Heating with Oil, Wood, and storage

[joecool85]

In this concept:

Wood Boiler set to 190F, pump runs any time storage is colder than boiler (pump wired so it can be switched to on 24/7 in the dead of winter/for vacations, as an anti-freeze measure)

Oil Boiler set to 150F Low, 170F High, pump runs any time storage is below 160F

 

[WoodNotOil]

Why is it piped hot to cold? I can't see how heat gets from the oil boiler to zones. Does it have to heat the storage? I wouldn't want to pay that oil bill...

I would consider something closer to the piping in Nofo's sticky.

 

[joecool85]

Why is it piped hot to cold? I can't see how heat gets from the oil boiler to zones. Does it have to heat the storage? I wouldn't want to pay that oil bill...

I would consider something closer to the piping in Nofo's sticky.

Oil heats the house the same way the wood does. Yes, oil will heat the storage as well - but only when the tank gets below 150. Its just as efficient as not having storage on the oil boiler. Oil furnaces do better on efficiency during longer burns just like wood stoves (to a lesser degree though). This also keeps my dhw hot regardless of whether it is running on wood or oil.

 

[webie]

Why is it piped hot to cold? I can't see how heat gets from the oil boiler to zones. Does it have to heat the storage? I wouldn't want to pay that oil bill...

I would consider something closer to the piping in Nofo's sticky.

+1

 

[WoodNotOil]

Why is it piped hot to cold? I can't see how heat gets from the oil boiler to zones. Does it have to heat the storage? I wouldn't want to pay that oil bill...

I would consider something closer to the piping in Nofo's sticky.

Oil heats the house the same way the wood does. Yes, oil will heat the storage as well - but only when the tank gets below 150. Its just as efficient as not having storage on the oil boiler. Oil furnaces do better on efficiency during longer burns just like wood stoves (to a lesser degree though). This also keeps my dhw hot regardless of whether it is running on wood or oil.

Say it is below 0* outside in January and really windy etc. Your baseboards are looking for 180* water to keep up with demand... Your tank is at 150*. Regardless of whether you are burning wood or oil, you now have to wait for the entire tank to get up to temp before your zones get anywhere near the temp they need to keep up with demand.

On the other hand... If you could pump directly from either wood or oil with 180* water, then your zones can be satisfied immediately.

 

[joecool85]

Why is it piped hot to cold? I can't see how heat gets from the oil boiler to zones. Does it have to heat the storage? I wouldn't want to pay that oil bill...

I would consider something closer to the piping in Nofo's sticky.

Oil heats the house the same way the wood does. Yes, oil will heat the storage as well - but only when the tank gets below 150. Its just as efficient as not having storage on the oil boiler. Oil furnaces do better on efficiency during longer burns just like wood stoves (to a lesser degree though). This also keeps my dhw hot regardless of whether it is running on wood or oil.

Say it is below 0* outside in January and really windy etc. Your baseboards are looking for 180* water to keep up with demand... Your tank is at 150*. Regardless of whether you are burning wood or oil, you now have to wait for the entire tank to get up to temp before your zones get anywhere near the temp they need to keep up with demand.

On the other hand... If you could pump directly from either wood or oil with 180* water, then your zones can be satisfied immediately.

In Nofos layout it has a similar issue. It uses the water off the storage tank unless it is below 140F. His designs only saving grace there is that if you are using the wood heat it will pump that through the system and any excess charges the tanks.

My set point may end up being 160, not 150. But regardless, I will make sure it can keep my house warm in cold weather at the lowest set point in my tank. Basically what it comes down to is either I need to heat my storage with oil periodically if the wood is out and temp is down, or I can change it so when the wood goes out it turns on the oil and then I would need a separate tank for my dhw and add a circ pump for it as well making it it's own zone.

As far as oil bill goes, it won't be that bad. Let's say it is a 500 gallon tank and that every day the oil has to run twice to bring the tank temp up from 150 to 160. With 150 days of this happening, and the house drawing 15,000btu/hr average while it's happening that would mean the oil would need to put 30,000 btu/hr for the house load and 83,000 btu/hr into the tank to raise the temp (twice) = 113,000 btu/hr every day. That means, at 85.5% efficiency (that my oil boiler tested out to last month), I would burn almost exactly one gallon of oil every day (need to burn 132,000 btu/hr worth of oil, oil is 140,000 btu/gallon). That would mean 150 gallons of oil a year.

