Oil vs wood BTU and storage

GARYL · Apr 23, 2008

In 2001 my old Thermo-Control wood boiler finally gave up after 25 years, so I installed an oil boiler with baseboards and said "NEVER AGAIN" would I have to put wood up! Since then I have averaged 1140 gallons between Oct 1st and May 1st each year. (I was burning about 6 to 8 cord a year and about 200 gallons of oil.) Is there a way to determine how many BTU's I am consuming and how that relates to storage? I am leaning toward a 1050 Switzer. I would ideally like to fire the boiler only once a day most of the time.

Thanks.
Gary

BrownianHeatingTech · Apr 23, 2008

GARYL said:
In 2001 my old Thermo-Control wood boiler finally gave up after 25 years, so I installed an oil boiler with baseboards and said "NEVER AGAIN" would I have to put wood up! Since then I have averaged 1140 gallons between Oct 1st and May 1st each year. (I was burning about 6 to 8 cord a year and about 200 gallons of oil.) Is there a way to determine how many BTU's I am consuming and how that relates to storage? I am leaning toward a 1050 Switzer. I would ideally like to fire the boiler only once a day most of the time.

1140 gallons of oil is roughly 158,500,000 btu's.

You can divide that by heating hours and get a rough idea of your average heat load. Heating hours are roughly related to growing zones, so if you know that, I can probably look up a good number for you. Your peak heat load is generally about double your average load.

The best method is to measure your house and do a heat loss analysis on it.

Then just take your hourly heat loss and multiply by 24 hours, and that tells you how many btu's you need to store in your tank.

Measure up the feet of baseboard and you can figure out what minimum water temperature will be required to provide that, then you can subtract that from the peak water temperature (170-200, depending on your storage system and such) and that will give you your temperature cycle (delta-T) for your storage tank.

Btu's required, divided by the delta-T gives you pounds of water. Divide that by 8.3 to get gallons.

Then decide if your tank size is reasonable. If not, you can go to a different sort of tank that will allow a higher maximum temperature (if possible) or add more baseboard (or other radiation) to lower the minimum temperature, or just decide that you can handle more frequent loading under peak load.

Joe

gorsuchmill · Apr 24, 2008

Joe -

I've done a heat/loss on my house and I'm coming up with a calc of approx. 110,000 btu. Based on your description, my average load would be approx. 55,000 btu per hour, for a total of 1,320,000 per day. I heat with radiators, which I suppose could provide ample heat down to 140*, and provided I could get my storage up to 200* would that indicate a delta-T of 60*? If so, I would need storage of approx. 2650 gallons (1,320,000 / 60 / 8.3 gal = 2,650 gal). Does that seem correct? It seems like a lot to me, and more than I could possibly provide.

For the record, I live in the Baltimore, MD area. Our coldest winter temp is in the low teens, with most days ranging from the mid-20's to upper 30's. Also, I currently use about 1200 gallons of heating oil from October thru April in an 85% efficient (rated), 135k btu, oil boiler. The boiler seems to cycle more than I would like, but doesn't come close to running non-stop on even the coldest days.

Thanks for your help.

Lee

GARYL · Apr 24, 2008

[quote author="BrownianHeatingTech" date="1209007120

The best method is to measure your house and do a heat loss analysis on it.

Then just take your hourly heat loss and multiply by 24 hours, and that tells you how many btu's you need to store in your tank.

Measure up the feet of baseboard and you can figure out what minimum water temperature will be required to provide that, then you can subtract that from the peak water temperature (170-200, depending on your storage system and such) and that will give you your temperature cycle (delta-T) for your storage tank.

Btu's required, divided by the delta-T gives you pounds of water. Divide that by 8.3 to get gallons.

Then decide if your tank size is reasonable. If not, you can go to a different sort of tank that will allow a higher maximum temperature (if possible) or add more baseboard (or other radiation) to lower the minimum temperature, or just decide that you can handle more frequent loading under peak load.

Joe[/quote]

Joe,

Thanks for your reply but I have a few questions.

As per your suggestion, I did an online heat loss calc and came up with 70778BTU/hr and 159.3millionBTU/yr. This appears to be correct because it matches up well with the caculation you gave of 158.5 millionBTU for 1140 gal of fuel oil.

The next step was to determine daily loss which is 1,698,672 BTU/daily.

I am afraid this next step is where you lost me. I have 204ft of baseboard and didn't know how to proceed from there. I know I don't have enough baseboard because when it gets below zero the house keeps losing temp. The circulator runs constantly while the furnace only comes on once in awhile. We keep the house at 68 all the time but last year when we had 5 days below zero in a row the house dropped to 52 brrrrrrr. I don't want to sacrifice anymore wall space to baseboard for the few times it goes below zero.

