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I Forge Iron

Forge from pool heater, newbe wants comments


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So I have an idea for a forge.

I have a 400,000 BTU natural gas heater for my pool that had the heat exchange develop a leak. I rarely use it so it is not worth fixing it.

It has a ¾ inch natural gas supply and the heat ex changer is stainless steel. It has a blower unit for the flame, and there is a “flame tube” in the assembly drawing. (item 11),

I am thinking of making it into a forge.

  1. Take the front plastic panel off, but keep the rest to keep it weather tight.  I will make the front panel so I can take it off to use the forge, but drop it back in to keep it weather tight.

  2. Take out the heat exchanger and use the opening for the door. 

  3. I have some fire brick to use for a door. 

  4. I have a Kilm shelf that I can use for the floor and top (just because I have and and do not want to spend money unless it works)  

  5.  I have extra light fire brick that I got from a kilm manufacturer when I was there for a class on kilm repair that can use for the walls

  6. I will keep  the existing exhaust setup. (it is already balanced to the blower) 

  7. I might replace the flame tube with a ribon burner. (item 11 in the parts break down below)   But for the first firing I can use the existing flame tube. 

I will  keep the existing controls including the self igniter, and gas control valve. I will just put the thermo couple that normally measures the water heat in the air (or a glass of cool water) and I can use the control to turn the burner on and off. I could try to rewire it to a switch, but the existing controls already turn on the ignitor and have the safety interlocks built in, so it is pretty easy to reuse them.

I will build a stand to move it up to a comfortable level. I may reuse the outside cabinet, just to keep it weather tight,. (since I do not have a shed built yet) 

In reality it will be a year before I get around to it, it is already 100F here in Texas, to hot to get a forge going right now. And I have other projects to get done first. 


A basic newbe question.  

If I want to replace the flame tube with a ribbon burner, I assume I need to keep the area of the holes for the ribbons equale to the area of the existing tube so the blower and flame front are balanced.  Is that correct?  Any other pointers for the ribbon burner design?  I have not taken it apart yet but from the general size of things, I assume that the flame tube is about 4 inches in diameter.  


The outside housing (which is stainless steel) is about 24 inches in diameter.  (rough guess.  I did not measure it) 


Am I wasting my time on this, or does it have possibilities? . 



Edited by wilsonintexas
fix spelling and corrected 40,000 to 400,000
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You have a 400 MBH burner fed by a 3/4" natural gas line?  What do you have commercial pressure there?  For typical residential pressure I would expect you to need at least a 1" line if not 1 1/4".  Are you sure you haven't misread and it is only 40 MBH?  I use a 1" residential pressure natural gas line for mine, and my burner is under 200 MBH.

In any event, you most likely will have to toss everything but the burner assembly, gas train, blower and controls.  None of the parts of your water heater are designed to handle temperatures that a forge may discharge (or leak from insufficient insulation, and putting enough insulation inside may reduce your chamber size too small.  Really hard to tell from a photo with no idea of dimensions.  Also you will almost definitely need a new mixing tube and burner outlet of some sort.  Hard to tell if the proposed ribbon burner holes need to be the same overall area of the exiting burner outlet or different.  A fire tube boiler is very different from a forge.  Kind of surprised it is a fire tube boiler, but I don't know much about pool heaters.

If the burner actually puts out 400 MBH it may be too large for a practical size forge as well, unless it has modulating output.  A typical hobby forge needs  more on the order of 100 - 200 MBH. If the burner is too large you will have trouble because your combustion chamber is too small, and some of the combustion will take place outside the chamber.

Also insulating fire bricks come in a variety of ratings.  I have used some that were only rated to 2300 deg. F and they literally melted in my forge.

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i could very easily have tne gas line wrong


i know that it is 400,000 btu

i will try to read up and see if i can modulate the burner


the boiler area is 24 inches in dia (from what i remember) and about a foot tall

i know that tne exhaust is hot but not to bad, so i assume that the combustion is complete inside tne combustion tube


the top is held on by a clamp to make it water tight, so i could use the top flange as the top of the forge and build a new insulated box for the forge making it as tall as i need to get the flame where i need it

if the burner is to big what options do i have to reduce it:

can i put in a smaller orifice on the  gas line and then put an adjustable opening on the fan   to adjust the air flow?

i would then plan on making a ribbon burner i need to read up on the design to build a mixing chamber



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A 3/4" line is pretty standard input size for a household furnace or gas water heater. Most are actually 1/2" pipe on the gas valve... they're reduced down to it.

