# Adding a wood boiler to oil boiler



## Camnikki (Sep 26, 2017)

Was given a HS Tarm. Looking for advice on installation of the plumbing and wiring of combining this Tarm with my exsisting oil boiler setup?? Thanks so much for any help. I am by no means a plumber or boiler tech.


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## peakbagger (Sep 27, 2017)

A few initial questions. 

Do you have two flues and is the spare one large enough to handle wood boiler? Maine does have an exemption that in some cases you can hook a wood boiler to a oil flue but its not a great idea and is not allowed in most states for really good reasons. 

Are you planning to install hot water storage?. Long ago people installed wood boilers without storage but it limits when you can use the wood boiler, it also introduces the potential for lots of chimney fires. 

About 20 years ago I got a wood boiler for free and did what you want to do. Until I added hot water storage I only used it during very cold weather. The problem is without storage, you are essentially adding an outdoor wood wood boiler in your basement with all the associated problems a OWB has. When the boiler is running and you need lots of heat, its fairly efficient combustion, the problem is once the house is up to temperature and there is no where to put the heat the air damper closes. When that happens the boiler is starved for air and doesnt burn efficiently. What you end up with is creosote in the chimney and lots of smoke up the stack. You may not notice it but your neighbors may. The alternative is to feed it very small loads of wood to try to match the heating demand. It sounds like a plan until you discover that most indoor wood boilers burn the wood completely so you have no coals between burns meaning you need to relight it quite often. My neighbor added a Tarm without storage and ended up having multiple chimney fires despite leaving a ladder permanently next to the chimney and cleaning the chimney every 4 or 5 weeks. He ended up destroying his chimney tile and melted a chimney liner. 

Once I put in storage, I cut my wood use by a third. I just run the boiler for a couple of hours at night to heat up the storage tank and then run off the tank until the next night. The key is to size the storage tank big enough to match the building heating load. 

Before you spend a lot of time on it, make sure the boiler is in good shape. You can get parts and manuals from tarm usa http://tarmusa.com/
They are good folks and should be able to help you figure out if its worth installing as Tarms do wear out.


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## Camnikki (Sep 27, 2017)

peakbagger said:


> A few initial questions.
> 
> Do you have two flues and is the spare one large enough to handle wood boiler? Maine does have an exemption that in some cases you can hook a wood boiler to a oil flue but its not a great idea and is not allowed in most states for really good reasons.
> 
> ...


Yes I have my boilers in separate flues. Wood boiler flue is a 8x11. I have burnt wood my whole life, so I get the whole creosote issues that could occur. I also have a wood stove and old fashion cook stove in my house, so heat isn't an issue. 
Storage?..... How much is my question I guess? The Tarm itself has storage or is that not considered storage? I'm also wondering connecting the wood boiler. Best way to do so, with and without storage? Also the best way to have the wiring setup, aquastats, circ. pumps exc? 
Thanks for your input peakbagger. I appreciate all ideas and thoughts.


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## Bad LP (Sep 27, 2017)

You have a couple of options for plans.

One is found in the sticky on this forum page. The other is to go on line to Tarm, find your boiler and download the manual. It has the plumbing schematics.

I wouldn't start spending the money if you are not willing to invest in storage. It's not a cheap exercise by any means.


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## maple1 (Sep 27, 2017)

Camnikki said:


> Yes I have my boilers in separate flues. Wood boiler flue is a 8x11. I have burnt wood my whole life, so I get the whole creosote issues that could occur. I also have a wood stove and old fashion cook stove in my house, so heat isn't an issue.
> Storage?..... How much is my question I guess? The Tarm itself has storage or is that not considered storage? I'm also wondering connecting the wood boiler. Best way to do so, with and without storage? Also the best way to have the wiring setup, aquastats, circ. pumps exc?
> Thanks for your input peakbagger. I appreciate all ideas and thoughts.



