Is EPDM for tank liner a catch-22?

[Rory]

I sure hope not, because I just started using my home-built tank-n-Tarm system. But, the real question involves storage strategy. The liner supposedly shouldn't be regularly heated beyond 170F, yet that is precisely the temperature at which my plumber claims my baseboards really do their thing well. Wouldn't we want to heat a bit beyond ideal and cool down not much below it?

For example, last night I elected to not stoke anymore wood because my tank had reached precisely 170. When I woke up this morning, it was down to 140. I wasn't confident that 140 would be enough to keep my house warm all day, so I had another fire this morning, and the evidence indicates that that was probably a good decision.

I guess I'm wondering what temperature regimes are typical for effective heating of a typical house in a cold climate.

 

[NHFarmer]

I have 1000 gallons of storage with an EPDM liner.I run my tank up to 170*-175*.I have gotten up to 180*,but that has been my max so far.I also load with smaller loads as I start to reach my desired temps.You can also turn down your thermostat on the Tarm and just let it idle.You will loose some efficiency but you will be able to extend your burn time

 

[pybyr]

this is the reason I decided to launch into a one-off 409 stainless tank;

the delta -T between the max temp of your tank and the minimum effective output temp of your heating system defines the useful capacity of your storage

the closer to boil you can push your storage (which you cannot do with EPDM), the greater net effective heat storage you can acheive

 

[Vtgent49]

I run 140 thru my baseboards all the time. Depending on ambient temp and the load it may be plenty. You'll have to develop a feel for it.

 

[Dave T]

The more BB you have the lower temps that you can run, if your house is like mine 160 and above is ideal but again I can install more BB and heat with cooler water..Dave

 

[Nofossil]

I run my baseboards all the way down to 120. I'm adding a radiant zone for the main floor to allow me to use water down to 100 or so to extend my storage. I have a stainless unpressurized tank, and I rarely get it above 170 at the top.

At lower water temps the baseboards can't keep up if it's very cold outside. For that reason, I plan my fires for late in the evening so that I'll have the hottest water at the coldest time of the day. I also heat the house 2 degrees warmer when I'm heating direct from the boiler vs. when I'm heating from storage.

 

[stee6043]

I'm able to heat with a water-to-air HX all the way down to 120ish on warmer days (upper 20's, lower 30's, a little sunshine). But once I hit low 20's and below my furnace has to run non-stop with the cooler water. 180 degree water running through the furnace is prime. If I could keep my tanks above 160-170 24/7 I would be in heaven...

 

[Eric Johnson]

Consider that EPDM is used for roofing, where I'm sure surface temps exceed 170, especially in the South, on a regular basis. And those roofs last for 20 years or longer, and see a lot more abuse than a tank liner. Just a guess, but I think the 170 spec (I thought it was 180) is probably a tad conservative.

 

[Grover59]

I get my tank up to 165 most of the time, however the reason I installed radiant heat is because I can get a lower tank temp and still heat the house, I do use my baseboard and it will keep the house warm even at 140* but may not take the house up to 70* The radiant floor heat gives me alot more useable BTUs out of my tank, and is more comfortable at lower temps. The way I have my system set up now is the circulator on my baseboard will only kick in if my boiler loop between the oil boiler and wood boiler is up to 180* and this is only when my wood boiler is fired and running. Most of the time I start a fire when I get home at about 5:30 or 6 and the base board will just start working once the loop gets to 180* all the rest of the day the house is heated with only the tank by the floor heat or my forced hot air set up. Because no one is home during the day we do set the temps back to 65 just to save a little, this so far has worked very well. I have been using nofossils burn calculator to set up my burn time and it works well, say if I have my tank at only 150 I can look at the calc for the next 8 hours or so just to see if I would have enough btus to get by until I get a chance to start a fire, I have skipped fires based on that calculator, and came home to see that my tank temp was just about where I thought it would be. Thanks Nofossil

Steve

 

[Nofossil]

I have been using nofossils burn calculator to set up my burn time and it works well, say if I have my tank at only 150 I can look at the calc for the next 8 hours or so just to see if I would have enough btus to get by until I get a chance to start a fire, I have skipped fires based on that calculator, and came home to see that my tank temp was just about where I thought it would be. Thanks Nofossil
Steve

Geez - somebody's using it! I guess I won't take it down.....

I'm glad it's helpful to others. I use it every day, and it's pretty darn good once you get the heat loss factors dialed in.