That was as far as I could get using your formula because I didn't know how to get from baseboard length to figuring out minimum water temp.

Sorry to be so dense. :red:

THANKS!
Gary

BrownianHeatingTech · Apr 24, 2008

gorsuchmill said:
I've done a heat/loss on my house and I'm coming up with a calc of approx. 110,000 btu. Based on your description, my average load would be approx. 55,000 btu per hour, for a total of 1,320,000 per day. I heat with radiators, which I suppose could provide ample heat down to 140*, and provided I could get my storage up to 200* would that indicate a delta-T of 60*? If so, I would need storage of approx. 2650 gallons (1,320,000 / 60 / 8.3 gal = 2,650 gal). Does that seem correct? It seems like a lot to me, and more than I could possibly provide.

For the record, I live in the Baltimore, MD area. Our coldest winter temp is in the low teens, with most days ranging from the mid-20's to upper 30's. Also, I currently use about 1200 gallons of heating oil from October thru April in an 85% efficient (rated), 135k btu, oil boiler. The boiler seems to cycle more than I would like, but doesn't come close to running non-stop on even the coldest days.

That does sound about right.

That's why in-floor radiant (which can operate at a much lower supply temperature) is a much better match for thermal storage systems. If you only needed 110 degrees, your delta goes to 90, and your gallons go down to less than 1800.

Again, that's to run for a full day without adding any heat. In most cases, it's better to plan for two fires per day, so you can halve the size of the tank. You will also have some "overlap" during which the fire is actually burning, so you can round down a bit on the tank size.

Joe

BrownianHeatingTech · Apr 24, 2008

GARYL said:
As per your suggestion, I did an online heat loss calc and came up with 70778BTU/hr and 159.3millionBTU/yr. This appears to be correct because it matches up well with the caculation you gave of 158.5 millionBTU for 1140 gal of fuel oil.

The next step was to determine daily loss which is 1,698,672 BTU/daily.

I am afraid this next step is where you lost me. I have 204ft of baseboard and didn't know how to proceed from there. I know I don't have enough baseboard because when it gets below zero the house keeps losing temp. The circulator runs constantly while the furnace only comes on once in awhile. We keep the house at 68 all the time but last year when we had 5 days below zero in a row the house dropped to 52 brrrrrrr. I don't want to sacrifice anymore wall space to baseboard for the few times it goes below zero.

That was as far as I could get using your formula because I didn't know how to get from baseboard length to figuring out minimum water temp.

Different manufacturers have different output curves, but 204 feet of typical baseboard should produce over 100,000 btuh, so I expect you either have a too-low supply temperature or a flow problem. Or there's a lot of bare pipe in the enclosure (when we say feet of baseboard, we mean the finned tube, not the sheetmetal jacket, just in case that's what you measured).

If your baseboard is "maxed out," then you will have a very low delta-T, and that means that a tank would have to be very large.

Panel radiators take up less wall space than baseboard, for similar output. In-floor radiant takes up no wall space. Either would let you lower your operating temperature without sacrificing wall space. Of course, there's a cost involved, so that's a factor.

Joe

GARYL · Apr 24, 2008

[quote author="BrownianHeatingTech" date="1209072774

Or there's a lot of bare pipe in the enclosure (when we say feet of baseboard, we mean the finned tube, not the sheetmetal jacket, just in case that's what you measured).

In-floor radiant takes up no wall space.
Joe[/quote]

Joe,

I do have alot of bare pipe down cellar and the many, many feet in the runs are not insulated. It helps to keep the cellar at 45 or 50 most of the winter. I checked and there is only about 9 feet that isn't finned inside the baseboard cover.

After reading this site over the last few months I have been wondering if I should take out the baseboard on the first floor and put in radiant. The joists are exposed in the basement and doesn't look like a tough job. I couldn't do the second floor without ripping things apart. I sure would like to get rid of the baseboard wherever I can! Gary Switzer said his boiler can go to 220* so my delta for radiant would be 100*? Even though both floors have the same footprint could I assume that the first floor where I would put the radiant would consume 60% or 70% or 80% of the BTU loss? We keep the upstairs about 65 and the downstairs about 68. The heat doesn't come on upstairs very often unless it gets very cold.

Is it logistic heavy to have a two temp system and does this add to the cost alot?

Alot of questions I'm afraid and I sure appreciate your help.

THANKS!
Gary

BrownianHeatingTech · Apr 24, 2008

GARYL said:
After reading this site over the last few months I have been wondering if I should take out the baseboard on the first floor and put in radiant. The joists are exposed in the basement and doesn't look like a tough job.

