can concrete radiant floor heat be substituted for storage?

[amh]

i love this site. So much great info, but couldnt find any on this.

to start off, i live in northeast pa. I built a chalet a couple years ago and it is very well insulated. Right now i have an englander stove 30nc, 75 k btu?, in my basement that heats the whole house until single digets. Then the electric heat pump kicks on every now and then.

I am looking pretty hard at an eko 25(seem to be the best for under 5k). when i called eko, the guy was making it seem like it wouldnt be worth the price for a gasser if i didnt get hot water storage. But once i told him i have concrete radiant floor heat, his whole attitude changed, like it would almost be the same as storage.

Will concrete radiant be comparable to storage?
does anyone have a boiler hooked up to concrete radiant without storage?
once the concrete is up to temp, how long will it go without kicking on?
and finially, any other boiler i should look at?

Any help would be greatly appreciate!
thx
aaron

 

[slowzuki]

It works ok in a shop. Not the best in a living space, once its heated, you have no practical way to turn it down so the temps rise and fall. If you don't mind varying temps or if other heat source works to level out the swings its ok.

 

[brant2000]

You mention you have a heat pump, is that central/ducted? If so, how close is the chimney to the air handler for the HP? Maybe a furnace would be a better option than a boiler.

 

[Hydronics]

I believe you'd be quite disappointed if you tried to use your living space slab radiant as thermal storage. As sluwzuki mentioned it works ok in a shop but you'd be cooking, then freezing in living space.
A 4" slab CAN absorb significant btu's - my eko 60 can't keep up to my garage slab radiant when on full flow, once the slab has cooled. The problem with banking up all those btu's in a slab is that you can't control the release.
I assume you're referring to a thin layer of lightweight concrete on the 2nd floor, not a slab on grade, correct? In this case I don't believe it could accept the btu output of that boiler.
On a side note: from a purely financial standpoint you're probably better off to just pay the electric bill when the temp drops considering the ROI. Of course, there are other considerations such as energy independence.

 

[__dan]

No.The only time boiler output would be close to matching the load is the first fire bringing up a cold slab from being off for a week or so. Maintenance btus on a warm slab are a lot less, maybe minimal, and the EKO would be driven into slumbering / idling with not enough load. You would need a heat source that could modulate from the 75K btu down to 10K btu, which the EKO is not set up for, if no storage.

I would be considering a pellet boiler ( a high end European type) that can modulate the firing rate and handle on/off cycles better or storage.

I am plumbed and tapped for future storage but am otherwise heating ~ 3300 sf including the basement using the 1200 sf basement slab for storage.

Radiant with storage would be a nice combo but a pellet boiler with radiant would be high on my list.

 

[skfire]

I played around with the slab in the basement as storage and as Dan said..nope. once there not much modulation unless u leave the doors open...now I use it as a secondary overheat dump.
where in nepa are you???
scott

 

[maple1]

You might be able to get some buffering action from the slab if it's in the basement - but along with that you would get temp swings in the house.

 

[Karl_northwind]

I use my 1000 sf basement slab as storage for off-peak electric heating when propane is expensive, and when it's cold out (0 or below) I can run 30,000 btu into it for 12 hours straight, and then let it coast for 12 hours. then do the same thing tomorrow. the temperature swing is substantial, and I wouldn't do it in someone's house.
I'm just using my example because the delivered amount of heat is easily documented (I adjust the amount of heat and run time manually)

 

[TimfromPittsburgh]

One room in our house has a 20' x 24' x 4" thick radiant heated concrete slab. Certainly concrete has good heat storage capacity (about 30% of the heat storage capacity of water), but the problem with using the floor for heat storage, especially if the room is living space, is that the temperature swings the room will experience as the floor is charged and discharged would be very uncomfortable to the occupants. Covering the slab with a thick carpet might improve the comfort factor. It is probably worth doing a calculation to see how many btus your floor can store at say 110 degrees to 70 degrees F.

 

[Rearscreen]

My experience: I installed Pex Al Pex, 4 loops in the basement slab. A Buderus boiler heated the water. The one time I had all 4 loops going (28' x 39') it sucked the life out of the boiler. Once up to temp, it was a sauna down there. In my situation just the radiant above on the first floor did a good job on the basement. I did put a timer so the water would flow through the pipe for about 4 hours in a 24 hour period.

 

[Floydian]

Quick math example of concrete vs H20.

1000 sf slab X 4 in thick=12.5 cu yds. or ~ 50,000 lbs. Specific heat of concrete is .18 btu/lb/. So 50,000 X .18 = 9000 btu/.

Lets figure a 10 T would be acceptable. So 90,000 btu's stored.

200 gallons of H20= 1666 lbs. Specific heat of H20 is 1 btu/lb/. So 1666 btu/.

Lets figure a 55 T. So that's 91,630 btu's stored.

 

[Fred61]

Quick math example of concrete vs H20.

1000 sf slab X 4 in thick=12.5 cu yds. or ~ 50,000 lbs. Specific heat of concrete is .18 btu/lb/. So 50,000 X .18 = 9000 btu/.

Lets figure a 10 T would be acceptable. So 90,000 btu's stored.

200 gallons of H20= 1666 lbs. Specific heat of H20 is 1 btu/lb/. So 1666 btu/.

Lets figure a 55 T. So that's 91,630 btu's stored.

But difficult to control the release of the stored heat. It will mostly be too fast or too slow but seldom just right.

