Radiant slab design info wanted

Tom G._2

Hello..
I am looking for info on designing a radiant slab for my new garage floor. Are there any good books out there on the process? I wanted to go with solar panels, but there is no domestic water to the garage for a heat sink. It will be a closed-loop system. Any input would be greatly appreciated, as I want to do it right the first time.

Thanks in advance for your help.

Tom

Constantin

There are a couple of books that I found useful.

"Modern Hydronic Heating" or "Modern Hydronic Heating for Residential and Light Commercial Buildings" by John Siegenthaler will give you an insight into anything radiant/hydronic. The former appears to be written more for the lay person than the latter.

Dan Holohans intro to radiant floor heat is a great primer.

The "Lessons Learned" solar hot water bible is pretty indespensible as well.

I don't mean to rain on your parade but what are you going to do with the solar hot water in the summertime? Are you going to use $$$ evacuated tubes or how mild is the winter in your conditions?

As far as the slab is concerned, you can't go really wrong with ½" tubing on 12" centers inside the space, 6" OC at te edge of the building. Ideally, all loops will have the same length (<250' long) and will terminate at one manifold. Hottest part to the exerior perimeter first (i.e. first part of each loop). The slab should be at least 6" thick, with the tubing suspended in the middle and resting on 2" of XPS foam + a vapor barrier.

Tom G._2

Thanks

Thanks for the reply...
I had thought about your point of the summertime issue with the solar panels. Probably not a good idea. I live in central/southern NJ, where it can get pretty cold in the winter. I am trying to educate myself so when it's time to build it {Springtime}, I will have somewhat an idea what I am doing!! The intent of the system will be primarily to keep the garage above freezing {40-50 F.} This will be an on-grade slab for a pole-type building. I have supplemental heat(woodstove} for when I am out there for extended periods.
Is a regular hot-water heater sufficient? I understand there is a special glycol-based solution used in these systems. Anyway, thanks so much for your reply, and I will look into those books you mentioned.

Tom

hr

One of the best solar posts here

I feel came from this solar hippie, a while back. Dale and his company have designed and built plenty of unique solar systems across the country. Wise advice.

hot rod


Date: October 02, 2004 11:16 PM
Author: Dale Pickard (pick@radiantengineering.com)
Subject: Seasonal heat storage


Thanks Bob, after the '70's, I went for a long time realizing that I knew an awful lot about something that few folks were interested in. I enjoy sharing what I know and I want to know that as an industry we don't repeat the same mistakes.

Seasonal heat storage is the holy grail, but really isn't practical or economical, especially high temp storage. Do some back of the envelope calculations on more precisely what is meant by "large" storage tank and "superinsulated". Then compare that to the radiation available, the weather in a given location and the heat load of the house as it varies over time. That is exactly what software like F-chart does. See http://sel.me.wisc.edu/programs.html

For hourly simulation software see http://www.mauisolarsoftware.com/

A "typical" residential annual space heating load in a northern climate may be 50 - 100 million btu/year with most of it occuring at night and on cloudy days. What you just produced in a week is about 1.5% of the 50 m number.

This is why solar domestic water heating, SDHW, is such a better application than solar space heating. It is also why passive solar techniques are so much more effective and efficient than "active" solar techniques. A south facing window doesn't require heat exchange and gets to work at room temp, which minimizes losses from the "collector". Of course there are other limitations to passive solar techniques.

With space heating, the largest load occurs at the worst time when radiation levels and collection efficiency are lowest. Also at that time, our hydronic distribution systems require the highest water temperatures to satisfy the load. This is why low temerature delivery systems are so important. You can't drop your tank below the "minimum useful temperature" for the load. This temp is a moving target, but for space heating it can never be any lower than 70 degrees f.

As we all know, even very efficient radiant floors require much higher water temps than this most of the time, especially under "design conditions", aka worst case conditions. So the practical "size" of your storage tank will vary with the conditions. The basic concept here is known as the "utilizability" of the energy produced.

Contrast this with domestic water heating, where the load is constant on a daily basis throughout the year. Heating 80 gal of dhw over 100 deg each day for 365 days will require about 67,000 btu/day or about 25 million btu/ year. Two of your collectors in your climate will easily produce 100% of this daily load in the summer months. As the seasons change the same two collectors will produce less so that in mid winter, you may get less than half of that. The annual solar fraction may be on the order of 70% of the annual dhw load. The same two collectors would produce only a minor fraction of a space heating load.

The minimum useful temperature for dhw may be as low as 40 deg with a cold tank and as high as 140 with a hot tank. The efficiency of the collectors along with the efficiency of the load heat exchanger will vary as the inlet temp to the collectors changes.