As for keeping the gas valve- and controls- no they wont work. They're built with a safety feature in them. They will shut down the valve/furnace/heater in two gas conditions- either over, or under pressure. 

Most are regulated in bars, or inch pressures. To reach forging temps, that's nowhere near enough fuel supply.

Using a high pressure- 20-30 psi regulator with a normal heater style gas valve will over pressurize the valve and cause it to shut down the solenoid and close off gas supply.

The fuel orifices in it are designed for low pressure -household gas propane pressure is normally around 3 bars ? in the US... to maintain a nice low, blue 1" or 2" flame on that flame bar/tube.

In my opinion... I'd plan on scrapping everything inside- and build your burners all new with forge pressures and temps in mind. Nothing in that system is designed, or built to come anywhere near 2,000 degrees. EVER. If they were to hit 500 degrees, I'd be amazed.

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Sorry Welshj, but I think you are making a lot of generalizations without the right information.

Most household gas furnaces and water heaters aren't rated for 400 MBH.  I have a very large heater in my house that is only 225 MBH and it heats both my house and domestic water.  A more typical size would be on the order of 40 MBH for a domestic water heater with storage tank (what you are used to seeing with a 3/4" natural gas line) and 60-80 MBH for a heating hot water boiler for a house.  A good rule of thumb for home heating, in climates that need it, is somewhere on the order of 35 BTUH/SF of home.  Natural gas line size is based on both the heat rating of the appliance and the equivalent length of line between that and the source.

What makes you think he plans on switching from natural gas to propane?  It doesn't say that anywhere in his post that I can see.  You can certainly run a forge off residential pressure natural gas.  I do, very successfully, and hit both forging temperatures and forge welding temperatures.  I do agree that nothing in that unit is designed for 2,000 deg. F temperatures.  That is why I said to scrap most of it.  Personally I keep my gas train and blower outside my forge, so it just experiences shop ambient temperatures.  Perhaps you do it differently.

To the OP: 24" diameter would give you a lot of space for thermal insulation.  You should do the math and figure out what the outer skin temperature will be if the inside is 2,400 deg. F and you have say 8" of insulation (giving an 8" ID forge - a good starter size).  I would use layers starting with insulation that can withstand at least 2,600 deg. F and progressing from there.  As far as the combustion chamber size, I don't know enough about that field to give you direction.  Might be a situation where you will have to do some experimentation.

Most likely by the end of this you will either have a catastrophic failure, have an under-performing forge and a lot of time invested, given up in frustration, or developed the next great homebuilt forge renovation/upgrade as "revolutionary" as the Lively washtub forge.  I sincerely hope for the last, but expect that in the end the cost in time, materials and degree of experimentation involved will have made it more practical to either go with a tried and true forge design or purchase one already built.  It all depends on whether you want to be a forge builder or blacksmith...

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Never said he planned on switching- just used it as a referance for... general pressure regulation. I spent 4 yrs working as a hvac and plumbing tech here in rural nw ohio. Mostly rural, alot of residential propane use. Everything is regulated before entering the residence... and he was talking about using the original gas valve and control parts, the original flame tube from the heater to heat the interior... planning on changing orifices to better the flame.

Every residential gas valve, propane or natural gas (in the last 40-50yrs or so.) has electric, and pressure safety over rides built into them.

I literally just went through this exact problem with a friend trying to hook up a space heater in a pole barn- directly to a propane tank, with no regulator at the tank. He thought that the gas valve in the heater would regulate the pressure... and wondered why he couldnt get it to fire up.

But... then again. I'm just trying to help someone not burn their house down or die.

I no longer have an opinion.  Good luck wilson.

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We are essentially in agreement that this is a mistake waiting to happen.  I just took exception to your assertion that he would need to up his pressure to 20-30 psi to be able to use a gas forge.  I know residential pressure natural gas forges work because I have one and have used it for over 5 years.  However it is all about the gas flowrate for a blower assisted burner, and a 3/4" line would most likely be too small.  It would also be too small for most 400 MBH heaters, which is apparent from review of the International Fuel Gas Code,  also the applicable code for Ohio (https://codes.iccsafe.org/category/Ohio?year[]=Current+Adoption&page=1).

