Actual Solar Gain in A House

Rich W

Kind of strange. I rigged an incident light meter to an LTT and found something similar. On a cloudless morning, the recovery from set-back took longer every time-as you might expect. Seems the whole house is acting like a solar collector.... Please keep your data flowing, I love this stuff.

Mike T., Swampeast MO

Is this possible?

I have accurate hourly insolation data available from a nearby source.

Condensing/modulating boiler with datalogs of operation. Fully TRVd system. Both north and south outside air temp logs.

If I compare a period of sustained operation at minimum modulation at night (no insolation) to a period with insolation but the same outside temperature is the difference in boiler output the amount of solar energy actually used by the house? If I'm lucky enough to find enough daytime periods of sufficient length and nearly the same outside temperature but varying amounts of insolation would such verify and even allow prediction of actual solar heat gain throughout an entire season?

Techman

Solar

> Is this possible?

>

> I have accurate hourly

> insolation data available from a nearby

> source.

>

> Condensing/modulating boiler with

> datalogs of operation. Fully TRVd system. Both

> north and south outside air temp logs.

>

> If I

> compare a period of sustained operation at

> minimum modulation at night (no insolation) to a

> period with insolation but the same outside

> temperature is the difference in boiler output

> the amount of solar energy actually used by the

> house? If I'm lucky enough to find enough

> daytime periods of sufficient length and nearly

> the same outside temperature but varying amounts

> of insolation would such verify and even allow

> prediction of actual solar heat gain throughout

> an entire season?

Techman

Solar

Mike T. WOW, what a question!I'm sure some of the engineering type's will have your answer.I am interested in their opinion's so I'll be quiet for now.

Brad White_61

I think your hypothesis

answers the question, Mike!

By eliminating outdoor temperature fluctuations from your night-time test case and with all other things being equal, one would suppose that daytime insolation (the best kind!) is the factor...

Hey, maybe this would be another thread, but I had a thought (which I am not sure I mentioned before but may have) about why heat loss calculations may differ from the actual experienced heat loss. Not the entirety of it but a factor.

Here it is: Cumulative infiltration.

Say you have a four-room house, equal quadrants. Each room is assumed to have one air change per hour infiltration to be taken care of by that room's radiation/emitter/air supply. Each room's heat input has to be sized for the 100% occurence.

But the building as a whole only has infiltration on two sides (for the sake of argument). The other two sides (rooms) have exfiltration pre-warmed by the other rooms thank you very much. This need of course rotates around depending on prevailing winds at a given time, but only two get the inflow.

As a result, the infiltration is overstated by 100%, it really would be half that.

I remember this coming up when teaching at Boston Architectural Center; I had my students trianglulate between the volume method, crack method, cfm per SF method. It came up in discussion the way I described it, did the cumulative sum of all rooms' infiltration really depict what was going on? One student settled on calculating the infiltration rate globally and adding it to the transmission loss which indeed is constant..

Short brain dump. Thoughts?

Brad

p.s.: Note my use if italics in exfiltration ,
Thanks to you, my friend!

Constantin

Some thoughts...

... I hope they are useful. We're just getting over our first housewarming party and have more leftovers in the fridge than you can shake a stick at. Contractor thank-you BBQ party is next weekend, should be fun also.

Coming back to your question, I would suggest that the difference may be insolation, but that you also have to account for other factors, such as time lagging due to the mass inside the thermal envelope, etc.. May not be a factor in a low mass home but should be thought about and accounted for.

Anecdotally, I notice a huge difference in the south vs. the north side of the house when it comes to insolation, with ancient (i.e. max solar gain) windows allowing south-facing rooms to shoot 8-10°F beyond their setpoint during the day unless the doors are left open and gravity convection does its magic. Even at 20°F and lower temps outside.

Given how variable solar gain is around here, I wouldn't try to predict anything from the outset... rather, I would look at the gain after the fact and look for good correlations, then establish parameters, then test them. I imagine it would have a lot to do with the time of day, the intended zone temperature, the envelope in that sector of the house, etc. Eventually, you may be able to recreate the factors that lead to fuel usage...

Given how local insolation can be (clouds vs. no clouds) I would consider investing in the sorts of sensors that allow you to instrument the house to collect insolation as well as HVAC data. To that point, I have bought Phil Malone some of the hobby boards out there to play with, hopefully, he will be able to interface the WEL to work with them.

Mike T., Swampeast MO

Back when I could predict my radiator temperatures accurately (still can't do this with the Vitodens), I once tested the effect of wind on a very windy night. Rads on the windward side were higher in temp; those on the leeward side unaffected.

Mike T., Swampeast MO

I wouldn't try to predict anything from the outset... rather, I would look at the gain after the fact and look for good correlations, then establish parameters, then test them.

That's exactly what I was planning Constantin.

I can certainly see the solar influence in the datalogs as the TRVs do their magic and keep sun-related temp differences to a minimum in the house which in turn affects boiler operation.

gasfolk

Baffling

Hi Mike,

Do you have interior baffles (i.e. walls, doors, partitions) to eliminate the gravity flow of air that Constantin describes above? If so, could an hour of observation with all baffles closed allow an estimate of 1) insolation effects and 2) passive air-flow balancing in your house?

gf

Mike T., Swampeast MO

Not really.

