DIY Stove Hot Water Systems can explode....

[webbie]

There has been some discussion here about adding domestic hot water capabilities to woodstoves. If you explore this option, make certain you understand the concepts involved...and especially the idea of having a Pressure Releif valve located as close as possible to the store WITH NO VALVES in between it and the stove.

There is a phenomonem called BLEVE, or Boiling liquid expanding vapor explosions - which can make your stoves into a giant hand grenade. Don't understand? Well, just look at the enclosed picture of a cookstove blown apart by a relatively small explosion of this type.

I've heard of wood boilers exploding and sending 1000+ lb units right through a cement block wall!

Not to scare anyone, but NEVER cap off leaking PR valves....replace them!. Most systems should have TWO PR valves on them, one at the stove and one at the heater tank that it ties into.

You also have to consider what is going to happen if and when the power goes out. I've seen some systems that would flash into steam within 30 seconds of the circ turning off. If this is the case in your situation, you might consider using a battery back-up (a heavy-duty computer one should work)....

Don't fool around with this stuff if you don't know what you are doing. Leave it to a pro and/or have a plumbing or mechanical inspector check it out.

 

[Eric Johnson]

Craig is way right on all counts. The computer battery backup is a good idea, since they put out 110 volts and can run a circ pump. You should also probably design a gravity heat dump loop into the system so that you can manually dissipate the hot water if all else fails.

One more important consideration with heating domestic hot water with a wood fire is water temp. Since it's difficult to control, you should have a tempering valve on your system that (at minimum) mixes cold water in with the hot before it goes up the pipes and into your house. This is especially critical if you have young children or old folks hanging around your pad. If you run the hot water from the boiler/stove coil into a regular domestic hot water heater, you're going to discover that the pressure relief valve is also designed to open up if the water temp exceeds a non-adjustable pre-set. The result will be a basement floor flooded with hot water, usually late at night when there's little draw on the heating system and the water temp rises as a result.

 

[Willhound]

YIKES! I sure as *&^% hope there was no one in the vicinity when this happened...

 

[wg_bent]

I would also add that if you look at all the systems built into a hydronic heating system, they should all be built into a hot water system in a wood stove. Think pressure relief valve, guage to watch the pressure, expansion tanks etc...

While doing some repair work on my furnice a month ago or so, the pressure relief valve popped (doing it's job), but let me tell you that was some serious amount of steam and a hell of a noise. I was glad it was there. I have no doubt that the pressure that might be built up inside a pipe is quite destructive if the pipe breaks. Just think of what pressure is required to break a stainless steel pipe inside a wood stove.

Good post.

I wonder if a different technology isn't better/safer. Take a look at the Tarm site where they discuss the use of heat storage tanks. Very space consuming, but also look like a nice option. (hate to get a leak in one though)

Warren

 

[BrotherBart]

Craig said:

"There is a phenomonem called BLEVE, or Boiling liquid expanding vapor explosions - which can make your stoves into a giant hand grenade. Don’t understand? Well, just look at the enclosed picture of a cookstove blown apart by a relatively small explosion of this type."

Two things. One, elk can probably have the cookstove rebuilt like new in a couple of days. Two, after reading this stuff I just went down and patted my new electric water heater.

 

[Eric Johnson]

Hey BB,

Electric water heaters can blow up, too. I read an article once about one that launched through the basement ceiling and into the first floor. Houston, we have a problem. The thermostats are designed to fail safe, but there's always the potential for disaster when water is being heated.

 

[begreen]

Steam under pressure is an amazing force. Redundant safety systems on hot-water boilers are a good investment and relief valves should be regularly checked. - Sh_t happens! - History is full of stories of catastrophic boiler failures. One very famous one was on the Great Eastern, considered to be one of the seven wonders of the industrial world and known for laying the first transatlantic cable. Only a few days into its maiden voyage there was a monumental steam explosion that almost destroyed the ship. It was caused by a valve left mistakenly closed during systems testing. There was a recent catastrophic boiler failure on a cruise ship that had equally spectacular and deadly results. Eric is right-on, though I would say that a gravity heat dump loop is a necessity on a wood-fired system. I would also always plumb the pressure relief valve to outdoors if at all possible, or at least into a drain. In an emergency, one doesn't want to have to contend with scalding hot water.

 

[webbie]

Steam under pressure is an amazing force. History is full of stories of catastrophic boiler failures. .

These explosions are one of the main reasons for modern safety codes and insurance company underwriting. It all started with steam engines and then railroad locomotives....seems there were no standards, so a lot of deaths and losses occurred.