I'm glad you stopped your friend from using propane without a regulator.  I'm a big fan of making hookups properly and have often posted that regulators should always be used in gas train hookups.  Even though a residential natural gas system has a pressure regulator at the entry to your home, all commercial appliances I am familiar with also have a safety regulator integral to their gas train system.  I expect his pool heater does as well, but can't tell from the blowup image in the OP.

As regards credentials, I'm a professional HVAC engineer who has been designing systems for over 25 years.  Still always learning, and don't present myself as an expert in every kind of combustion system.

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On 6/12/2020 at 9:36 AM, wilsonintexas said:

I have a 400,000 BTU natural gas heater

Natural gas, not propane. So high pressure regulator involved or available. 

Non HVAC guy here so I reread the OP and the misunderstanding seems obvious to me.

PAX Brothers.

Frosty The Lucky.

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natural gas connected to house gas meter

the heater has the regulator, igniter, fan for the burner, and all safety interlocks. those are the reasons i want to repurpose it.  all safety interlocks and a fan and burner tube assembly sized to match

Edited by Mod30
Remove excessive quote.
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I wasn't responding directly to your question I was pointing out a simple misunderstanding between two HVAC professionals to hopefully avoid an argument or bad feelings between a couple of my friends. 

However if you really want to know what I think, read on if not delete now.

You may WANT to salvage the thing but even I a non HVAC guy know enough to know it's a waste of time and energy. This is the first and only necessary deal killer. A water heater is what's called a "transparent" heat exchanger meaning that there is little if any resistance to the flame's passage through the plates or coils. If you want to make 400,000 btus worth of heat stay in a small volume it takes pressure to force it in and to keep it in.

A transparent system is high volume low pressure. Won't work in a forge.

Frosty The Lucky.


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sorry i missed some of the discussions

this is a pool heater designed to heat a 10,000 gallon pool

it is 400,000 btu per the model and verified at the web site

it is a high efficiency burner with a blower motor on top of the boiler.

i know that when it went in they had to put in a bigger gas regulator on tne house to keep up with it

the line going across the yard is 1-1/2 inches it drops down at the heater

i was planning on gutting everything insulat it replace the burner tube with a ribbon burner to avoid problems with increases temps

basically i have a control system with ignitor, safety interlocks, and a fan blown burner.  

but i am asking and a the consensus is bad idea

not sure why more specifics please on why bad idea

so far the one that makes since is that 400,000 but is just to big the problem of the heating coils going bad is common

there is a pool maint guy in the building i work in. i may have hil keep an eye out on a smaller unit in tbe 100,000 to 200,000 btu range

that is beginning to make since thanks


Edited by Mod30
Remove excessive quote and double spacing.
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Latticino - re-reading and thinking... I owe you an apology. It was a long day that day, and I was tired and cranky. I didnt mean to be that short, and I'm not usually. Sorry sir... I wasn't suggesting that a residential natural gas supply wouldnt run a forge, but instead- that using the/some/any of the equipment in a heater like that probably or likely wouldnt.

Wilson- i think what i was trying to get at, and not doing in a very clear fashion... is that i don't see attempting to use the original valve, ignitor, controls etc... working without a major amount of work.

Theres alot of variables in equipment like this... from water heaters, to furnaces, to boiler systems. But they all have similiar components and functions.

And without your specific heater in front of me- Here's  some basics why.

The gas valve- basic commercial/residential- no matter what pressure they operate at... as I said they have safeties built in. You may be thinking or seeing that as a benefit- I see it as a downfall and hindrance... the best machine operates with the least moving parts.

There can be a basic thermo couple, that detects the presence of a pilot flame, that will not allow the valve to open unless it's in a certain heat range. That thermo couple is rated for a certain range... at a forging heat range- it probably wont last long at all. I've seen them burned/corroded up from just months in a simple pilot flame. A heavier more industrial one might last, might not.

Some have photoelectric eye sensors that detect ignition and or flame.

Electric controls- in for example, a home furnace- you have combustion/exhaust air blower pressure switches, temp limit switches, flame sensor, that all work together to complete a circuit to keep the valve open and supplying gas. If one drops out, no gas.

