurized tank that supplies both domestic hot water and provides some space heating. It could provide more space heating with minimal system cost increase if I had more space available for collector.
The system is designed to be robust and requires little maintenance.
The version of the system that does DHW only (with a smaller collector) has an honest payback period of less than three years.
The version with space heating payback is a bit longer depending on your climate etc.


Its important when you think about payback for solar space heating that you consider the full heating season. Its true that in many places the solar fraction for January is not great, but think about the return for the full heating season. Places with long tough winters actually do better on rate of return that "easy" places with mild winters.
Around here, we have some heating 8 months of the year.
You can actually get a fairly good idea what your solar fraction and heat savings will be by going through this method:
http://www.builditsolar.com/References/HowMuch/HowMuch.htm
It will give you a pretty good idea for your climate and your house how much a given amount of collector will get you.

The other thing to think about on payback is that for solar thermal there is a very large saving for doing the system yourself. For the system above, my out of pocket materials cost was about $2000 -- a commercial version of such a system would like be north of $15000. For some reason I don't really understand, commercially done solar thermal systems command very high prices.

Wondering why you are steering away from drainback -- it sounds like you might have a pretty good setup for a drain back system. If done correctly, I think they are lower maintenance and provide excellent freeze protection without concerns about cooked antifreeze? I have two drainback systems here with no problems, and we get occasional -30F lows.

There are quite a few more DIY solar systems on these pages:
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm

You might find something you like.

Gary

 

[semipro]

Hi,
Agree with others that first thing to do is work on insulation and sealing, but it sounds like you have done that.

Solar thermal systems that you build can be very cost effective -- this is the system I use on my house:
http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/Main.htm

Its a very simple system that is not difficult to build (but its not a one weekend build). Its a drain back system with a large, non-pressurized tank that supplies both domestic hot water and provides some space heating. It could provide more space heating with minimal system cost increase if I had more space available for collector.
The system is designed to be robust and requires little maintenance.
The version of the system that does DHW only (with a smaller collector) has an honest payback period of less than three years.
The version with space heating payback is a bit longer depending on your climate etc.


Its important when you think about payback for solar space heating that you consider the full heating season. Its true that in many places the solar fraction for January is not great, but think about the return for the full heating season. Places with long tough winters actually do better on rate of return that "easy" places with mild winters.
Around here, we have some heating 8 months of the year.
You can actually get a fairly good idea what your solar fraction and heat savings will be by going through this method:
http://www.builditsolar.com/References/HowMuch/HowMuch.htm
It will give you a pretty good idea for your climate and your house how much a given amount of collector will get you.

The other thing to think about on payback is that for solar thermal there is a very large saving for doing the system yourself. For the system above, my out of pocket materials cost was about $2000 -- a commercial version of such a system would like be north of $15000. For some reason I don't really understand, commercially done solar thermal systems command very high prices.

Wondering why you are steering away from drainback -- it sounds like you might have a pretty good setup for a drain back system. If done correctly, I think they are lower maintenance and provide excellent freeze protection without concerns about cooked antifreeze? I have two drainback systems here with no problems, and we get occasional -30F lows.

There are quite a few more DIY solar systems on these pages:
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm

You might find something you like.

Gary

Thanks Gary. I was hoping you'd chime in.

Our heating season is about 7 months (though before I sat down and figure it out I would have sworn it was shorter).

I've been spending a lot of time on your site and the Home Power site lately (with online subscription).

I'm reconsidering a drain back system after the research. My reasons had to do with the type of (homemade) heat exchanger
I was contemplating.

Andy

 

[ChoppingAccountant]

I am going to make the assumption in your location, you need a fair amount of A/C during the summer. With that, try looking at the evacuated tube solar hot water and pair it up with an absorption cooler to (a) shed the summer heat load and (b) as a by-product provide cooling to your house.

Example only:
http://www.solarpanelsplus.com/solar-air-conditioning/

This way you can make use of the panels all year long.

 

[semipro]

I am going to make the assumption in your location, you need a fair amount of A/C during the summer. With that, try looking at the evacuated tube solar hot water and pair it up with an absorption cooler to (a) shed the summer heat load and (b) as a by-product provide cooling to your house.

Example only:
http://www.solarpanelsplus.com/solar-air-conditioning/

This way you can make use of the panels all year long.

Absorption cooler? I'll have to look into that.

Actually, we use the AC very little . Our house is well shaded to the south by large maples and it cools down at night, usually. We're at about 2000' elevation.