How the load coincides with the supply is important. DHW systems that supply their load completely at night and in the morning get to meet the peak solar production hours in the afternoon with the coldest tank temperatures which maximizes the efficiency of the heat exhangers, both in the collectors and in in the tank, so the higher solar temps are produced under the best conditions.

Seriously, this is a complex subject. The many variables involved affect the results of many other variables. The best way to evaluate all of this is through hourly computer simulations where an exact answer for each variable can be calculated for every one of the 8760 hours in a year.

The information that comes from such simulations is pretty simple. Achieving high annual solar fractions and good economics for active solar space heating is difficult at best . Appllcations like SDHW, residential and commercial can yield high annual solar fractions and good economics with practical systems.

Dale



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Bob Gagnon plumbing and heating

solar works

couldn't you just cover the panels with a canvas boat-type cover? it seems like there are simple solutions to simple problems. if a superinsulated tank doesn't exist, couldn't you make one, rubber lined with 1' of foam. or 2'? my evacuated tubes are making a lot of heat. yesterday it was cloudy and it pumped 125 degree water all day! my boiler has been off since I put the tubes in, and the solar is providing 100% of my domestic and a lot of my heating. Bob Gagnon

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hr

Bob

sure that works and tinkering with solar as you and I do is one thing. But when you start adding costs of building a system like this for a customer the numbers just don't work out.

The cost of enough panels and insulated storage and piping controls, etc to use the system maybe 6 months of the year for heat??? That's the catch in my opinion spending thousands, maybe 10,000 for a system that just gets used 6 months IF the sun shines enough... hmmm.

That is the point I think Dale makes the economics may not pencil out for the homeowner with a budget.

hot rod

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Tom G._2

Thanks for the info..

Solar was always my first choice, but I know it can be pricey (I realize it will eventually pay for itself). There is no domestic water in my garage, as it is 80' from my house. I am just trying to design a simple, yet efficient self-contained system. Thanks again!!

Constantin

Consider a small Munchkin

or similar heater. The T50 has a very low minimum output (it can modulate down to 10kBTU/h), very high efficiency, etc. You could still use solar as a supplement, but solars best role (as other have said before) is for potable hot water.

Or attach the garage to the house via underground, insulated pipes. Has been done also. Might be simpler than putting a gas line or oil tank into the garage.

Bob Gagnon plumbing and heating

solar costs

are high but for how long? 30% increase in oil prices this year alone. prices could come down if materials and installs become more popular. a lot of people are paying $20k TO $50k just for radiant. at the fuel cell class at Boston ISH they told us $50K- $100K for a residential fuel cell that may last only 5 years or so. a quality solar install should last decades. if you have enough collectors and a big enough storage tank you should be able to get through the cloudy days, like I have for the last couple of months.
The guy who just built the plane and flew it up 65 miles into the atmosphere and collected a $10 million dollar prize spent $20 million on the plane. lets get the solar heating down pat, and worry about costs later. Bob Gagnon

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Kevin_in_Denver

Keeping garage temp above freezing

Think about this:

1. Most garages are drafty. Heat loss from infiltration is likely the largest of all the heat loss mechanisms. Seal the garage as you would your house. (Keep CO safety in mind at all times)

2. Insulate the walls of the garage. (less than $0.50 per sq. ft.)


3. The slab is coupled to a free heat source that is always above freezing. Therefore don't insulate the bottom of the slab.

4. Most garage slabs get colder than freezing anyway. This is due to perimeter losses. Therefore install perimeter insulation to a depth of at least 3'. Use at least 3" of EPS. Some horizontal buried EPS can also help a lot (study the topic "freeze protected shallow foundations").

5. South facing windows are all it takes harvest some solar.

I guarantee this garage won't freeze anywhere in the lower 49 states. I just built a sunroom like this, and it is staying above 60 all the time, even when our nights are going down to 10F.

Weezbo

It makes me wonder what state your in.....*~/:)

then here is another thought ,what do you want to use the garage for ...say it was to park a car, the car would be happy and never complain..:) you could buy books and leave them in the garage maybe it will learn to read:)And you wouldnt need insulation ,solar panels,radiant floor heat ...a heat loss calculation or insulated garage doors And..you can buy some cheap books for the car hey its gonna be lots cheaper than buying all kinds of construction how to books:)

Unknown

it is dangerous to advise not insulating under entire slabs without paying attention to site conditions.

I have personally seen slabs that could not heat the space because they were losing so much heat downward in wet springtime weather.

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Kevin_in_Denver

garage slab heat

The details for a shallow frost protected foundation also help against groundwater problems. My proposed scenario is primarily to prevent freezing for zero fuel cost (no pipes in the slab, no boiler). Groundwater is obviously always above 32F even if ambient is less.