 

[Grover59]

I have been using nofossils burn calculator to set up my burn time and it works well, say if I have my tank at only 150 I can look at the calc for the next 8 hours or so just to see if I would have enough btus to get by until I get a chance to start a fire, I have skipped fires based on that calculator, and came home to see that my tank temp was just about where I thought it would be. Thanks Nofossil
Steve

Geez - somebody's using it! I guess I won't take it down.....

I'm glad it's helpful to others. I use it every day, and it's pretty darn good once you get the heat loss factors dialed in.

Yes when I got the right numbers in it was very close, I did modify my heat loss a little, is this written in perl on your server?

Steve

 

[DaveBP]

This thread is a good illustration of why it is so difficult to answer the frequent question "How much storage do I need?".

Different heat emitters work best and are sized to work at different temps.

Baseboard is the most problematic for wood boiler/heat storage systems, both because it is so common and typically is expecting the highest temps. Those buildings that heated adequately on the coldest nights when supplied by fossil fueled boilers @ 180F are going to struggle when the woodboiler storage tank temps drop below that.
For this very reason, some on this forum are being wedged into a cold corner now that January is doing its stuff.

While this is an argument in favor of pressurized storage ( it can be used to higher temps ) it's no help for those who already have unpressurized rubber-lined tanks or simply can't get a steel pressure vessel into their basement (not everyone is so eager to put another taxable structure in their yard).

If you can't get enough heat out of your storage tank to use with the baseboard you have you can simply put in more baseboard. A very well known radiant heat designer here in Maine uses regular Lowes Depot finned-tube baseboard where radiant heat can't be installed. He uses them down to 110F. It requires using more than double the linear footage but that can get a lot of usable BTUs out of you storage tank. Back in the glory days of local town dumps (sigh) you could find usable baseboard piled up for the taking. No longer free but it's still relatively cheap. For lower temp usage it's best not to install them one over the other up the wall.

 

[tom in maine]

I think EPDM has proven itself over the years. However, there are so many variants out there, that I think some EPDM pedigree would be nice to have for heat storage.
There are a lot of additives that can be put into roofing material that might not be very good for a storage tank for long term water storage.
Occasional roof wetness is a little different than a fully wetted tank that goes to 170 or 180 every night.
That being said, EPDM is everywhere and is reasonably priced.
I am somewhat concerned about longevity, both of the EPDM and its breakdown and how it affects copper heat exchangers.
Once again, you need to buy a liner from someone who knows about the use and has some track record.
I guess there are not too many people out there who have used EPDM for long enough to answer that question. And there is also an issue as to what the EPDM
was produced for.
The issue with the wood boiler storage marketplace is that it is a tiny, tiny market when it comes to EPDM. I suspect not many people are producing for us.

That being said, it does work. For how long and how hot, we get to figure that out!

 

[BioHeat Dealer Guy]

we recently had a customer who had been using an EPDM lined tank for seven years that was now weeping. He had regularly run his tank to 180+* and the liner had become soft - kind of breaking down a bit - in some places. As I understand it we would normally expect at least a ten year lifespan out of the EPDM liner with lower temps. This is only one example, but gives some insight, I hope.

Others have mentioned the argument for pressurized storage - pros and cons - and I agree. I would also like to point out that our square tanks offer a welded and fitted PVC liner that is more tolerant of higher temps in an unpressurized storage tank. Not so accessible to DIY'ers though, I guess.

One last thing is that the tank/heat storage system is not typically playing a major role in the heat delivery during the coldest days of the year when highest water temps are required. Most if not all of the heat being generated would normally be going straight into the house with a much more frequent wood loading too. Is this what people are typically seeing?

 

[free75degrees]

This whole epdm issue is my only concern/regret with how I set up my system. In the end I am not sure if I had any other options though. Are there any other options for the do it yourself tank builder with the constraint that the parts have to be small enough to fit through a standard doorway? The only other thing I can think of, and I wish I has pursued, was a solid concrete storage mass. Concrete has half the heat capacity as water per volume, but I'd be willing to make it twice as large if it meant that I'd never have to worry about it again. My thought is a large mass of concrete with 1/2" embedded pex and with no more than 2 or so inches between each run of pex. I'd also add lots of strips of aluminum flashing embedded in the concrete to help spread the heat.

Anybody ever do anything like this? Any thoughts on whether it would work? I definitely push my tank to 180* so if it starts to weep in 7 years I may do this.

 

[BioHeat Dealer Guy]

free73, I don't think you should have serious regrets about having used EPDM. We have sold lots of tanks for years with EPDM liners. From a price / practicality perspective for the DIY'er, I think it's hard to beat. Also not too bad to replace when the time comes.