The trees actually complicate things a bit. They shade the house very well. I'll have to find a location for solar collectors that is not shaded by the tress. I considered mounting them at ground level on the north side of our garden but dislike the idea of trenching and insulating the lines.

 

[semipro]

Based on comments here I'm again investigating improving the house's shell (as well as solar DHW heating).

Right now from inside to out the wall is plaster veneer, 2x6 studs with kraft-faced fiberglass insulation, then 1/2" (vinyl?) faced iso foam, then cedar lap siding. The fact that we have both a kraft paper under the wallboard and some sort of vapor barrier on the existing foam concerns me a little.

The reason I think the shell is a problem is because the foam was poorly installed. It was not taped well or sealed adequately where it meets masonry or fenestration.

I'm considering a retrofit with adding another layer of foam and house wrap on top of what exists already, then installing thin vertical furring strips and the reinstalling the cedar siding after staining both inner and outer sides.

There's a pretty good document on this at the Building Science website if anyone is interested.

http://www.buildingscience.com/docu...ential-exterior-wall-superinsulation-retrofit

I'm going to have the energy audit with blower door test done first to establish an infiltration baseline and make sure I'm not wasting my efforts on the wrong areas.

Thanks to all.

 

[woodgeek]

I think the audit (done well) is a key first step before you decide what to do. No envelope is perfect, and it seems that builders are often the least concerned with your future heating bills. Your envelope does have a complex shape compared to most, and cathedral ceilings are notorious for heat loss if not done well.

I guess my point is that until you have a handle on how much energy your house is using relative to a theoretical 'best practices' house of the same size/vintage/framing/windows, you don't even know how big a problem you have to fix. In other words, you don't really know how good or bad (or costly to run) your envelope is until you measure it.

If you know the BTU output of your geo, and can time it cycling on and off at typical winter temps (with other large heat inputs 'off'--cloudy day, wood stove cold, etc.), you can compute your BTU demand per degree day, divided by your conditioned sq ft. This number, the specific heating demand, aka the 'energy benchmark' is a useful way to compare different houses' envelopes. Houses in heating climates in the US have a median score of about 10, with most homes in the range 7-13, and very few homes below 5 (mostly recent construction that took pains to be built efficiently).

If you **know** you are using buckets of kWh to heat your house, the problem could be a simple as an incorrectly sized or malfunctioning geo. Can you tell if it is firing backup a lot? Is it old?

 

[semipro]

Woodgeek,
The GSHP is about 22 years old. It seems to be in good working order though. I check the heat exchanger for mineral buildup and monitor the system pretty closely as far as water flow and temp differential inlet to outlet. Efficiency is dependent upon water temp and flow rate from the well so actual heat input would hard to determine (I think). There are actually two units. One for the upper floor and one for the 1st and basement. We use the upper only for AC and typically use only the lower for heat.

I like the idea of using the HP to monitor heat input and inside and outside temps or somehow establishing a heat usage baseline. However, with lots of vertical stratification, an uninsulated basement, thermal momentum from masonry (like my fireplace), high ceilings, lots of windows etc. I think it would be hard to get a handle on heat demand per degree-day. In fact I've often wondered how anyone could get a handle on "absolute" energy usage when there are so many variables in play. It seems even the smallest breeze could have a major impact on heating requirements because of the disruption of the boundary layer at windows and elsewhere. I also seems like you'd need inside and outside temps to stabilize for quite a while by the house reaches "steady state". Maybe I'm over-analyzing it. I'm sure my wife would think so.

I pulled some trim off the exterior yesterday and found that someone had installed trim boards and doors/windows before wrapping the house. I now feel that I need to pull the siding and re-wrap just to hold off water damage if nothing else. I think my only question is whether I want to add some foam board as added insulation at the same time. It would complicate the trim work for sure given the complicated exterior of the house.

As you pointed out the cathedral ceilings are an issue also and one I don't know how to address reasonably. The roof is ventilated and there are several strange attic areas where you can see the kraft paper on the bottom of the roof insulation. I'm sure that kraft paper is not an adequate seal to keep roof ventilation air from leaking into the house. If I had the resources I'd pull the roof and sheathing, pull the fiberglass and insulate with expanding foam, then reinstall an unventilated metal roof, closing off soffit and ridge vents.

Thanks for you input. I'll update this thread after my energy audit.

 

[SolarAndWood]

I am on gut/rebuild while we lived in it number 4. Your house looks much too nice to start tearing apart. FWIW, the felt/1" foam/wrap/siding has been a relatively easy install. Seems a lot less complicated than the 4" sandwich.