Right now I am burning 650 gallons of oil per year. On top of that, I'd have to REALLY ignore my wood boiler for all that oil consumption to happen. In the dead of winter I should be only needing to fill the boiler 2 times a day, and most of the winter will be once a day or so.

So worst case scenario, I burn 1/4 the oil I do now. Best case, I burn virtually no oil.

 

[stee6043]

His designs only saving grace there is that if you are using the wood heat it will pump that through the system and any excess charges the tanks.

Hmmm....this is a pretty big deal when you start approaching design loads for your heating system. Coincidentally these "design load" type days will be the days you care most about your system running properly.

I can see there being days when you aren't heating with wood, tank temps drop and you don't really want to heat storage with oil. Temporariy heating the home with oil makes sense until you can use wood to get your storage back up to temp.

There is a reason many (most) systems are plumbed with loads first and storage second. But I guess you didn't really ask any questions in your first post so maybe you didn't even want this kind of feedback???

 

[webie]

So let me get this straight for year around includeing summer your storage tank will be heated with your oil boiler to provide you with DHW ?

 

[joecool85]

So let me get this straight for year around includeing summer your storage tank will be heated with your oil boiler to provide you with DHW ?

If the wood wasn't running, then yes. I was trying to keep things as simple as possible plumbing wise and what not...maybe I should rethink after all. As much as I was originally against adding a tank and a third zone for the DHW, maybe it would make sense afterall.

 

[Nofossil]

In Nofos layout it has a similar issue. It uses the water off the storage tank unless it is below 140F. His designs only saving grace there is that if you are using the wood heat it will pump that through the system and any excess charges the tanks.

Almost. It will not draw anything from storage if the wood or oil boiler are operating UNLESS the flow from the wood boiler is less than the total flow that the heat loads need. In most cases, the wood boiler flow will be greater than is needed for the heat loads, and the excess will go into storage.

Starting from a cold boiler and cold house, 100% of the boiler output will go directly to the house, not through storage.

It draws from storage if the wood and oil boilers are not providing heat.

In all fairness, there is some tweaking need to balance flows, and variable speed boiler and load circulators would be an improvement. Some people have moved the oil boiler and its circ to the right of the load circ - that's an improvement, and I should have done it that way. In that case, the load circ would be disabled when the oil boiler is running.

 

[joecool85]

Interesting about moving the oil boiler circ. So, what happens in your setup if the storage is at 150F, the house calls for heat and the wood boiler is out of wood? Are you stuck using 150F storage water?

 

[Rory]

My system is basically what you described, but there's a mixing valve AND a Termovar involved in the piping between the tank/boiler loop and the oil furnace. I have my oil setpoint at 135 in the storage tank. Some days when I get home from work it comes on before I have the wood boiler pumping again, but it doesn't run very long before the furnace jacket is 180 and it shuts down. Perhaps it doesn't shunt to the tank as fast as it should, I don't know, but everything works fine and I'm using hardly any oil at all.

 

[joecool85]

My system is basically what you described, but there's a mixing valve AND a Termovar involved in the piping between the tank/boiler loop and the oil furnace. I have my oil setpoint at 135 in the storage tank. Some days when I get home from work it comes on before I have the wood boiler pumping again, but it doesn't run very long before the furnace jacket is 180 and it shuts down. Perhaps it doesn't shunt to the tank as fast as it should, I don't know, but everything works fine and I'm using hardly any oil at all.

As I described or as NoFossil described?

 

[Rory]

My system is basically what you described, but there's a mixing valve AND a Termovar involved in the piping between the tank/boiler loop and the oil furnace. I have my oil setpoint at 135 in the storage tank. Some days when I get home from work it comes on before I have the wood boiler pumping again, but it doesn't run very long before the furnace jacket is 180 and it shuts down. Perhaps it doesn't shunt to the tank as fast as it should, I don't know, but everything works fine and I'm using hardly any oil at all.

As I described or as NoFossil described?

As you described, if I understand your plumbing correctly. I have the design developed by Revision Energy. There is basically a loop of 1" pipe going from my wood boiler to my oil boiler with the passive storage tank in between. The primary heat exchanger in the tank is 2 120' coils of 3/4 inch copper in parallel, and the DHW comes in via 3/4" pipe, plunges to the bottom of the tank and returns through a 180' coil. I use about 1 fire a week in the summer to heat enough hot water for my wife and I, but I could leave the oil boiler set to kick in if I desired. I also have a toggle switch on the burner and I just leave it off for the summer. In fact, my oil was out of commission until last month, but since then I've left it set at 135 and, as I already mentioned, it runs a little, I can tell my wife "we're covered" no matter what, but my gauge hasn't moved enough to notice.