Thus started boiler standards and the "Hartford Steam-Boiler and Inspection Company" to make certain that the public and industry would be safer. Our modern insurance and standards grew out of this.

Let's not go backwards in this case. If you love disaster, you can read all about the power of these explosions by googling the subject.

 

[Eric Johnson]

You know BG, I did put a gravity heat dump into my system this summer by running an independent supply and return line into two old cast-iron radiators in the attic right above the boiler room. That's so I can dump the heat in the event of a power or circ pump failure. The only problem currently is that I have to go out there and open the valves to make it work, which means I have to be home and aware that the power is out. What I would really prefer is a spring-loaded valve that opens automatically when the power is cut. Do you have any idea if these things are available and if so, what they're called? My local P/H supply guy got hung up on the flow check valve, saying that would prevent flow, but the flow check on the rest of this system is not a factor in this loop.

Also, it probably goes without saying to people who get this far into such a project, but be aware that pressure relief valves for hot water boilers and water heaters set to different pressure ratings, and are thus not interchangable. Most boiler PR valves are set at 30 psi while hot water heater blow-offs are set much higher, though I don't know exactly how high. They look virtually identical, but you don't want to put the DHW unit on a boiler by mistake. Could be a fatal one.

 

[webbie]

Do you have any idea if these things are available and if so, what they're called? My local P/H supply guy got hung up on the flow check valve, saying that would prevent flow, but the flow check on the rest of this system is not a factor in this loop.

.

Wouldn't a regular zone valve do this, or at least a Normally Closed one? This means while power is to it (usually 24 volts off a transformer) it will stay closed, but open on a power failure.

 

[Eric Johnson]

Yes, a zone valve would work just fine, so long as it opens when the power is cut. You say it's called a "Normally Closed" zone valve? And if it's 24 volt, then I would need a relay as well, right?

 

[begreen]

That's was what I was thinking Craig. A simple off the shelf NC zone valve should work. Usually there's no relay required, it can be directly wired to the thermostat transformer as long as it can supply the needed current.

 

[Eric Johnson]

I don't know if I believe it or not either, Dylan, but it was in the Bangor Daily News about 15 years ago. A lot of strange things happen up in Maine, so who knows? I suspect that in the unlikely event that the thermostat failed to cut the power or flame and the somewhat more likely possibility that the pressure relief valve failed to open, coupled with the very likely possibility that the hot water heater was rusted out on the bottom, it could generate a pretty good head of steam before the tank bottom failed, sending the tank up, up and away, Romex and copper tubing notwithstanding. And having seen some Maine construction techniques first-hand, I wouldn't be surprised if the floor lacked the integrity to stop it.

Good to see Dylan's bullshit detector is still fully charged up.

 

[webbie]

Eric,

Sometimes I just don't believe the 'articles' that I (or others) read, and this might be one of them.
Dylan

http://www.mass.gov/dps/HotWater.htm

Look at what a 5 gallon heater did - looks like a cinderblock wall!

According the article (by Mass. Government) "Boiler or hot water heater failures with water temperatures exceeding 212°F can result in catastrophic explosions that can level homes."

-----------UPI-----------
Ballistic Water Heater

From the United Press International comes this report:

The cost of heating water went through the roof for one man recently when his hot water heater exploded and rocketed through the roof of his home. Authorities said Anthony Sobaski Jr. and another adult in the house were not injured even though the 50 gallon electric water heater tore a 4 ft hole through the floor, 3 ft from where he was sleeping, as it shot from the basement into the sky about 6:15 a.m. It landed about 150 ft away across an alley in a neighbor's yard.

"If it had hit them, they could have been killed. It was just like a rocket,'' said Lt. Greg Anderson of the South St. Paul Fire Department. "It over-pressured and just blew off. Nothing was going to stop it at all. It was a major explosion.''

The blast knocked the $77,000 house off its foundation and killed a pet dog. Fire officials blamed the accident on an inoperable pressure relief valve.

Lesson: The pressure relief valve is there for a reason. Make sure it works. Also when wrapping a water heater, cut a hole in the wrap to ensure free movement of moving parts in the valve and release of pressure.

 

[begreen]

Damn. I'm going down and checking the alignment of my hotwater with the floors above it right now. Might just have to move the bed! Talk about bad Feng Shui!

 

[Eric Johnson]

I don't see what's wrong with the government posting information that could save lives. I mean, there are actually some people out there who don't believe a water heater could do that. Really. Hard to believe, I know, but it's true.

