Lightweight concrete?

Unknown

I have a printout of a page on the Wall dated Jan 23, 2000 where Sven has replied with two formulas for lightweight concrete. One Siggy's and one "Youker" mix courtesy Dan H. Can anyone tell me how many cu yards these make? I can repost them if I have to. The other question would be like how much does one square foot 2.5" thick of each weigh when cured?

gregrehn

are you refering to a thin slab application? True lightweight concrete is a poor choice for radiant. Due to its low density, its ability to transfer BTU's is very low. You can reference this info in Siggy's Modern Hydronics. As for your area problem convert to inches^3 from yards^3 and insert into L x W x H = volume.

Unknown

Thanks,

We started out wanting to do steel structure / steel decking with tubed 6" concrete floors. It was supposed to provide thermal mass. This has proven to be a pretty expensive idea. Now we want to think about regular wood (LVL / TGI) construction with a tubed lightweight thin pour on each level. I'd love to use 2-3" of regular concrete but this is too heavy for wood construction, from what I'm being told. Any ideas are welcome! Typical job, "What's the best way to do this?" So you give them an answer, then, "Ok, now how do we reduce the costs?" (By doing it a way that is less than the best, that's how.) LOL...

gregrehn

define less than the best. do you want thermal mass? how about thin slab concrete or gypcrete. Stored energy in a slab has few benefits.

Unknown

Storing heat...

this house will be a hybrid solar/air source HP house. We want to warm the concrete during the day so that it can release that energy at night. In the summer I want to cool it at night during the off peak / cooler night time temps so it has a head start on the following day. Only by a degree or so. Like, in heat season I'd boost the slab temps at 11 am by 1* till 5 pm so that the system is "forced" to do the bulk of it's work when it will run at it's highest efficiency.

"less than the best" = Wood and 2" of gypcrete instead of steel and 6" (or even just 4") of concrete. Actually I think 6" is too much but the arkeythechs don't.

Unknown

The interesting part...

is I have 5000 sq ft of house that only wants roughly 40,000 btuh to heat and 21,200 btuh to cool.

Unknown

Bump...

Kevin_in_Denver_2

Got Engineer?

It helps to think things through before construction starts.

A few hundred dollars for a mechanical engineer might be worth it.

For example, pumping off-peak electric energy into a slab sounds worthwhile, but it is very rarely done. Therefore, there are no off-the-shelf controls available for that scenario.

Unknown

Sure there is...

A programmable T'stat is already serving this purpose on another job I did. By boosting (or lowering) the slab temp 1* during the correct time of day it makes it more likely that the system will do the bulk of it's work when it is running most efficiently. The architects have an engineer. We haven't gone over it together yet. This is all still in the very preliminary stages. Incidentally, looks like they're going to compromise and give me my dense concrete at 2"-3". I told them density was more important than thickness for our purposes.

Tom Manton

Thin slab

I did thin slab pours in my house. 1.5" home made lightweight concrete: 1 bag portland 1 bag perlite 28 shovels of sand, yeild a little more than .5 yd.

I don't know where the idea comes from that concrete is not a good storage medium, I know of many applications where it is used for storage and buffering. I know in my case, I used it to buffer against frequent power outages and on a cold January day my floors will maintain consistant livable temps for many hours 18+ and the house will maintain livable conditions for over 24hrs.

greg_47

Modern Hydronics has a great explanation of using light weight concrete. It references possible striping due to its reduced ability to diffuse heat.

greg_47

5000 sqr ft at 6" depth would take about 73,500btu to change the temp 1*. This does not even take into account the heat needed to maintain the current temp. high mass has limited benifits.

hot_rod

Too much of a good thing

high mass slabs can pose some real control issues. If it will see any passive solar gain at all, over heating can be something to think about.

large thermal flywheels are of limited use unless you can harness the output well.

hr

Unknown

Yes, excellent points.

By design some of it WILL see passive gain. The idea is to capture that energy with the water in the tubing and dump it into the rest of the system, thereby distributing it throughout the parts of the structure where no passive gain is seen. I'm thinking this is a way to help control the overheating problems you're talking about. Pull that heat out of there and put it to use elsewhere. The best part is it will all happen as a matter of course. I'll have the capability of dumping some excess heat if need be.

We're also looking at vacuum tube heaters for DHW supplement and even though Viessman says no, I think we can use them to put heat to the system when there is no call for DHW. Hence the trying to store heat during the day story.

I wasn't happy with 6" myself but the architects were insisting on it at first. Then they figured out how much it will cost to support a couple 2500 sq ft x 6" slabs and they started changing their minds. I've been thinking 3" sounds more reasonable? I just like that number for some reason. 2" seems a little light. This will NOT be lightweight concrete, as far as I know now, just to reiterate.

Chances are we'll run the floor water temp somewhere between 80*-90*, as low as we can. We may end up going as low as just a couple degrees above desired space temp. The place has already been described as a big walk in cooler. We will have small HV air handlers that will stage in to pick up some of the load for the coldest times. They will also humidify and perform mostly as dehumidifiers in the summer, if we actually do cool with the floors. So far it looks like we will.

hot_rod

with your numbers

of 5000 sq ft and a 40,000 load, at design I assume? That is a load of 8 btu's per square foot.

