Considering building a boiler... Water jacket VS Water Coil?

[Mr4btTahoe]

Hey guys...

So I'm toying with the idea of building a boiler. I bought a used Reznor RA235 (235k btu) waste oil furnace that is in need of having the heat exchanger rebuilt due to burnout. I got the entire unit and pump for $200.

Now.. I know this isn't the forum for waste oil fueled heat... however I trust the info I get from this forum vs others.

With that said... fuel source doesn't make much difference for my questions.

The plan for this up coming season is to only heat my shop and patch the current heat exchanger just to make sure everything functions as it should. Plus it's a little late in the year to start a major heating project... so maybe this coming spring. Problem is the unit is MUCH too large for my shop... so it will be short cycling quite a bit... but it wouldn't be too large to heat my shop.. AND my home...

Anyways.. I'm thinking about gutting the unit this spring and using the burner, etc... to fire a home built boiler. I'm in the design phase and am trying to figure out what the best method of heating the water in the boiler would be. I have access to a large steel tank which could easily be turned into a water jacket... cut a hole in the tank.. build and weld in the burn chamber... insulate... etc..etc.. just like an outdoor wood boiler.

Or.. (I've seen this a few times too)... wrapping a burn chamber (which I could use the one that is in the furnace now as it's in great shape) with copper tubing and insulating around the entire chamber using sand or similar... and pumping water from a large heavily insulated storage tank through the coil. I've also seen a few where they place a small water tank inside the burn chamber (small air compressor tank or similar) and use that as the heat exchanger... I can see a potential issue here with a long run of copper or stainless tubing... possibly getting the water too hot creating steam inside the coil. Not sure if that can happen being outside the burn chamber... but it's a concern none the less.

I like the simplicity of building a simple burn chamber and wrapping it with a water coil as there are very few chances for leaks... less fabrication.. etc.... and the water tank could be built to any size. I've seen the same thing used in solar hydronic heating systems and they seem to work well. My issue here is I'm not sure how efficient it would be and how much heat would be transferred into the water via the coil.

As far as the water jacket method... I think this would probably give a more efficient transfer of heat to the water (could be wrong...) but comes with the cost of MUCH more fabrication (which I don't mind).. and a bit more cost.

So... what would work better in the real world? Water coil (or tank) or water jacket with everything else being the same?

On a side note, I currently heat with a wood stove and have an all electric furnace for back-up... so this will mainly be supplementary. I also have a LOT of waste oil... so no issues there. Also, in either case, the system will be an OPEN system... not pressurized. The burner will be controlled via aquastat to maintain water temp between 160-180. Keep in mind the unit is starting life as a stand alone furnace with all of the safeties and thermostatic controls in place (including over fire protection, etc..etc). It will also be in it's own out building.

Thanks for the input guys.

 

[maple1]

I don't think either one would be very efficient - not sure one would be better than the other. Jacket should have the more direct contact for HXing, but ?.

How much is a LOT (of used oil?).

 

[salecker]

Hi Mr4
I have the same waste oil furnace.Have been using it for 18 years and at the height of my shop being busy we would burn through around 1750 gallons a year with it.
One thing to make sure you do is make your water heater cleanable,every tube or corner needs accesses because of the ash produced by the waste oil.I have to clean my burn chamber about every 500 hrs.I can get up to 750 if i cut my oil with gas at 20%. And get up to 1000 hrs when burning waste veggi oil.
Reznor makes a waste oil boiler/water heater i am not sure if it is a pressurized system.I would try to find one of those units to copy if i was going to build a waste oil water heater.Reznor seems to have the beat waste oil furnace that i have seen,and i would copy their boiler/water heater to avoid the pitfalls that i am sure they came across while designing it.
I know that i wouldn't hesitate buying another Reznor product.
From memory my furnace has got over 20000 hrs run time on it now. I have had to replace the inside wall of the burn chamber 3 times,it's a heat resistant material and i have found a place to source the product so i can cut my own now.The heater needs to be cleaned once a year.The sensors on the burner give out once in a while.All filters are cleanable.The nozzles can be taken apart and cleaned. And i have had to rebuild the compressor a couple of times.I found a site to buy the compressor parts from as well.
We have no local dealer,the dealer i used to buy parts from has stopped dealing with Reznor and they were just parts pushers.I have a number from an guy in Manitoba i call for info now.
The tube that is in the burner that the oil flows through plugs up in time as the oil cokes inside it.Not many people know this and never clean it.Many burners get tossed because of that little tube.
If you have any questions i will try to help.
Thomas

 

[blades]

i kinda like your coil idea of one type or another vs a full water jacket, seems those last about 5-7 years and then the leaks start, course there are a couple reasons for some of those leaks one being excisive 02 in the water due to lack of treatment and just plain over firing for starters.