 

[elkimmeg]

Having fun with this: We just discovered the weapons they were in our basements

Simply wiring another zone valve into your transformer, there is a little more thought that goes into this
Many transformers can only support 3 zone valves one has to daisy chain in another to add additional valves
I have failed boiler inspections for this reason. These transformers can not supply proper voltage or adequately support more than their capacity. What happens is they start burning out. There is another posibility some transformers are designed to support 5 zones. Probably replacing the 3 zone transformer to the five is best. Too many times I see the dual transformers wired incorrectly meaning 48 volts instead of 24. I had one where each zone recorded different voltages 24 18 48 all over the board. It seems even some electricians have trouble figuring out how to wire them.

I think a sub plot theme of this post, is knowing when not to tamper with things one knows not enough about. Posters actually get angry at me for pointing out the dangers of cutting holes in their floors to distrubute heat. or using the HVAC system for a use it was not designed to do. Every week there is a post about this. Where some one figures he can re- engineer a system. None have admitted to being Mechanical engineers. I think Eric had it right with the pressure blow off valves one being 30 PSI and the other rated mush higher I think 50 psi. Well there is a lot of difference between 30 and 50 under pressure. To the unsuspecting eye they look identical. I also read the gages to see operational pressures. I have actually found exactly what Eric explained. Your boiler should not be charged more than 18 psi usually in a range of 12 to 18psi
I observed one at 45 psi It should have blown off around 30 psi if the correct valve was used.

 

[Eric Johnson]

That's an interesting point about the PR valves. In theory, they ought to be set for the same pressure. I mean if it's dangerous to run a boiler up past 30 psi, the same ought to follow for a water heater, which is just welded sheet metal compared to 1/4-inch boiler plate. But you need more pressure in a domestic water system to get adequate flow, so the water heater has to be (apparently) designed to withstand higher pressures. Presumably too, there's less room for error on the water heater.

As to the transformer & zone valve wiring, my nearest transformer or relay is more than 100 feet from the "dump zone." Actually, I don't even have any zone valves on my system--just circ pumps wired into relays which are wired into 24-volt thermostats. So I'll either have to get a NC zone valve wired for 110 current or wire it up to a relay. I want it on the same circuit, in any event, as the circ pumps because if something shorts out and trips the breaker, the zone will open.

 

[Eric Johnson]

The electric hot water heaters I'm familiar with have two elements--one at the bottom and one in the middle of the tank. The cold water feed extends to the bottom of the tank via a plastic tube. If you screw in a copper connector and then try to solder it directly, you'll melt the tube and ruin the water heater. That's why you should always do your soldering before making the screw-in connection. And yes, scale will definitely cost you efficiency, presumably because lime is a pretty good insulator. Don't ask me where the heat goes, 'cause I don't know. Maybe it scales up the elements, insulating them from the water. Also, as your tank fills with scale, there's less room for water, so your capacity is effectively decreased.

On the first point, I really don't know how to respond because I'm not clear on what you're asking. What happens is that when you open the hot water tap, the water flowing out of the tank is displaced by cold water coming in from the system, which is under pressure. There's no bladder; it's just a big tank with a pipe going in one end and another coming out the other. When the tank overheats, the pressure from the expanding water opens the relief valve, which remains open until the pressure goes below the set point. Sometimes that can take awhile. Steam is usually formed at 212 degrees at atmospheric pressure, but when it's pressurized above atmospheric, the flash point is higher. Since most PR valves have a temperature-sensitive trigger, they open up before it gets hot enough for steam to form. You get steam when the PR valve fails. That's when your water heater becomes a Delta 5 rocket.

 

[Ray4852]

You can put on a Honeywell safety valve if you have a wood or coal boiler. I have the valve on my Harmon coal/wood boiler now. This valve has never failed to work. Every time my power goes out this valve open up and heat trickle through the house. If you have a boiler now that uses wood or coal I strongly recommend you to put one on now. Don’t gamble with safety.
(broken link removed)

 

[webbie]

Eric,

It would take eighty pounds of water at 213 degrees-F to develop ONE pound of steam when any system is breached; I feel this is not a particularly scary amount of energy.
Dylan

Well, let us know when you decide to experiment and I'll be looking out the window here is western mass, from CT it should land somewhere near here!

The thing about these explosions is that the water is often at VERY high temps, not 1 degree about boiling. Water when under pressure can be heated to very high temperatures.....of course, what happens is the pressure then rises until the weakest link in the system breaks....

So, in some ways, a pressure vessel and piping which is not well built is safer! If the heater is very solid and the piping also continues to hold, the temp and pressure can go higher and higher....