I think you will have a hard time controlling such a slab with such a low load. At 70 degrees room temperature a 74 floor temperature will output that 8 btu/ft. It just seems at any day less than design you may have some over heat issues? That much thermal mass doesn't change temperature quickly, even with a back room to dump it.

Comes a time when radiant floors, especially high mass radiant, just isn't the right tool for the job. You sure don't want to have to open windows to control the temperature mid winter the old "double hung" thermostat.

hr

Doug_7

Adding Thermal Mass

Increasing thermal mass is a good idea for improving energy efficiency by evening out the daily temperature cycles. I have not seen documented cost savings.

But is the floor the best place to add more thermal mass to a home ?

I say 1 1/2 inches of PEX in Gypcrete is enough thermal mass for a floor.

If you increase the floor weight beyond this, you get temperature control problems, slow response time and the structural weight problems. Then someone lays a carpet on the floor and screws up the whole concept.

I would prefer to add thermal mass using a feature wall of concrete, stone or brick. Wall would run up through the living space from the foundation to the roof. Add as many tons of concrete as you want.

If you really want to store heat when the sun shines for use at night - put in a solar panel and (big) water storage tank in the basement.

Doug

Unknown

Water temp,,,

is the key. A water temp close to desired room temp should negate overheating problems. You're right, it will take minimal input to keep the place comfortable and that's the idea, minimal input to the floors. If I can heat the place with 70*-72* water, I must be doing pretty good on fuel consumption. Especially if on a sunny day the water from the sunlit areas comes back at 80* or more. The idea of using the floors as thermal mass is not mine. The architects have designed things so a portion of it will absorb and hold heat from the sun when it's available. That's why they were leaning in the direction of 6". They also want to heavily plaster the walls and ceilings for even more mass. I'm just the one who wants to tube the floors and try to redistribute some of that solar gained heat to the rest of the slab(s)in the process. It was already in the plans, I'm just trying to put it to better use. They want to tube the walls and the ceilings too. I'm telling them it's overkill. As far as catching up quickly when needed, that's where the AHs come in.

I won't have a back bedroom to dump heat into, I'll have 12,000 gallons of H2O stashed under the garage. That water will come into play for cooling. I've been thinking about using it to store heat as mentioned but figure the same thing as above with too MUCH mass it'll take a lot of heat to warm that water up and some will be lost to ground. Is it worth it? I could insulate the tanks but I would rather have them losing heat to ground in the summer for the cooling aspect.

Unknown

I should add...

I misspoke when I said raise slab temp 1* I should have said raise AIR temp 1* from 11 to 5. The idea being to cause the system to run during the warmest part of the day but not overdo things. We set back at night and let the stored energy bleed back out into the space. Shade will come into play somehow as well. I'm thinking if things start to overheat, say my floor system water temp exceeds a certain #, then we somehow automatically shut the sun off to the floors. With the circ running it will help to redistribute the heat from the still warm concrete and I'm thinking it will even things out quite nicely. We're tubing the basement and garage too. Plenty of places to put that heat. Essentially, I can heat that place on a cold but sunny day using only 4 - 007 for a good portion of it. I think that's an amp. Or then again, there's that Grundfos VS that I kind of like.

hot_rod

A reset control

with outdoor, and indoor sensors would be a good idea. Maybe a way to read the slab temperature and crunch all 3 numbers to get a good control. tekmar? or a custom designed PLC type of control.

I still think the slow responsiveness of the large mass will make it hard to react quick enough to temperature swings.

With loads that low the building may be heated by the appliances, lighting, and occupant load

hr

Unknown

I was talking to my Wirponor...

rep and we though a few cats or an energetic dog would probably do it.

The AH's make up for that. AHs are second stage. They'll receive at least 110* water and will be used to overcome the lag you're talking about, we'll need to humidify anyways. Given that, I would think that floor water temps closer or AT desired room temp would work. The closer the floor temp is to desired room temp, the more the AHs will run to compensate. Think of it as floor "warming" not "heating". We're tempering the structure with the floors, not heating or cooling it.

Besides, the thing has +40 walls and at least +50 ceilings so the house itself won't react all that fast to the temp swings either. I WAS thinking about a certain Robur GAHP for my chiller/heater but they only offer one size and it's waaaay too big for the load. We may turn it around a bit, go water to water electric HP and use the cisterns that way. Wells have already been planned for conditioning them so it's not a stretch at all. Now, you hook me some of them fancy vacuum tubes into the mess somehow and where might that take me? I know you're involved with those but forget what brand. I'm interested seeing some different brands. I've seen the Viessman stuff. It's cool looking. The architects are having trouble "reconciling the array to the outside appearance of the structure".

Another side benefit, because we're tubing these sunrooms, on the cold cloudy days when they can do nothing, the tube will warm them.

I came up with 22,000 btuh to cool this place. 5000 sq ft. I did a rough calc on Horgan's Omniclac. It hasn't failed me yet so I have to believe I'm close. It's mind boggling to an old brain like mine. LOL...

Unknown

Anyhoo,,,

I got to go tube up a boring old 13 SEER split now. Sigh...