 

[Bob Rohr]

Look at the Clean Burn boiler design, it's basically a few lengths or 1-1/2 steel pipe curled up, a simple water tube design.

I highly recommend a buffer tank or dump zone with the Clean Burn, they tend to over-temperature when they start filling with ash. All that residual heat needs to be pulled out when the fire shuts down. The pump needs to run to post purge the heat, of course.

Ash removal is critical, in some shops that I installed CBs in it is a monthly clean out requirement. Depends on the oil you burn, most drain oil has a lot of contaminants, sometimes antifreeze mixed in. Synthetics are tougher to burn also, takes more pre-heating.

Of find an old Buderus cast iron boiler and stick the oil gun in it. I believe Reznor sold a re-baged Buderus boiler with their gun in it. Some models of Buderus also received pellet burner ad ons.

 

[Mr4btTahoe]

Am I reading that correctly.... 1750 gallons per year?? I mean... at the rated 1.7gph nozzle flow rate.. thats over 40 days of around the clock burning...

How many cold days do you get a year??

In our area... I run our wood stove around the clock for 3-4 weeks. After that, it's just when we are home. Our winters are fairly mild in comparison to some. There is basically only 1-2 weeks of single digit temps.

The guy I bought it from said he kept his shop t-stat on 70 all winter and would burn through 6-800 gallons per season. His shop was fairly large and uninsulated.

Ive currently got 2k gallons on hand.. but as I said we process fuel out of it.. so I don't want to be burning all of my fuel source every winter. As supplement heat, if I could get away with 500-700 gallons.. I'd be ok with that.

I like the idea of the heating coil. I'm doing some digging on other waste oil boiler designs. The plan is to use the boiler to heat a storage tank (275 gallon) which will be heavily insulated. A lot of solar heating guys use the same idea. With this method, they heat the tank during the day.. tank stays warm throughout the night and keeps the house warm.

 

[maple1]

I think I would take Bobs suggestion, and look for a used boiler to put the gun in.

 

[Mr4btTahoe]

Ive been looking for a bit... nothing within 200 miles.

Used boilers very rarely come up for sale in my area... and I don't mind building one myself. I'd certainly think twice if I was talking about a pressurized system.

I also like the storage tank as I can build a solar heating system as well and tie them together to heat the same tank. Would reduce oil usage that much more during sunny days. Solar water heating has always been something in the back of my mind for a future project.

 

[maple1]

Some have gone thru evaluating of solar thermal & decided it wasn't worth the trouble & expense for the amount of heat you could get out of it. I came to the same conclusion for DHW. I suspect having big DIY skills & desire to use them for that though could change the beans on the water - I didn't have any inclination or patience to DIY any of it.

 

[Mr4btTahoe]

I've seen some builds on the solar side that have worked very well. Collector size is a big part of most people's issues I'd say. That and expectations.

I built a solar air heater last fall. Still in use... Puts out 120-130° air temps any time the collector is in the sun... Even in 10° ambient temps. The air heater is plumbed with it's own duct and duct fan and heats my son's room as it's the farthest from our stove.

I'm a big DIY guy. Love building and tinkering... Especially when those projects save me money.

 

[Bob Rohr]

You are wise to consider what happens when things go wrong in boilers. There is about a stick of dynamite's worth of energy when a gallon of water flashes to steam. Maybe you have seen MythBusters superheat a water heater and send it towards the moon. Pressurized vessels need to be respected and protected from dangerous operating conditions.

As for storage to get thru the night, you need to determine the buildings load under design condition and do some math. 275 gallons might only buy you a few hours worth. Using low temperature emitters or radiant allows the best draw down of a tank.

Some good reading and formulas in these journals. Click on any issue for a PDF download. Issue #10 and 17 are about renewable energy and thermal storage options and formulas.

 

[salecker]

Am I reading that correctly.... 1750 gallons per year?? I mean... at the rated 1.7gph nozzle flow rate.. thats over 40 days of around the clock burning...

How many cold days do you get a year??

.

My shop is 80ft x 30ft with a 16 ft ceiling.Not really insulated that good The ceiling has maybe R30 and the walls maybe R12
When it is -40 the furnace will run 12 hrs out of 24 hrs.
When i had a mechanic and apprentice working in the shop the doors would open and close a lot.They never really got the hang of trying to save heat.
Now that i am working by myself in the shop i keep the temp way down,usually around 50 F.I am in Insulated coveralls all day anyhow.no need for a bunch of heat.