As far as your concrete storage mass, my only comment would be that I don't think concrete and aluminum go well together(?). With the kind of spacing you are talking about for the PEX, I doubt you would have any heat transfer problems. Very cool idea.

Chris

 

[WoodNotOil]

This whole epdm issue is my only concern/regret with how I set up my system. In the end I am not sure if I had any other options though. Are there any other options for the do it yourself tank builder with the constraint that the parts have to be small enough to fit through a standard doorway? The only other thing I can think of, and I wish I has pursued, was a solid concrete storage mass. Concrete has half the heat capacity as water per volume, but I'd be willing to make it twice as large if it meant that I'd never have to worry about it again. My thought is a large mass of concrete with 1/2" embedded pex and with no more than 2 or so inches between each run of pex. I'd also add lots of strips of aluminum flashing embedded in the concrete to help spread the heat.

Anybody ever do anything like this? Any thoughts on whether it would work? I definitely push my tank to 180* so if it starts to weep in 7 years I may do this.

Concrete doesn't last forever either and I would be skeptical of what would happen to it over time expanding and contracting. It would definitely last longer than an EPDM liner (no question about that), but in the end would be much harder to get rid of and replace. Imagine what someone 50-100 years from now would have to deal with. Also, I think it is much slower in transferring the heat when you try to draw it back.

As a side note I am the 8th generation of my family to live on my land and we have to deal with several concrete structures from the 30s and 40s that are large crumbling messes taking up space. Most of them have to do with cisterns for holding water. Think about your legacy when building things of concrete. Especially big things. We are often short sighted in our building techniques these days.

 

[steam man]

This whole epdm issue is my only concern/regret with how I set up my system. In the end I am not sure if I had any other options though. Are there any other options for the do it yourself tank builder with the constraint that the parts have to be small enough to fit through a standard doorway? The only other thing I can think of, and I wish I has pursued, was a solid concrete storage mass. Concrete has half the heat capacity as water per volume, but I'd be willing to make it twice as large if it meant that I'd never have to worry about it again. My thought is a large mass of concrete with 1/2" embedded pex and with no more than 2 or so inches between each run of pex. I'd also add lots of strips of aluminum flashing embedded in the concrete to help spread the heat.

Anybody ever do anything like this? Any thoughts on whether it would work? I definitely push my tank to 180* so if it starts to weep in 7 years I may do this.

I think the biggest problem you'd face is that concrete would probably not like the temperature cycling and especially higher temps of up to 180 F. There are electric boilers that use off peak rates to store heat in high temp ceramic mass. Forget the aluminum plates. They would probably just corrode.

Mike

 

[tom in maine]

A thicker slab or storing heat under a slab is a very valid concept. You cannot heat it much over 120, without comfort/concrete degradation issues.
However, a small temp swing in a massive slab yields a very large storage effect, much larger than most tanks. Obviously, you must set it up to not thermally shock
the boiler, but it has some legs. Do the math. You can assume the specific heat of sand or concrete at about .2

 

[free75degrees]

Ok, so i think i am being convinced that concrete is not so good. I would have tried to do it mostly under my basement slab to avoid the issue of a huge chunk of nuisance concrete 50 years from now, but the high temp degradation concerns me. However, maybe degradation doesn't matter in this case. If it is all below grade so that the concrete does not need structural integrity, maybe some cracks and crumbling would matter. It might reduce the heat transfer a bit. I dunno.

How about sticking to water and an alternative to epdm? Can anybody think of an easy wat to attach sheets of metal to form a good, permanent seal?

 

[DaveBP]

Using any affordable solid for storage will always run up against 2 disadvantages.

The first one is thermal conductivity. How fast the heat can be drawn away from the supply water from your boiler. Fast enough to keep your boiler from idling its life away? Fast enough to supply the heat demands on these January nights?

But even if you had a 20 ton block of copper cast around your copper heat exchanger tubing you would still encounter the very great advantages of water. With careful design it can stratify as it stores heat and remain stratified as it gives up heat to the loads. With any solid ( assuming it can conduct the heat fast enough) you will have the temperature of storage rising evenly throughout with the amount of heat stored. The hotter the storage the lower the temperature difference between the boiler and the return water from storage. So it becomes harder to extract the heat from the boiler as the burn continues. With a stratified liquid the hottest fills in at the top and squeezes the coolest at the bottom out to return to the boiler because they don't mix. You have the coolest water returning to the boiler for the most efficient heat transfer out of the boiler up until the end when the hot water makes it down to the bottom and the tank is full of hot water. How perfectly this works depends on lots of variables but that's the ideal.

