I am reading Dan's book and have a question.

Mike T., Swampeast MO

You can't use 165 degrees as a basis. Why? Because that's the amount of heat above 0 degrees. The water could NEVER cool to 0 degrees!

If you want to know the <I>absolute</I> amount of energy contained in the boiler you have to use the Rankine scale which is based on <I>absolute zero</I>. For 165F it's about 625 Rankine! Again, obviously you could NEVER recover this much energy!

To know how much energy is available in the boiler you have to know how fast that energy will be used. Such is a function of the flow rate and the ability of the emitters to liberate heat.

While this is WAY over-simplified and will probably never be exactly true, assume that 20 degrees of the supply temp will be used.

So instead of multiplying by 165, multiply by 20 and you'll get a VERY rough estimate of how many btus are available in the boiler itself.

ralman

In Dan's book

I read that based on a 20 degree temperature drop, 1 gpm of flow will transport 10,000 btu.

Does each gallon of water stored in my boiler contain 10,000 BTU's?

Mike T., Swampeast MO

Absolutely not.

That 10,000 btu number is based on an hour. In other words that's 10,000 btus in one hour with 1 gallon per minute of flow and a 20°F drop in temperature.

Most btu ratings for boilers and emitters are based on btu/hr but the "hr" part is often omitted.

Heat transfer is based on flow rate and temperature drop (or delta-t). "Delta" means "change in".

A very simple formula allows you to estimate when you know two of the three factors:

Btuh = 500 * gpm * DT

gpm = gallons per minute

DT = delta-t

The 500 constant is based on the weight of water. Weight of water changes slightly with temperature, but for heating system purposes 500 is quite accurate.

One other thing: a BTU (British Thermal Unit) is the amount of heat required to raise one pound of water one degree Fahrenheit. It's frequently stated that one wooden match provides about one BTU of heat.

Empire_2

NO

1 GPM is the required flow rate to convey that 10,000 BTU as it moves thru the boiler. Dont forget that 10,000 is related as BTU/HR. If you ran your pump for 1 hour @ 1gpm it would transfer 10,000 BTU's in that time, at your designed 20* "Delta T". Effectivly you would have moved 60 gal. of water in and out of the boiler to move the 10,000BTU value.

ralman

Thanks!

I was trying to figure out how many BTU's are stored in my boiler between firing cycles.

I don't know how to figure my flow rate and DT.

JimGPE_19

One gallon of water is 8.333 pounds, so you have 8.333 BTU of heat stored per degree F (Not 8.33 BTU/Hr, just BTU's).

The question is how many BTU's compared to what? Room temperature? Then you have gallons x 8.333 x (water temp - room temp).

We're having trouble getting our hands around your question because the BTUs in your boiler are not real useful unless you are moving the water around.

If you want to know how much useful heat you have if you were to shut down the burner and keep circulating the water, you have the above BTU's. The problem is, its going to take you a LONG time to wring out the last few BTU's because the temperature difference between the water and the room temperature (the driving force to move heat) gets smaller and smaller as the water cools down.

JimGPE_19

Don't confuse BTUH with Btu's. We get lazy sometimes and refer to heat (Btu's) when we mean heat RATE (Btu/hr), but when you are talking about GPM (gallons per minute) you are talking about a heat transfer RATE.

Its like confusing miles per hour and miles - one is a rate, the other is a distance. BTU/Hr is a rate, and BTU's is a distance!

ralman

What I am trying to understand is this.

I have 17 gallons of water stored in my boiler. Based on my incorrect interpretation - 17 gallons of water X 10,000 BTU = 170,000 BTU. Every time my thermostat calls for heat 170,000 BTU circulated through the system whether I needed this much or not. My calculated heat loss is 70,000 BTU/hr at 5 degrees. Heat loss is less as outside temp goes up. I get the same 3 minute burner cycle at 25 degree outside temp or 35 outside temp. It seemed wasteful to me. I am basically trying to figure why the fuel bill is so high. Since my understanding was totally wrong, Maybe I am not wasting fuel.