 

[Nofossil]

Interesting about moving the oil boiler circ. So, what happens in your setup if the storage is at 150F, the house calls for heat and the wood boiler is out of wood? Are you stuck using 150F storage water?

'Stuck' is a funny word in this context. When you design the system, you decide the lowest storage temp that's useful. When storage drops below that point and the wood boiler is out, it switches automatically to the backup heat source. You're only 'stuck' using 150 degree water if you decided that you'd rather do that than fire up the backup heat source.

Here's a schematic. Ignore the inside / outside labels. In this setup, the load circ runs when heat is from either wood or storage. It has an integral check valve to prevent reverse flow.

 

[joecool85]

Ok, the more thinking I do, the more and more I really like my original post (in this thread, not from other threads).

Yes, the water needs to heat up before the house gets heat, BUT the tank can't get below 160 or the oil kicks in and holds it there. Contrary to what it seems, this isn't that inefficient really. Also, I could lower it to 150 for the tank minimum. I've done some math, and once I add more high output baseboard to my living room and upstairs (doing this before adding storage anyway), I will be able to heat my house with storage down to 150 if it is above zero outside. I will need 160 or a little more if it's colder than that. And that is to keep it 68-70 in here. Worst case, I don't turn the min temp up on the tank, it gets to -20 outside, and my house can't get warmer than 60 degrees till I go outside and start a fire in the boiler. How is that any worse than waking up to a cold house when you are running just a standard wood stove?

With 500 gallons of storage, I should get a 12 hour run time during most of the cold parts of the winter, 8-9 hours at the very coldest (-20F) times.

Please, someone correct me if I'm wrong. I'm just sharing my thoughts/ideas at this point.

**edit**
As to summer oil bill, the oil furnace would be able to be shut off entirely. I would only need to fire the wood boiler once a week to keep up with dhw - I can live with that for sure.

 

[Rory]

You know, I don't really get the concerns with using oil to heat storage. #1, it's set to kick in at a point we choose, which isn't going to be anything close to the temp it comes in from the well. #2, if the storage is insulated really well, any heat made will ultimately be used for its intended purpose. If the storage is inside and it's heating season, any loss goes into the house anyways. Where's the problem?

 

[Nofossil]

You know, I don't really get the concerns with using oil to heat storage. #1, it's set to kick in at a point we choose, which isn't going to be anything close to the temp it comes in from the well. #2, if the storage is insulated really well, any heat made will ultimately be used for its intended purpose. If the storage is inside and it's heating season, any loss goes into the house anyways. Where's the problem?

Like many things, it depends. Most of us are trying to burn as little oil as possible, usually for brief periods when we're not there to start the wood boiler. In my case, when that happens I want to use just enough oil to keep the house and/or DHW at temperature. The expectation is that I'll be starting the wood boiler again at some point in the not-too-distant future.

The oil boiler won't come on at all unless storage is too cold to heat the house. If the oil boiler has to heat storage before it can heat the house and/or DHW, then I'm going to have to burn a lot of oil to get a little bit of heat into my house.

I'll agree that he heat is not lost, but if I'm heating storage with oil I'm burning oil to reduce the amount of wood that I'll burn in the future. That's not what I want to do.

 

[joecool85]

You know, I don't really get the concerns with using oil to heat storage. #1, it's set to kick in at a point we choose, which isn't going to be anything close to the temp it comes in from the well. #2, if the storage is insulated really well, any heat made will ultimately be used for its intended purpose. If the storage is inside and it's heating season, any loss goes into the house anyways. Where's the problem?

Like many things, it depends. Most of us are trying to burn as little oil as possible, usually for brief periods when we're not there to start the wood boiler. In my case, when that happens I want to use just enough oil to keep the house and/or DHW at temperature. The expectation is that I'll be starting the wood boiler again at some point in the not-too-distant future.

The oil boiler won't come on at all unless storage is too cold to heat the house. If the oil boiler has to heat storage before it can heat the house and/or DHW, then I'm going to have to burn a lot of oil to get a little bit of heat into my house.

I'll agree that he heat is not lost, but if I'm heating storage with oil I'm burning oil to reduce the amount of wood that I'll burn in the future. That's not what I want to do.

