Slab Radiant Loads

[mtnmizer]

I have the new econoburn 150 up and running and have
been testing it trying to learn how it all works.
So far, so good, ol man winter gave us a parting shot
of arctic temps so that was good to have for loads.

My basement is aprox 2500 sqft and has 9 300 ft pex loops
in the slab over 2" blueboard base. It's fed via a grundfos 15-58.

Yesterday I dialed the lp boiler down to 90 and ran up the econburn
to 175 then set up the thermostat started moving water.. It took less than
ten minutes to stall the wood boiler!! By stall I mean the boiler temp dropped
from 175 down to 145 and shut off the main circ.


Granted I had maybe a quarter wood load but it was gassifing good.
I was kinda shocked at how much load it takes to bring up the temps..
I'm guessing it will take fine spits loaded to the max a couple of times
over to move this.. I ran three smaller loads into the slab last night
and barely got warm water out of the return, finally dialing down
the mixing valve down to 100 degrees, the basement felt very
hot even tho the thermometer said only 73 degrees. It looks like
good storage as the whole house was warm this morning even thou
the wood boiler went out sometime during the night..

Any insights on this would be very appreciated..Is it better to
heat a storage tank or the slab? Will the storage tank pull down
the boiler as much as the slab does? Is there a good way to put
in a small flow all the time? tnks

 

[DaveBP]

A number of radiant floor control schemes use almost constant flow through the floor by varying the temperature to match the heating demand. The slab responds to conditions so slowly it's hard to hit a steady comfort with on-off control.

The only setup I am familiar with uses a microprocessor (Tekmar brand, but there are others) to compare outdoor temps and indoor temps and a number of fudge factors that you can dial around to suit your setup. This adjusts the position of a mixing valve to vary the temp through the floor.

Radiant floors use relatively low temp water but if you're circulating all the time it might be tricky feeding even low temp water with a wood boiler unless you feed it all the time.
It would be another good argument for heat storage for the sake of your own convenience, if nothing else.

If you don't want to hire a professional radiant designer to draw up a system for you that you could build and want to do it all yourself I can't recommend any better start to learn the principles than Siegenthaler's book, "Modern Hydronic Heating". It's not light reading. It's a tech college textbook. It's not cheap.

The best place to get it (and I think the cheapest, also) is from the author himself.

http://store.hydronicpros.com/category.php?qcc=1002&qpg=1

 

[mtnmizer]

Thanks for the info, education always seems to have it's price. No matter
where one gets it. There's a bunch of hydronics specialist guys
in a nearby town, I bought a Caleffi valve through them and need to
get back with them and get a professional opinion on my system.

I played with the boiler last night for a few hours.
I loaded the woodboiler to the max with small splits then fired it to 190 and then ran it through
the slab. It didn't stall but it was interesting to watch the temps fall to 130 over a half
hour.

I refilled on a big bed of coals and brought it back to 180. On the return side
of the pex lines there a valve on each one. I closed the zone into 1/3's and the difference
was amazing. No thermometer to determine the delta but it had fairly warm water coming
back. In all I ran 2 full wood loads into the slab and could feel the location of the loops in bare feet
very easily.. In the middle of the night my wife wakes up and says it's too hot and go open
a window or something

Once I get my storage running I think it will make this a perfect way to heat the house..It
will need to be rezoned/tuned with it's own control system..

 

[rowerwet]

from what I have read, it can take days to get a slab up to temp, it will take a day or two for the slab to "run out" of heat, the slab is your storage.

 

[cguida]

We find that we can either charge up the tank, or raise the temperature in the slab (hot brick), but that it is often difficult to do both at the same time. The sequence for warming things up in anticipation of a cold night goes something like this -- 4pm make a fire, and lower the slab thermostat so it doesn't grag all the heat. Get the rest of the house up to temperature, and start charging the tank. By 10 or 11,the tank ought to be up around 70 degrees. So now adjust the thermostat so extra heat goes into the slab. The slab is never actually cold -- just less warm. It will be warm in the morning. And the tank won't be that far down The slab serves as a kind of 2ndary storage.