Ground floor is a very open floorplan--particularly towards the south where the two front rooms have 8' permanent openings to the long center hall. One of these openings is right at the foot of the stairs.

One staircase from the ground floor leads to another long central hall on the 2nd floor. Doors there (all with operable transoms) of course to various rooms but staircase is completely open.

Other staircase from the ground floor leads deep into the master suite, no doors and again staircase is completely open.

For fire safety reasons, every room of the house (except the upstairs hall bath) has at least two routes leading to different exits. (I picked up that idea from a 1900 mansion in Hannibal, MO. That's also where I first saw individual proportional radiator control--an old Johnson Electric pneumatic system with the most incredible thermostats I've ever seen.)

gasfolk

Thermal Mass

Constantin wrote: "...time lagging due to the mass inside the thermal envelope."

If an RC circuit is a useful model for heat flow through a structure...

Could you estimate thermal mass by shutting down your boiler and plotting the decay of indoor temp over time? If a low mass house should cool faster than a high mass house, would measurement of the "time constant" of a house be useful not only for research but also for practical purposes?

gf

Constantin

Right on...

... control theory applies whether one plays with electronics, mechanical thingies, or thermal ones, that's the beauty of it. All of them share analogues of capacitance, resistance, etc. that manifest themselves differently but can be modeled using the same laplace transformations.

Coming back to real-life benefits, figuring out the time constant for thermal decay and gain could allow you to program the Vitotronic's OR system time constant to be better matched to the home. One issue though is that the no OR system I am aware of accounts for insolation or wind, just thermal conditions. Then again, perhaps the Vitosolic with its external insolation sensor might come close to that since it can communicate with a Vitotronic via the KM bus.

jp_2

trv's

it seems to me that mike's use of trv's will take care of the wind and solar variables.

it would be too far complex to design/program that info into the system. wind speed, direction and its effect on the house would have to be known.

also, with solar, I do not understand why people can't be happy with "free" heat. If the room over heats, dang! go put on a pair of shorts and pretend is july in feburary! crack a window, its free!

gasfolk

Solar Plexus...

jp wrote: "Mike's use of trv's will take care of...solar variables"

Hi JP,

We completely agree with your embrace of free, solar heat. Would you agree with these (and are there other) additional benefits of better understanding solar gain:

1. Discrepancy between Manual J heat loss estimate and "Real-Time Load" (based on actual fuel usage) is often explained away as benefit from solar gain, thermal mass, etc. Better quantification of these effects may reveal the true degree of safety factors (i.e. oversizing) built into Manual J.

2. To avoid uncontrolled overheating, some on The Wall have advocated excluding solar gain (using low emissivity coatings, awnings, overhangs, etc.). With better understanding of solar gain, perhaps system design could accept, redistribute, and store (if necessary) this free heat.

Worthwhile? Other benefits?

gf

Constantin

Best approach depends on the climate, I wager.

For example, up here in the NE, it makes a lot of sense to have overhangs and windows that allow solar gain. Keep the summer sun out, let the winter sun in. Even a thin drape shade outside would do the trick.

For homes where there are no overhangs to take advantage of, it may make sense to have some solar gain resistance or the AC will be running... a lot.

Alas, I have no options re: overhangs due to the historic district we're siuated in. However, we do benefit a lot from 50'+ trees casting their shade on the house, hopefully for years to come.

Mike T., Swampeast MO

Thanks for that graph Rich. Quite interesting. Am still working on the programs--the stuff in the "Eurocave Numbers" post is the first step and I'm getting better and better at using the computer (instead of my eyes) to analyze the graphs.

jp_2

agree

i agree gasfolk.

1.) I myself would not use software where I did not know the how and whys of the calculations, I understand the 'over-rating' or c.y.a. part. doing it long hand is not that hard. I have not heard of a comparsion here between software and long hand.

2.) from what I have read here, overheating seems to come from after-the-fact and not part of a passive solar design. then its kind of too late. ceiling fans are propbably the best methods for trying to redistribute passive solar heat, trying to move it around by pipe is not heat practical.

once again, why would you say NO to free heat.

by the way, I've seen impressive solar gains on real cloudy days in my cabin. i say impressive because I expected zero gain and saw a 15F difference between inside and outside with very little building mass to speak of.

gasfolk

“It ain't so much the things we don't know...

...that get us in trouble. It's the things we know that ain't so.” Artemus Ward

Hi JP,

2. Solar heating (even passive approaches) is often dismissed from standard (fossil fueled) systems because the sun is "unreliable." Yet it rises every day. Your observation suggests a host of simple experiments. Maybe Mike T.'s experiment will inspire others to forget what they "know" to be true and simply observe the systems in their own home.

1. Manual J (MJ) and MJ software are widely recommended, yet simple, empirical verification (such as comparison to the Eatherton or Boilerpro real-time loads) seems rarely to be done and almost never to be published. True, MJ never yields a "too small" unit, but you're not a good surgeon if every appendix you remove is obviously inflamed; the good surgeons occasionally remove a normal appendix.

If MJ was developed for Air Conditioning Contractors (and perhaps the "on/off" nature of forced air systems demands larger units for faster pickup), maybe the hydronics industry could at least come up with its own corrections to MJ. Mike T.'s ongoing experiment is heartening in its possibilities.

The Emperor specifies an oversized boiler...because he has no clothes.

gf