btu question

[huffdawg]

Hello all, im just trying to decipher my design summary for my radiant heating system .
It says the total radiant load is 39500 btu's per hour.

My lpg. boiler puts out 110,000 btu per hour.
It also supplies heat to my 60 gal. DHW heater

So im wondering how they sized my boiler from the 39500 btu per hour radiant load.

Im also trying to size a wood gassification to the same system as well as 1500 sq' shop.

Thanx in advance for any help.

Huff.

 

[Nofossil]

When sizing fossil boilers, the normal approach is to make sure that it's too big. As a customer, you'll never know that it was too big, but you'll be very unhappy if it was too small. Also, DHW typically presents a very high load when it calls for heat.

Since fossil boilers can go from full output to no output very quickly, there isn't much of an efficiency penalty for oversizing, either.

Wood boilers are a different animal. They burn most efficiently at full output. Idling causes creosote, smoke, and reduced efficiency. If your peak load is 35,000 BTU/hr, that means that your average load is somewhere around 15,000 to 20,000 BTU/hr. That means that even if you could perfectly match your wood boiler to your peak load, it would still be idling half the time.

There are two solutions: First, you could get the smallest boiler that's big enough to satisfy your peak demand. That's probably going to be in the 80,000 BTU/hr range, which translates to an average output of somewhere around 60,000 BTU/hr. You can then operate it aiming at some compromise between convenience and efficiency. Most convenient is to load it up and let it idle as necessary (about 75% of the time on average in this case). Most efficient is a series of small, hot fires with the boiler going out or very nearly out between fires.

The second solution is heat storage. There are only a million threads on heat storage here. The basic idea is that you run the boiler flat out to heat the house and storage, then let the boiler go out. Heat the house from storage until it's time for the next fire. In that case, you can get a boiler that's a bit larger - especially if you have plenty of storage.

In my case, I burn an average of 7 hours a day. Right now, I'm still living off of heat in storage from a fire I built on Wednesday.

 

[heaterman]

Hello all, im just trying to decipher my design summary for my radiant heating system .
It says the total radiant load is 39500 btu's per hour.

My lpg. boiler puts out 110,000 btu per hour.
It also supplies heat to my 60 gal. DHW heater

So im wondering how they sized my boiler from the 39500 btu per hour radiant load.

Im also trying to size a wood gassification to the same system as well as 1500 sq' shop.

Thanx in advance for any help.

Huff.

I hate it when that happens.............and it happens a lot. I would have tried to get you something in the 50-60,000 btu range tops. If your heat loss is from an actual calculation even the 39.5 number is probably high because Manual J calcs do not consider or factor in heat gains from within the building like appliances. lighting and cooking. With a heat loss that low running your oven for an hour will probably shut down your heating system.
Lot's of contractors think that if they have a water heater connected to the boiler they need to drive it with enough btu's for max recovery rate. Not true. They will work just fine at lower btu inputs. Consider that a standard gas water heater has a 35-40,000 btu burner underneath it.

Now as to efficiency of your gas boiler, it will be reduced because of short cycling. While it's true that gas is instant on instant off peak efficiency is obtained at a continuous burn condition. This is why modulating/condensing boilers show efficiency improvements far beyond the difference in AFUE rating when compared to a standard on/off boiler.

The answer to your wood sizing is to use storage of some type and size the boiler to the that, not the heat load. In that scenario the boiler is "disconnected" from the actual load and you can operate it however you need to establish a clean and efficient burn plan.

 

[huffdawg]

Thanx heaterman and ripe and nofossil.

I gave the contracter that built my boiler board and designed the sytem my house plans with the r rating of the insulation and windows . I think he used this program to calculate the numbers.( Uponor Advanced Design Suite - 6.6.1)

I copied and pasted the sheet . Its kind of haywire but the numbers are on the bottom of this post.
It seems according to heaterman he overshot the boiler size by quite a margin.
This might explain why im using so much propane.
I burned 257 litres of lpg in 19 days and the temps here are averaging about 5 degrees c.
granted we are a familly of five and use a lot of hotwater " two teenage girls use a lot of hot water"
I dont know if he included my attached garage in his calculations it 550 square feet with 11 ' ceiling and a 8" radiant cement floor with styrofoam underneath , but has not been on yet


Cheers Huff.


YOU WII NEED ONE 6LOOP MANIFOLD AND ONE 2LOOP MANIFOLD
I WOULD PUT THE 6 LOOP UNDER THE STAIRS AND THE 2 LOOP IN THE WALK IN CLOSET.
YOU WILL NEED 1 THERMOSTAT IN MASTER, ENSUITE,DINING WALL BY THE KITCHEN END.
THE UPPER FLOOR 1IN THE BATH AND ONE IN THE HALL ALSO STORAGE.