Control, or circuit board- exactly that. It controls timing the combustion/exhaust fan(s) to start the operation. Then when sensed, it will ignite. Then, in a furnace, at higher temps, it kicks on a blower fan. Without the proper voltage, limit switch, sensor readings, etc... getting a control board to work without certain parts, or the wrong parts in the loop... not impossible, but difficult... and well above my knowledge of electronics.

Limit switches. Any heater of any type has temperature limit switches in it. They're designed to operate within a certain temperature range. Above or below that- they drop out, shutting down or cooling the system. I would think that they're not going to get you to forging temps.

Ignitors- depending on what type- pilot light, piezo/arc style, electric glow type... electric types are designed to operate on a certain voltage, often for a short period. They can be very fragile and easily broken... they can be under powered by lower voltage and not work. Over voltage, and they burn up.

So, when I said that using the original controls wouldnt work- I had in my head why, but didnt figure out how to explain.

Your heater, like any other- gas, electric... is designed and built to operate within a certain temperature range. Safety protocols and manufacturer specific equipment keep it from being a law suit in process. The equipment and parts in are designed for a specific use, and using them in a different way can be a deadly, or costly mistake. Let alone a headache to figure out.

Building a forge out of that heater case, properly insulated can be done... if it were me- the case is the only thing I'd use. Period. 

But, other than a financial... consideration... being that you have the parts on hand- I personally, with access to that kind of gas line and pressure... would research a little more and figure out how to efficiently use it.:D

Turn gas valve on, light fire, heat steel... beat steel is much less of a headache to deal with.

I built a forced air burner in my forge. It has the tank gas valve, a regulator, a secondary inline gas valve closer to the forge for safety shut off. Its fed by a squirrel cage style industrial blower that is almost too much air for it. It plugs directly into an extension cord.

I kept it simple as possible, getting information and help from the folks here. Got me forging much faster, and much cheaper.

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On 6/15/2020 at 4:08 PM, Latticino said:

I'm glad you stopped your friend from using propane without a regulator. 

As am I.

I walked in blind- to troubleshoot a "non-working brand new space heater".  Climbed a ladder in a corner, and stood looking at/into a service port as he powered it up. Watched it cycle, fan kicked on, gas valve opened and ignitor lit up. Four inches from my face.

If the valve had not done its job and stopped it from opening on a full non-regulated pressure... I don't know for sure what would have happened, but I'm sure it wouldn't have been a good day for me.

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1 hour ago, Welshj said:

Latticino - re-reading and thinking... I owe you an apology

No worries, like I said, we are basically on the same page.  I've got a little bit of a background in controls and tend to expect that others can gimmick systems in the same way I do.  I do agree that none of the parts in the heater are likely to be rated for the temperatures they would experience if he expects to keep the whole thing essentially intact and just line the combustion chamber with some insulating brick.  You have carefully laid out a lot of the potential drawbacks in detail.  Some of which can be worked around, but some would be difficult to address. The only insurmountable issue would be the size of the burner, if it doesn't have a large degree of modulation.  Again we are essentially in  agreement.

Most likely this system will have a flame rod, not a UV sensor for flame proofing.  A UV sensor can work if relocated, a typical flame rod may be seriously problematic if the unit ever cycles and the forge heat projects back into the burner assembly.  I've used industrial UV sensors in similar circumstances, but mine had a mica shield.

The igniter could also be a major problem.  Spark igniters will certainly fail under regular cycling and the newer electric meshs won't respond well to the heat.  A pilot light should be fine, but most newer gas trains don't come with them, and if they do it is fitted with a basso style valve,  which also won't take the forge IR backfeed temperatures.

My thought was that with a long enough mixing tube you could keep the gas train far enough away from the forge to protect it, but it is still not ideal.  Some of the safety system would still be useful, like the integral gas pressure regulator, gas high pressure cutout, gas solenoid valve and the blower pressure airflow proving switch.

4 hours ago, wilsonintexas said:

line going across the yard is 1-1/2 inches it drops down at the heater

Now that makes much better sense.

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ok i am beginning to understand


the safety interlocks and controls which is what i wanted to reuse are probably not going to work


it is a glow plug ignitor so that is probably out as well

i may be able to use the blower  and possibly tne regulator

the stainless tank may be reworked into a shell and insulate it

thanks for the comments and help

i will think on it some and when the fall comes i may take it apart and see what i have to work with

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