 

[jklingel]

Dave: Siegenthaler's book. Amazon. $123.50. 2nd Edition, new. Believe it is the same one as on his web site, though I just took a quick look there. Just ordered one; have heard before that he knows his stink on this topic. j

 

[DaveBP]

When I bought mine last year Amazon was a couple bucks more. These things change daily.

If it isn't a whole lot more money I still prefer to buy from the source. The ones who did all the work get the reward, and all that. But Amazon ships free for a lot of their books and I buy a number of books through them, myself.

For anyone planning to put in their own hydronic heating system and spend the kind of money it's going to take to do that, this book will be the best bargain of the whole project. Just don't try to read the thing all in one sitting. Your head will explode.

 

[in hot water]

a cold concrete slab will really overpower that, or any boiler on start up. Consider the amount of mass and the energy required to change the temperature even 1 degree.

A yard of concrete weighs about 4000 lbs. A yard covers about 110 square feet. So you can calculate how much mass you have to "rev" up.

a cold slab will also keep a boiler in condensing temperatures for a long time. You need a means to assure the boiler never runs below 140F for extended periods. Sounds like you have that covered.

hr

 

[Fred61]

Start up can be deceiving. Run it for a few hours. I have a staple up system with no more than 400 feet of 1" tubing per zone. The returning cold water from the cold start cools the boiler rapidly with that small amount of tubing. Also for the first few minutes the mass of the tubing and immediate surrounding surfaces keep the returning water about 25* lower than supply. After 30 minutes or so I only have about a 10* drop then it continues to drop slowly from there to about a 6* differential. I wouldn't worry about finding the correct temperature and flow to keep constant circulation.

 

[SE Iowa]

mtnmizer, what is your design temp of your primary loop? Are you running directly from the boiler into the slab? I have multiple zones that cover over 4500 sf. Occasionally my NG boiler will electronically kick off and I won't notice for 8 to 12 hours. By then 4 or 5 zones are calling for heat so we have to heat alot of water and sometimes overshoot the thermostats by 2 degrees. It seems very hot when you overshoot by 2 degrees with radiant heat. My design temp for my primary loop is 136F max. Once the temp goes over that the boiler shuts off so that we can not circulate too hot of water. This helps the pumps stay on when its cold outside and there is a good call for heat (i.e. the only way for air temps to rise is for the zone to stay ciculating). I would think that if you are running 175F water into the slab the air temps would fluctuate wildly (even in concrete) as you "pulse" with high water temps?

 

[mtnmizer]

I'm sorry to say I can't give the design temp of the primary loop.
This a new install and was done by a master plumber.. I think they
did it by seat of the pants. It has 3 zones of baseboard, a DHW zone
and one large radiant slab zone. All the heat zones are close spaced tee's.
I know the plumber has a copy of Siegenthaler’s book as I asked him
some questions.

The LP boiler is a 150k weil mclean phd and the output was set on 180. The primary loop is
1" copper but the manual for the lp boiler spec's 1.25" and prints that in capital
bold letters...I asked a different boiler guy about that and he didn't seemed too concerned
as the primary is maybe 6' in length and has a Taco14 pushing it.. All the zones
are fed off grundfos 15-58's. I reset the output to 130 and it works good so
far, off of the wood boiler.

I really don't think the general contractor or the heating guy paid a lot(or any) of
attention to design on this job. This biggest shortfall I see is in the
radiant slab system, it feed direct off the primary and has a taco
tempering valve that was set on 130, now on 100. I think the
whole thing could benefit from some proper design.. Just by
getting it up to temp is seems to heat the whole house
by itself.. What I'm seeing is that when I set the thermostat
on is suck down the propane is massive amounts the overheats.
I'm hoping to get a steady feed via the woodboiler storage and get
it stay at a steady 70 degrees.??