 

[amh]

thanks to all who responded, it's great that there is so much knowledge here.

brant, good thought about the furnace, but the air handler is probably 40 feet from chimney.

hydronics, the ROI is my biggest obstacle. the only way it would be close is if i could get away with no storage and build the outbuilding myself and even then its not that good.

 

[maple1]

What's hooked to your radiant floor now?

 

[amh]

dan, i considered the pellet boiler and a corn boiler, but with free wood on the property, i'm trying to stay that route.

skfire, i'm in benton, pa

and thanks to all the others for the info!

the boiler and outdoor wood boiler is out cause of price. and the wood furnace is out because of placement

so i'm now thinking of replacing my nc-30 with a blaze king princess or chinook.
with the blower, princess $3000 and chinook $3750
20 to 30 hr burn times, so they claim, i think the englander nc-30 is a great stove for the $800 i spent, but
just getting tired of getting up at 3am to reload with these temps and the heat pump still kicks on.

any thoughts?

 

[amh]

nothing is hooked to my radiant right now maple1.

the original plan when i was building was to use an amaizing heat corn boiler to heat the basement floor.
but had a buddy install a heat pump for me at cost and with money being tight when i was building, i then decided
for only $800 i should add the englander stove for the winter, it payed for its self the first year.
the stove is still in great shape, but now that i have some money and am sick of loading 4 times a day and cleaning ash
every day and getting up at 3am, im just trying to look at all my options.

gasser in an outbuilding with storage would be the best from what i have read, but with the cost of that, i can probably
run my heat pump every winter for 8 years

 

[maple1]

Bit of a shame to have that radiant & not be using it.

Where were you going to put the Eko if you were to get it & run without storage?

 

[ihookem]

I have an EKO 25 with radiant heat in the floor in half my house. It is not concrete though. It is a new house with OSB 3/4" with carpet on about 500 sq ft. , 3/4" maple floor on 350 sq ft and 150 sq with tile. A lot depends on what is on the floor. The tile keeps a lot of heat, the wood keeps a fair amount of BTU's and the carpet not so much. I can tell you the radiant floor is really nice, albiet not the best. It holds heat in the right place and that helps. I can load the EKO at dinner time , run a few loads for about 10 hours with 2 burns . By morning the floor is still noticably warmer, especially the tile. I keep the temp @ 165 or so. It is a strange set up but it works as aomewhat of a heat tank.

 

[amh]

Bit of a shame to have that radiant & not be using it.

Where were you going to put the Eko if you were to get it & run without storage?

around 20 ft from my basement door

 

[amh]

I have an EKO 25 with radiant heat in the floor in half my house. It is not concrete though. It is a new house with OSB 3/4" with carpet on about 500 sq ft. , 3/4" maple floor on 350 sq ft and 150 sq with tile. A lot depends on what is on the floor. The tile keeps a lot of heat, the wood keeps a fair amount of BTU's and the carpet not so much. I can tell you the radiant floor is really nice, albiet not the best. It holds heat in the right place and that helps. I can load the EKO at dinner time , run a few loads for about 10 hours with 2 burns . By morning the floor is still noticably warmer, especially the tile. I keep the temp @ 165 or so. It is a strange set up but it works as aomewhat of a heat tank.

Thx for the info, but seem everyone is i agreement not to do it without storage, plus the return on investment isnt there.
so, my 2 options are to keep my 30nc or replace with blaze king princes. Havr to see how these to compare.

Guess ill post in different forum, or if anyone has any info on both of these, please reply here.

Thanks again for all your help!

 

[Highbeam]

Since your nc30 can't keep up, the princess won't either. It's a smaller stove and you will not get the long burn times if you're running it high. Perhaps someone here can share wood furnace experience? If the nc30 can't do it easily then you are beyond stove territory.

Many wood furnaces are cheaper than the 3000$ budget.

 

[maple1]

If you're just considering ROI, you'd likely be better off just to keep doing what you're doing, or add a second stove.

But there's so much more to a good boiler/storage setup than just ROI. Your call in the end though, since it's you that will have to pony up for it.

 

[JP11]

For the money... add PV solar to keep running the heat pump?

it's something to look at. Depends on your state's 'net metering' laws.

There's MANY ways to skin the cat here. Do you WANT to be cutting and splitting wood? Otherwise.. have someone come selectively harvest the wood on your lot every 10 or 15 years.. and do something with that money.

JP

 

[Bob Rohr]

For the money... add PV solar to keep running the heat pump?

it's something to look at. Depends on your state's 'net metering' laws.

There's MANY ways to skin the cat here. Do you WANT to be cutting and splitting wood? Otherwise.. have someone come selectively harvest the wood on your lot every 10 or 15 years.. and do something with that money.

JP

I agree with pv to offset some of the heat pump energy. I'm hearing 65 cents a watt for modules in some markets. Calculate how many KW of PV you could buy for the cost of a wood boiler, and run some simulations for your area. I think the NREL site has a free solar calculator to get you some ideas.

 

[JP11]

yeah.. do a search for PVwatts. good calculator. I was running about 10percent over this fall. Had EPIC snow this winter, so I've been blanked out a while. weather is 30 year average in their calculations.

The company I bought from did an estimate for me. Guy on the roof, camera and GPS.. they can show you the path of the sun every month of the year. Very cool for them to figure out shading issues.

It won't work for everyone.. but it might be a good solution if you like the heat pump. Maybe that, coupled with the wood stove for the coldest few months (or as a nice, no power needed solution for outages)

JP