BASEBOARD FOR UPPER FLOOR
STORAGE 1- 6FT.
FAMILY 3- 6FT
BED4 1- 6FT
BED3 1- 6FT
BED2 1- 6FT

I WOULD DO THE BASEBOARD IN 5/8 KITEX PIPE
ALSO PUT 2 8FT PIECES IN CRAWL

Project #:
Project Name:
Design Data Location:
Outdoor Temperature:
Wind Speed:
Total Area:
Heated Area:
Construction Quality:
RFH Glycol Level:
Design Temp. Drop:
Radiant Tubing Volume:
Volume Water:
Volume Glycol:
250-287-1359
DALE
Vancouver, British Columbia
24 °F
0 mph
1,236 ft²
1,236 ft²
Best
100% Water
10 °F
15.8 gallons(US)
15.8 gallons(US)
0 gallons(US)
Total Flowrate:
Maximum Head Loss:
Total Loops:
Total Manifolds:
Total Zones:
Min. Tubing Required:
Total Load:
Total Radiant Load:
Total Supplemental Load:
4.9 USGPM
10.1 ft(H2O)
10
34
1727 ft
19,525 Btu/hr
19,525 Btu/hr
0 Btu/hr
Project #: 250-287-1359
Date: Jun 01, 2008
Radiant Design Summary -
Manifold View

 

[Nofossil]

In all fairness, it's worth mentioning that heating system contractors probably can't be expected to do the depth of analysis that it would take to optimize the system to the extent that we fanatics might desire.

With a fossil fuel system, there's not an enormous efficiency penalty for having an oversized boiler, and most people expect a fast recovery rate if the house is cold. A boiler (and baseboards) sized to just exceed the peak load will provide VERY slow recovery if you're coming off of setback temperature. Would make for unhappy customer.

The whole idea of setback comes from the first energy crisis in the 70s, and the assumption is that most houses are poorly insulated and drafty, and that heating systems are oversized. If those assumptions are true, the setting back the thermostats at night and when the house is unoccupied makes good sense. The house temperature will drop quickly as soon as the thermostats are set back, reducing the heat loss. When the thermostats come off of setback, the temperatures will recover in a short amount of time and life will be good.

Newer homes don't always fit this model. The ratio of house thermal mass to heat loss is MUCH higher, so the house doesn't cool very fast. If it doesn't cool down, then setback doesn't reduce heat loss. If the heating system is well matched, then the heating system can't change the temperature of the house very quickly, either. In our case, the house will lose about a degree per hour on a cold day, and the heating system recovery rate is about the same. Setback just doesn't make sense.

 

[DBoon]

If I'm reading all the data right, your house size and BTU/hour usage is about the same as mine. I have a 1250 square foot house with a 22,500 BTU/hour need at 0 degrees F. I heat with cast iron radiators.

In any case, the oil boiler I have has a rating of 87,500 BTUs/hour max. It will cycle ON for about 6 hours a day on a really cold day. My BTU/hour estimate about matches expectations of seasonal oil usage when converted using seasonal degree days and boiler efficiency assumptions. So all the info seems to agree. I don't think ON to OFF ratio is a problem, but again, the cast iron radiators provide a huge degree of thermal mass.

Now that I have a wood stove, I'm not usually running my boiler except as a "pick-up" heater on a really cold day, or to even the heat out in the house. I've been looking at new oil boilers and it is tough to find one that is sized much smaller than what I have. Natural gas boilers are sized smaller, but I have no natural gas availability in my area, so this is not an option. So your contractor may be defaulting to what may be the smallest boiler available for your available fuel in a brand that he has used before.

 

[huffdawg]

If I'm reading all the data right, your house size and BTU/hour usage is about the same as mine. I have a 1250 square foot house with a 22,500 BTU/hour need at 0 degrees F. I heat with cast iron radiators.

In any case, the oil boiler I have has a rating of 87,500 BTUs/hour max. It will cycle ON for about 6 hours a day on a really cold day. My BTU/hour estimate about matches expectations of seasonal oil usage when converted using seasonal degree days and boiler efficiency assumptions. So all the info seems to agree. I don't think ON to OFF ratio is a problem, but again, the cast iron radiators provide a huge degree of thermal mass.

Now that I have a wood stove, I'm not usually running my boiler except as a "pick-up" heater on a really cold day, or to even the heat out in the house. I've been looking at new oil boilers and it is tough to find one that is sized much smaller than what I have. Natural gas boilers are sized smaller, but I have no natural gas availability in my area, so this is not an option. So your contractor may be defaulting to what may be the smallest boiler available for your available fuel in a brand that he has used before.

dboon my house is actually around 3000 sq. ft of heated space . 1200 down stairs 1200 upstairs plus a 550 sq.ft attached garage. which im not heating right now.
My boiler is a buderus gb142 . Maybe the efficiency isnt that bad. And its the price thats killing me 64 cents a litre.
I have always heated with a wood stove before I built this house . with elbow grease being the only cost.
I am waiting for some money from the sale of a small busines I own .
I am hoping to get gassifier and storage all setup and going for around 14k doing all the install myself.
Im just trying to get an idea of what im going to need and how Im going to do it .


Lots of great info here aswell as a lot of heat junkies with the know how.