While I do agree with your theory entirely, in real life I doubt that it would equal out to that many gallons of oil. I suppose it all comes down to how many times the oil furnace has to kick in. If it only runs a few times during the winter, then either way it won't use much oil. If it kicks in every single day once or twice, then my way will use substantially more oil than yours.

 

[joecool85]

How's this?

It would still only run the oil when the wood isn't keeping the temp in the tank up, but now, when the oil runs it kicks on it's own circ pump and effectively bypasses the tank entirely. I've moved the dhw into it's own zone so you would still have dhw in this case too.

 

[Gooserider]

I don't think it is a good thing to be running the wood boiler the way you have it - where you are mixing the WB output with load return water before sending it through the tank. If the tank is cold, you will be reducing the temp of the water going to the load. If the tank is hot, you will be raising the temp of the water going to the WB, which will lower your thermal transfer ability...

My first pass at a fix would be to run the WB supply and return to the manifolds the same way that the oil burner is hooked up - supply to the hot side, return to the load return. (i.e. reverse the way the WB is currently connected)

If the oil burner fires, water goes through that loop, no problem.

If the WB is hot, run the pump on the WB side, and if the house loads are running at an equal demand, the water bypasses the tank and goes through the WB. If the house loads are less, the extra water goes "backwards" through the storage tank and charges it.

If the WB is not hot, then the loads pull through the storage tank, bypassing the two boilers.

Note that this does assume that the flow resistances of the loads and the two boilers are enough higher than the tank flow resistance that you don't get significant "ghost flow" through the loops when you don't want it, you might end up needing some zone valves or spring checks to enforce this.

Gooserider

 

[WoodNotOil]

I agree with Goose on this one. The wood should be switched for hot to go to zones and when no call for heat, it should pull through the coil in reverse. Make sure the top of the coil points to the hot side and the pipe to the bottom of the coil is to the cold side. That will make certain you maintain stratification.

One way to go would be to put a ZV on the oil piping and one on the wood so that when the tank supplied heat there would be no ghost flow. Another option is to tee the wood piping into the oil piping before the manifold and put only one ZV between the tee and the manifold. That would shut off flow to both boilers and reduce parts cost and simplify controls a little.

 

[joecool85]

Good thinking guys. I draw up a new one later and put it up here. I don't think ghost flow will be a problem. The lines to the wood boiler and oil boiler are going to be 1" and the manifold on the storage tank is going to be 1 1/4" and the coil will be 5 x 1/2" copper coils, so it should flow better than the loops going to the boilers. If it became an issue I could always add some zone valves later like you guys mentioned.

 

[joecool85]

Finally got around to re-doing my layout.

Probably won't go this way afterall, but only because my oil company won't let me keep my service contract on the oil boiler if I tie in a solid fuel boiler (indoors or out). So I will probably keep the two systems entirely separate and just have two thermostats, leaving the oil one down 4-5 degrees cooler than the wood.

Regardless, if I was to tie it in, this is how I would do it. Surprise, surprise: the more I learned and tweaked my layout, the more it looks like a "standard" install lol. The big thing I did is to use a single heat exchanger rather than one to charge and one to remove heat from the tank.

 

[Gooserider]

Finally got around to re-doing my layout.

Probably won't go this way afterall, but only because my oil company won't let me keep my service contract on the oil boiler if I tie in a solid fuel boiler (indoors or out). So I will probably keep the two systems entirely separate and just have two thermostats, leaving the oil one down 4-5 degrees cooler than the wood.

Why? You married to that Oil Co. or something?

If they don't want to keep your business why not just tell them that you'll be happy to take it somewhere else? (One of the nice things about oil co's is that there are lots of them, and they tend to be pretty competitive - unlike our natural gas or electric company where you have ONE vendor to get service from, whether you like them or not...)

Unless the oil co in question wants to pick up the tab for installing a fully redundant system, I certainly wouldn't let them force me into such a setup... I'm willing to bet they are hoping to price you out of doing the WB install so they can keep selling you oil...

Regardless, if I was to tie it in, this is how I would do it. Surprise, surprise: the more I learned and tweaked my layout, the more it looks like a "standard" install lol. The big thing I did is to use a single heat exchanger rather than one to charge and one to remove heat from the tank.

Actually your setup does look pretty much like a standard install - it isn't often that we would suggest someone do duplicate HX's, as generally it is far better to save the extra expense, and instead set things up so that you get flow in both directions through the same HX...

Gooserider