What the boiler holds itself is not considered 'storage', no. 'Storage' would be added capacity in the form of storage tanks. I have 660 gallons in 2 tanks - others have different amounts.

You should start with the manuals as suggested. There should be info in there on hooking to systems. A lot would depend on your exact existing system configuration - they do vary. Everything you need to know to start to figure it out should also be in the stickies. You might need someone there with eyes on to get completely figured out.

A wood boiler is a lot different wood burning unit than stoves or furnaces so prior experience may differ. A different perspective is that rather than a water heating unit - they are a unit with a water cooled firebox. Which changes things a bit, operational & creosote wise - with storage being one way to make them work closer to their potential.


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## peakbagger (Sep 27, 2017)

The low end of storage is usually 500 gallons. If you want to do the math here is some basics

What type of radiators do you have ? Typical baseboard radiators are really not very effective putting out adequate heat below 140 deg F. If you have radiant heating or modern radiant wall emitters you may be able to get down to 90 deg F. Storage temp is usually limited to less than boiling temp with some margin so lets set the max storage temp at 190 deg F. So subtract the lowest temp your radiators will work adequately of 140 from 190 and you get 50 degrees.

The specific heat of water is about 1 btu/lb per degree F. Convert gallons to pounds at about 8.3 gallons per pound so every gallon of water potentially holds 415 btu. Therefore a 500 gallon tank will hold 207,500 btu of useful heat. That sounds like a lot but that's roughly 2 gallons of oil.

The next part is bit more difficult which is what is heat loss of the structure over 24 hours?. It you have newer tight house, you may use less than two gallons over 24 hours in the coldest part of the year but older homes may use more. If you need a lot of heat go big but take into account what you have to heat it.

A way to back into storage volume could be to look at the Tarms rating plate which may be 100,000 btu/hr. That's optimistic but with a 500 gallon tank that means you can bring it up to temp in 2 hours (plan on three). Of course that means you or someone needs to be around to feed it every 24 hours in cold weather but that's what oil backup is for.

You can go two ways on storage, you can drive down and see a Hearth.com member Tom in Maine at American Solar Technics in Searsport and buy an insulated non pressurized tank or you can go with a homebrew pressurized design by using an old propane tank modified for storage use. This choice determines which way you need to plumb it up. There are pros and cons for either one and I happened to have one of Toms tanks so I am biased. I didn't have the option of rigging a propane tank into my basement. The AST design was carried down my basement steps in pieces by me and assembled by me and takes up less room in my basement as its is a square design. It is made out of insulation boards reinforced with metal so no need to have to insulate it after its installed.

You can add storage in after the fact but its lot easier to install it at the same time as otherwise the plumbing get messy (mine was done that way and I would love to clean it up but cant justify it).

It does sound like you have good flue so the next step is see if the boiler is in good shape. You will need a buy a device called a thermovar or equivalent to keep the return water from damaging the boiler but that's details you will need to get into


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## NateB (Sep 27, 2017)

I agree with Peakbagger on everything, but the sizing storage to match your house.  I would size your storage to match your boiler not the demand for heat.  The reason I say that is if the storage is to small you will have to short cycle your boiler, or if it is to large you will have many reloads to get your storage up to temp.  I highly recommend the tanks from Tom http://www.americansolartechnics.com/.  I also recommend the heat pump water heater, so you can have hot water all summer, and a cool dehumidified basement.

I would recommend the primary secondary for connecting everything, and supplyhouse.com for your plumbing needs.  Look at the webstone purge tees, and get a good torch for soldering.  

I would guess if you hooked everything up with storage and domestic hot water you will be in the 4k-5k range.

I would also recommend you use a insulated stainless liner in the chimney, because the low temp exhaust is acidic, and that will eat away your chimney liner. (this explanation may not be exact, but it is to the best of my memory.)


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## peakbagger (Sep 27, 2017)

I dont think acidic exhaust is an issue with an older Tarm unless there is some downstream heat recovery. My burnham is similar vintage and have never seen a trace.


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