Adding wood boiler to existing oil burner question

[ejb]

Hi guys....

Anyone have on of these?

http://www.thermocontrolheating.com/model400.htm

I want to add a boiler to my existing oil fired burner to save oil costs in the winter, this unit (the model 500) has 125000 BTU, which I suspect is a little small for my 3000 sf house...but wouldn't that mean it just won't work on the really, really cold days? Would the only impact of having a slightly undersized boiler daisy-chained to the oil burner be that on those really cold days the wood boiler would be running full bore, but also the oil would be picking up the slack?

I'd like to cut my heating costs by about 75% (my wood is free), I like this little unit because it is relatively cheap (will pay for itself in one season) and will fit down my cellar stairs...some of the larger units I am not so sure of....

Besides this model in particular, anyone have good or bad stories about the impact of daisy chaining a wood boiler to an oil burner?

Thanks all!

 

[Birdhunter1]

I assume you are talking about a wood fired furnace that heats water and sends it to a coil in the ductowrk. I also assume from ytour question that you are putting this in your house basement? If you are worried about if it will handle the load of your house or not why not just get a wood furnace and hook it to your duct system. If it were me I would get a wood burning furnace that blows air across your wood chamber and then hook this into your duct system. To do it the way I would describe you would actually hook your wood burning duct above your oil furnace (in the supply duct) and you would actually pick up you return air from underneath the wood furnace. If you would like I could send a picture of the wood burner in my house or draw out a schematic of what I am talking about.

 

[beenthere]

I am assumming you have hot water heat, as I do, and want a wood fired 'boiler' in combination with a separate oil-fired 'boiler', which is also similar to what I have.

Mine has been operational for almost 20 years now (after next season anyway). I used to have oil, but now have gas. When I put the gas in, I designed the system so I could heat the house with all wood, or all gas, or wood backed up by gas. Learned that most of the time, even at -20 with wind, that the wood could keep up. I have a 9 gal water jacket around the combustion chamber, and it works fine. The water temp is kept at 100 - 180 degree F in the wood heater (doesn't really boil the water, but needs relief valves to protect against high pressure if something fails to shut down). Plumbing into the three zone (pumps) house system was fairly straight forward. I have it set up so if the wood heated water gets below 100 deg F, then the gas will fire until that water exiting the wood heater gets up to 100. Just a safety measure to keep warm water circulating in the house if the wood fire goes out for some reason.

Sorry, but am not familiar with the system you inquired about. Mine is not mfg anymore, unfortunately.

 

[ejb]

OK, a few more details to clarify what I have, and what I want to do....

I already have an oil burner. Its is a forced hot water system, ie. heated water circulates around the house thru radiators (not the big old fashioned steam ones, regular baseboard ones...).

I also already get my domestic hot water off the oil burner...its a tankless hot water off the furnace type setup....

I want to add a wood boiler, but also keep my oil burner for when it gets really, really cold, or if I go away, or if I run out of wood....

My understanding on this is that in order to pipe one in, I would take the output pipe of the new wood boiler and feed it into the cold water side of the the oil burner (i.e. the side of the oil burner where the water goes into to be heated)...and I would take the input to the wood burner and pipe it to accept the water as it returns from the radiators....

SO in other words, when the wood boiler is running, it is heating up water and sending it into the oil burner, where the thermometer in the oil burner senses that the water is already hot enough and it just circulates the water to the house w/out firing the boiler...the water circulates thru the house, returns to the wood boiler and the process repeats....

The benefit of this, as I see it, is you don't need an extra set up thermostats, and if the fire goes out, the oild burner just kicks in because the water is cold....when the wood burner is not running it is just extra pipe for the water to pass thru....

I am pretty sure I more or less have the concept down, but am looking for feedback from people that have personal experience with this type of setup, and if there are any "gotchas" that I should be on the lookout for before I get this up and running....

Thanks!

 

[beenthere]

I hear you, and have the same system (gas instead of oil) except I don't have the domestic water heater in the loop (almost true, as I have a 30 gal pre-heat tank on the cold water input side of the domestic hot water tank, that the wood heats up some). This second picture shows the gas boiler on the right, the three pumps, the dom. hot water ss preheat tank, and the wood boiler on the far left.

The only problem I see with your design is getting the right controller of water temp at the oil burner. If the wood heats the water to 180, and the oil burner kicks it another 10 degree, that water may come back around to the wood burner, which checks the temp of that return water and says "its already hot, I don't need to add any wood heat", so keeps the wood damper closed. So, effectively the wood heat is out of the loop. I found that I couldn't get a water temp control for the gas fired boiler to have the right range of temp to do what you want to do.
My wood boiler has a temp sensitive draft control, which closes the damper at about 180 deg F. That is because the wood fire has residual heat that will keep heating the water from hot coals, etc. after the damper is closed. If one makes the mistake of throwing in a lot of wood in early, and there isn't a demand for a lot of hot water, then the damper shuts early, and the wood just smolders away (don't make the mistake of looking at the fire when it has been smoldering, because a big flash-back can occur, i.e. explosion of gases getting some oxygen).
I put the gas boiler under control of a separate temp senser, that is set to have the gas come on when the wood boiler output temp gets below 100 Deg F. But it doesn't take much for the gas boiler to kick the circulating water to 100, at which time the gas shuts off.

If the hot water in your system recirculated back to the wood boiler cooled down to, say, 100 deg F, then the wood would heat it up and the oil could kick it some more. But I don't think you will get that much cooling from your circulation system. You may know that better than I.

Your dom. hot water being tied into the system makes the design much more difficult, IMO.

 

[tc35dforme]

Plan for a dump zone to get rid of heat when the house doesn't need it and the wood stove isn't on idle ...a unit heater in the garage is always nice.....

Watch the size of your relief valves...you can't shut off a wood stove as quickly as you can a boiler. This will be especially important if a pump fails and you limit circulation to gravity flow.

Have someone check the size of your expansion tank. The added volume in the system and the potentially rapid heating by the wood stove may require an additional tank to keep the system pressures stable.

Consider pumping the wood boiler separately as an injection loop into the main boiler loop. By doing this, you only circulate water through the wood boiler when its hot and don't use it as a reverse radiator ( stealing heat ) from the system when its idle.

Watch the smokepipe routing. Mass code generally doesn't allow the two ( oil fired / wood fired ) in the same stack.

Be mindful of the increased need for combustion air.

 

[beenthere]

tc35
These are all considerations in the design, however, none have been any problem with my system (only 9 gal water jacket and about a 10 gal expansion tank) in the years it has been in. But they are important to have in mind at all times.

Never have needed to dump heat. When the water temp gets to the high limit, the damper shuts off the air supply and the fire 'sleeps'. If a power failure, the damper is spring loaded and closes. Very simple but has been very well designed system.