When you throw in the fact that water is still pretty cheap and stores more heat per pound (or per kilo if you use metric water ) than almost any other material around... water looks pretty good. If it just didn't have the nasty habit of looking for a place to leak it would be almost perfect.

There are materials called eutectic that can throw in some extra heat storage per volume into the design. If you had one that liquified at 160F or so you could store more heat at the higher temps that baseboards prefer but they are expensive and maybe corrosive. I haven't followed that stuff for decades now.

Water is pretty much where it is at. But the image of a 20 ton block of copper is awesome, isn't it?

 

[leaddog]

years ago I built a HASHA which was a fire box surrounded with sand enclosed with concrete. The fire box had a steel top on which there was a copper hx going up into the sand. The spacing for the tubes were about 6in and there was also a pvc hx with 6in spacing about 6in above the fire box top. The whole thing was about 8ftx8ftx8ft. I insulated it with fiberglass and blown insulation 10in thick. The instructions said to build SMALL fires as the heat transfer was slow. I need more heat than I was getting so I started to build larger fires. Pvc WILL exspand to the size of a softball at 20lbs pressure and leak. My temp. just above the 4in layer of sand got to be 220* and the sand wasn't able to absorb the heat fast enough. Ended up taking the pvc out, putting in all copper. I did work but you need a fire going most of the time and getting the heat out wasn't fast either. I had to shut it down when I got a leak in Jan and the middle of the tank was still over 50* in May. Water still is the easiest to work with.
But on the same thought if you wanted to build a huge heat sink under the floor and could insulate it with out having any water infiltrating the insulation it could be done but you would need lots of pex and would take a long time to get up to heat. You would have to keep the temp down but with alot of thought it might work BUT if you had problems I wouldn't want to be the one digging it up and reengenering and repairing. It would be a one time thing and hope it works.
leaddog

 

[Fred61]

I built a passive solar house in 1980 with 360 sq. ft. of site built collector in the roof. Heated air was ducted down into the basement and pushed through 1000 concrete blocks laid on end and back to the collector. Bear in mind now this was the whole floor. On top of those blocks was about a foot of concrete with six inch duct work laid in it about every two feet. Total weight estimated to be about 50 tons. The whole idea was to capture the heat from the collector with the blocks and run the house air through the ducts, thus tempering the living space air when the living space was overheating during the day and bring it back when the house started cooling.
It worked to a certain extent! The problem was that I couldn't transfer the heat fast enough either in the heating cycle or the retrieving cycle. The air from the collector came down at about 140* and returned at about 139.9* and the house air stayed too warm on the storage cycle and wouldn't grab enough heat on the warming cycle. Concrete is a poor conductor.
Water is the standard and has a specific heat of 1 and everything else (with the exception of some eutectic salts) is less than 1 and it is a good conductor.
Eutectics have been used in some experiments and in some wealthy person's projects but with the high price and still having the problem of conducting the heat toward the salts remains a problem. Eutectics are usually sealed in a pvc or other plastic tube and some medium, whether air or water must carry the heat to them. Water will probably still be used because these phase changing salts usually freeze at 100*f to 130*f and it takes a temp of a little higher than that to turn the salts into liquid in the storage phase.
The only advantage is a lot of storage in a little space. Not worth the money!

 

[DaveBP]

How about sticking to water and an alternative to epdm?

The Holy Grail of unpressurized heat storage.

There are polypropylene tank liners that take higher temps than EPDM. Way more money and not as flexible. Shipping is more complicated if something that large can't fold down neatly. It's industrial stuff.

You could look into welded stainless steel sheet metal. I spoke with a guy at a maple syrup equipment manufacturer about it. They make thin stainless sheet metal tanks all day long. You would need to build the frame around it to take all the weight of the water same as rubber and shipping would be expensive if you didn't pick it up yourself. But if would take any temp and last a long, long time. You could have welded fittings that won't leaks put anywhere you want. I never got any price quotes. I came up with some propane tanks.

 

[Fred61]

Wanna stanless tank? How much do you want to spend? One call to these guys ad you'll have a quote the same day and have a photo of the tank. You decide if the price is worth it.

http://www.jlmercer.com/

I've talked to polypropylene tank manufacturers and they say they will tolerate the heat but they run about twice the price of polyethylene.