Some of these stories have come to me first hand, since I used to import boilers into the USA from Denmark. One in particular blew out the side of the boiler and propelled it through a wall and across the basement. Luckily, we were not the ones who sold or installed that boiler!.

Here's a good wikipedia entry: http://en.wikipedia.org/wiki/BLEVE

 

[Eric Johnson]

Given my soldering skills, Craig, I would expect the piping to give out long before the vessel. But I'm not taking any chances. Next time I have a problem, I'm going to have Dylan come over and do the work. Apparently he's impervious to the laws of nature.

 

[slowzuki]

Electric heater elements are always 100% efficient. No other possibility really. They do lose heat output as they get scaled up. This results in higher element temperatures and eventual burnout.

The hot water tank thing has me thinking, if you have thirty psi in the the tank, it isn't going to boil at 212, it will keep heating and heating, and the pressure will climb and climb preventing it from boiling until the tanks yeild strength is reached and the whole thing flashes to steam. The quality of the steam will depend on how strong the container is. The place the tank yields will act as a nozzle for the steam which increases in volume by what 500 time at room temp, even more when heated. This propells the tank quite nicely.

Propane tanks do a similar trick when exposed to fire, they are worse because the propane has a gas space in the tank. The liquid phase portion of the tank starts to boil only at the surface and the gas insulates the wall from the cold propane so the steel wall starts to heat up and lose strength. Eventually they become bleve's. Tank manufacturers have been approached to install reinforcing rings on the interior of tanker cars to be used in highly populated areas. The rings would act as heat transfer fins so the mass of propane could be used as coolant for the tank walls.

 

[TreeCo]

I heated domestic hot water for 15 years with a Rightway 37 that had a factory one gallon stainless steel water tank inside the firebox (7.5cu. ft. box).

My system had the correct relief valves, two water tanks, one a hooked up gas 30 gallon water heater and the second an 80 gallon electric water heated used as a tank. It was designed as a gravity fed system and had no pumps, thermostats etc. In other words.......it didn't know if the power was on or off because it used zero electricity. It worked flawlessly for 15 years and then I moved. It's sitting in the shop out back now.

I'm going to fool with MSpaint for a few and see if I can draw it.

 

[cbrodsky]

Interesting thread, and has me thinking some of the same principles used for solar thermal systems could be applied here. I have been considering doing something for winter hot water generation that I would tie into our recently installed solar thermal system.

The system is in the basement below the stove. This presents a problem in that you can't use a gravity feed system. However, I am thinking that a "drainback" approach just as we use with the solar panels could be applied to a stove heating system. In these systems, you have a small 20 gallon tank that sits just above the main 120 gallon water storage tank. Whenever the solar collectors reach a temperature 12 degrees above your storage tank, the fluid in the 20 gallon tank pumps up to the roof, fills the panels and starts a circulation loop that trickles hot water back down from the roof. At the same time, a second very small circulator flows house water through a pair of heat exchanger coils that transfers heat from the 20 gallon tank to the 120 gallon storage tank. This means the 20 gallon tank is at atmospheric pressure with some airspace above the fluid, and it isolated from the house water supply and pressure. You can use glycol in this setup, but even in a cold climate, you don't need to. If you lose power, the pump stops, and all the water drains back from the roof. Naturally, a lot of thought goes into the piping to ensure this happens cleanly!

Now, with this system already sitting under the stove, I am thinking a second loop could be added, I think using the exact same solar controller, that would do the following:

1. Verify temp at stove is higher than main storage tank temperature
2. Verify temp at tank is not at high limit
3. As long as conditions 1 and 2 continue to be met, start circulator pump, flowing fluid from 20 gallon tank up to wood stove.

It could be setup to view the wood stove heat exchanger just like a second solar panel array - many controllers enable this for cases where you may have them on two sides of your house, for example, and they heat at different points in the day.

In either application, I believe the same fail-safe issues apply... a solar thermal system can easily overheat if you don't have either a heat dump, or a drainback that eventually stops extracting excess heat and fails-safe draining the panels in a power outage. There may still be a pressure relief valve on the roof at the high point of the solar loop, but I'm not even sure that one exists because the possibility is deemed too remote that you could overheat to those levels. Maybe that would be a good extra addition in general if you linked in a wood stove circulation loop. I definitely think this offers significant advantages over trying to route house pressure water directly to your stove in a way that requires constant circulation while your stove is running. The only way I can see the battery backup being safe is if you can be sure it will always outlast the heat duration from your woodstove fires.

Thoughts on this as a safer approach to generate wood-stove hot water?

-Colin