If I would done some more research pryor to building my house I would of went with the wood boiler heat. But I guess the lp back up is a nice option too.

cheers Huff

 

[huffdawg]

In all fairness, it's worth mentioning that heating system contractors probably can't be expected to do the depth of analysis that it would take to optimize the system to the extent that we fanatics might desire.

With a fossil fuel system, there's not an enormous efficiency penalty for having an oversized boiler, and most people expect a fast recovery rate if the house is cold. A boiler (and baseboards) sized to just exceed the peak load will provide VERY slow recovery if you're coming off of setback temperature. Would make for unhappy customer.

The whole idea of setback comes from the first energy crisis in the 70s, and the assumption is that most houses are poorly insulated and drafty, and that heating systems are oversized. If those assumptions are true, the setting back the thermostats at night and when the house is unoccupied makes good sense. The house temperature will drop quickly as soon as the thermostats are set back, reducing the heat loss. When the thermostats come off of setback, the temperatures will recover in a short amount of time and life will be good.

Newer homes don't always fit this model. The ratio of house thermal mass to heat loss is MUCH higher, so the house doesn't cool very fast. If it doesn't cool down, then setback doesn't reduce heat loss. If the heating system is well matched, then the heating system can't change the temperature of the house very quickly, either. In our case, the house will lose about a degree per hour on a cold day, and the heating system recovery rate is about the same. Setback just doesn't make sense.

The other night the power went out and we lost 2 degrees f. in 5 hours with the outside temp. at about 1 degree c.
I leave my thermostats set @ 70 degrees f.
It will take my house around 6 hours to get back up to temp if starting from a cold house. I Think thats when consumes a lot of fuel .

Cheers Huff

 

[DBoon]

I misread your data - not easy to understand in that form.

If I had propane (lpg) heat and a spare flue, I would have put in a wood boiler a long time ago. Good luck to you - you sound like you are on the right track.

 

[huffdawg]

Ya I misread it aswell I thought it was 19,525 x2 for btu's but it was 19,525 total.
I dont know why the data pasted like that.

 

[huffdawg]

[.

The answer to your wood sizing is to use storage of some type and size the boiler to the that, not the heat load. In that scenario the boiler is "disconnected" from the actual load and you can operate it however you need to establish a clean and efficient burn plan.[/quote]


Heaterman . Im pretty new at this stuff, how would I go about sizing a wood boiler to a storage tk. for my place.

seems like 800 to 1000 gallon storage tk. is pretty common from the posts I have been reading here .

huff

 

[heaterman]

[.

The answer to your wood sizing is to use storage of some type and size the boiler to the that, not the heat load. In that scenario the boiler is "disconnected" from the actual load and you can operate it however you need to establish a clean and efficient burn plan.

Heaterman . Im pretty new at this stuff, how would I go about sizing a wood boiler to a storage tk. for my place.

seems like 800 to 1000 gallon storage tk. is pretty common from the posts I have been reading here .

huff[/quote]

Storage size should factor in two main things. The firing rate of the boiler and how long you want to work from your storage before firing again. There are of course a host of other factors that may determine or limit the size of your storage such as physical size constraints for example.

Basically, if you have a load of roughly 20,000 btus, (which is a beautiful thing) to run for 24 hours at that heat loss you would need 480,000 btu's of capacity in your tank. The actual gallons will be determined by how high you can drive it and how low the system will go while still providing that output.

Remember that btu's = weight of the water in pounds x temp drop. If you have 500 gallons for example and can drive it to 180, then use heat from it until refiring at 120 you have a 60* drop. 500 gallons x 8.33 (weight of 1 gl of water) = (4,165 pounds x 60 (temp drop) = 249,900 btu's . In your case 500 gallons would carry your load for 12 hours with no fire in the boiler.

 

[Leonard]

I am in the same boat, condensing boiler LP fired, 80 gallon dual coil water tank, fireplace insert and am looking at wood boilers, my plan was to add solar for the DHW first but I have seen a few nice deals on used Buderus wood boilers. I too am looking to size the wood boiler where did you come across the calc's and what not?

 

[heaterman]

I am in the same boat, condensing boiler LP fired, 80 gallon dual coil water tank, fireplace insert and am looking at wood boilers, my plan was to add solar for the DHW first but I have seen a few nice deals on used Buderus wood boilers. I too am looking to size the wood boiler where did you come across the calc's and what not?

I have been accused of having water on the brain so basically that stuff is just floating around in there.

A Buderus will do a nice job for you with storage. It's a decent non-gasification unit. Adding storage to one will help immensely with keeping it from creosoting into a large black blob of tar.

The basic calc for btu's into storage is just one of the laws of hot water heat that anyone in the biz should know. If you're talking about a heat loss for your house, that has to be done on an individual basis. The only way I could do it for you is with a set of blueprints of your home. And that's also the best place to start. A heat loss calculation will tell you everything from how much storage you need to the sizing of pipe, tubing and circulators. A rough rule of thumb as a starting point would be 30-35 btu's for every sq ft of floor space. That's assuming design conditions in your area of -15* and a normal older house. Please note that I said ROUGH.
Sounds like you have a few options as to how a person could integrate a wood boiler into your system.