It's labor-intensive, but not particularly difficult. You need to calculate the temperature and flow rate properly, and keep your loop lengths to reasonable levels (no longer than 300 feet per loop for 1/2" tube). It's worthwhile to invest in a proper injection mixing control setup to get accurate water temperature regulation.

GARYL said:
Gary Switzer said his boiler can go to 220* so my delta for radiant would be 100*?

The limitation is more based upon the sort of storage tank you use. Some commercial electric boilers are pressurized so they can store water at almost 300 degrees without it turning to steam, letting them heat the water at off-peak rates and then deliver it during peak times.

Typically, we keep storage tanks at or close to atmospheric, so we tend to limit the temp to 200 during normal operation in order to have a safety factor. EPDM-lined tanks tend to require the temp to be limited further in most cases.

GARYL said:
Even though both floors have the same footprint could I assume that the first floor where I would put the radiant would consume 60% or 70% or 80% of the BTU loss? We keep the upstairs about 65 and the downstairs about 68. The heat doesn't come on upstairs very often unless it gets very cold.

Calculate the heat loss for each floor. Window sizing and such has a large effect. Typically the first floor consumes less than the second floor, because it is below heated space. If you install radiant, you will have to insulate below the tubing, so account for that in your calculations.

GARYL said:
Is it logistic heavy to have a two temp system and does this add to the cost alot?

Not really. You basically have a "standard" temp and a reduced temp. It only starts getting expensive when you start adding multiple different reduced temps...

Joe

GARYL · Apr 25, 2008

Joe,

Thanks for your help!

I believe at this point in time I'm gonna get a 1050 Switzer and install radiant under my first floor only. I don't think I am going to hook into my existing oil boiler system or alter it at all. Doing it this way seems to me to be the easiest and least expensive way for now. If I can heat the first floor up to 68* like my oil unit does, then the second floor won't require heat until it gets real cold out and the oil unit can satisfy that. Maybe in the future, once I fully understand the controls necessary to integrate the two systems I will do it, but I will need more storage.

The Switzer (like the Garn it's water storage is internal) will have 1050 gallons at 220* yielding a delta T of about 100* for the radiant. Assuming no losses in the system that should give me about half a days needs if I were supplying the whole house (1.7MillionBTU). I would guess 5 cord of wood and 200 to 300 gallons of oil should carry me through the winter. A 900 gallon savings @ $4 per = $3600 saved. The ROI is about 4 to 5 years. NOT BAD !!!

!! (a few less camels for someone)

Does all this make sense to you or am I dreaming?

Gary

BrownianHeatingTech · Apr 25, 2008

GARYL said:
Joe,

Thanks for your help!

I believe at this point in time I'm gonna get a 1050 Switzer and install radiant under my first floor only. I don't think I am going to hook into my existing oil boiler system or alter it at all. Doing it this way seems to me to be the easiest and least expensive way for now. If I can heat the first floor up to 68* like my oil unit does, then the second floor won't require heat until it gets real cold out and the oil unit can satisfy that. Maybe in the future, once I fully understand the controls necessary to integrate the two systems I will do it, but I will need more storage.

The Switzer (like the Garn it's water storage is internal) will have 1050 gallons at 220* yielding a delta T of about 100* for the radiant. Assuming no losses in the system that should give me about half a days needs if I were supplying the whole house (1.7MillionBTU). I would guess 5 cord of wood and 200 to 300 gallons of oil should carry me through the winter. A 900 gallon savings @ $4 per = $3600 saved. The ROI is about 4 to 5 years. NOT BAD !! (a few less camels for someone)

Does all this make sense to you or am I dreaming?

Gary

Sounds reasonable. You can get a two-stage thermostat to control both heat sources. That way, if you set it at 68, but the wood can't keep up so it cools by another two degrees (or whatever you set the differential at), it will automatically turn on the baseboard using the oil boiler.

Joe

DenaliChuck · Apr 25, 2008

Wow Joe,

A two stage thermostat is exactly what I've been needing to help figure out how to set up my soon-to-be system but I didn't know such a thing existed. Do you have a model number you recommend, or a manufacturer?

Chuck

BrownianHeatingTech · Apr 25, 2008

DenaliChuck said:
Wow Joe,

A two stage thermostat is exactly what I've been needing to help figure out how to set up my soon-to-be system but I didn't know such a thing existed. Do you have a model number you recommend, or a manufacturer?

Honeywell FocusPro TH6220D1002 (-D1028 if you want a larger display).

Joe

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