17 gallons x 8.33 rounded to 140 for discussion = 140 pounds of water raised 25 degrees in the boiler controlled by aquastat is 140 x 25 = 3500 BTU contained in the 17 gallons of water stored in the boiler.

Is this correct?

ralman

I was checking over this. I think I had it wrong again.

More like 17 gallons X 8.333 = 141.661 x 165 degree boiler temp = 23,374 BTU stored in boiler.

ralman

I have been advised to add a reverse indirect buffer storage

as a way to save money on fuel bill and I am trying to figure out the effect on the fuel bill before I make changes.

My first thought is more water to heat up takes more fuel.

With 23,000 btu stored in boiler and 55,000 btu stored in a 40 gallon storage tank, it looks like I would have less burner cycles per hour (I don't know how to do the math).

But, I don't know how long it will take to heat the extra 40 gallons from the tank.

I get five 3 minute burner cycles per hour for a total of 15 minute run time with current setup.

If I add the storage tank and extra 40 gallons, And it takes one 20 minute burner cycle per hour there is no fuel savings for me.

Mike E_2

You are correct in that with a buffer tank you'll get fewer but longer burn times. This will benefit fuel econony because boilers are the most efficient during steady state running.

If your boiler is set up with pre and post purges, you're wasting alot of heat running 5 cycles per hour. Each pre or post purge turns the boiler into a radiator that is blowing heat straight up the chimney.

Also, with five 3 minute cycles per hour, your boiler is probably oversized. What is the outside temperature when you get five 3 minute cycles? Replacing with a new, correctly sized boiler, or better yet a modulating boiler would save quite a bit of money.

Is your boiler located in a heated area, or in an unheated area? If it is in a heated area, you're not really loosing that much heat when the water cools down since the "lost" heat is going into the room. The only real losses are during purges, and how much draft you have through the boiler during idle times.

Mike T., Swampeast MO

That works out to about 2,800 btus.

Say your emitters can deliver 40,000 btus per hour. 40,000 divided by 60 minutes = 667 btus per minute. That means the boiler stores about 4.19 minutes worth of heat IF the heat is leaving at a rate of 40,000 btu/hr AND IF the temperature drop was 20°F. Those are BIG ifs, but as you can probably see, the boiler itself does not store much heat that's usable to the system.

Mike T., Swampeast MO

Hope I haven't and don't confuse you too much here...

But a boiler is a heat PRODUCTION device--not a storage device.

While your boiler may have been at 147° when it stopped firing, it IMMEDIATELY begins cooling--and guess what--most of the heat goes right up the flue! The rest is given off to the room via the boiler jacket. By the time the boiler fires again, the temp is likely at or below the return temperature of the system. So...any heat you think you might have "stored" has actually been lost and must be replaced in the next firing cycle.

When your boiler fires, it first has to bring the heat exchanger up to temperature. Once that happens it begins heating the water. After that, the emitters come into play. If they cannot liberate heat as fast as is being supplied to them, the temperature of the water rises.

If the boiler is providing btus MUCH faster than the emitters can give it off, the temp rises quite rapidly and the boiler shuts down on some limit control. The faster the succession of these events, the less efficient the system because you're getting LOTS of cycle loss.

This is most likely why a buffer tank was recommended to you. The goal here is to actually STORE heat away from the boiler to be used by the emitters over time. Provided the buffer is large enough, you'll get better system efficiency. Depending on the nature of the system and the degree of oversizing, buffer tanks can get VERY big to be truly effective!

An alternate is a modulating boiler. Such boilers vary the level of their output and do their best to match output with the load at each moment of time. While such perfection is impossible, if you also use proportional control on your emitters, you can get pretty damned close.

JimGPE_19

No!

Okay, your heat capacity being delivered to your home during a heating cycle is the less of either your burner's capacity to burn gas per hour or the ability of your radiators to give off BTU's per hour at the temperature water being supplied. The boiler could be a million horsepower, but if it is connected to 78000 BTU/hr of radiation, you get 78,000 BTU/hr to your home. If your entire home is crammed with radiators and your burner can only produce 78000 BTU/hr heat, you tet 78000 BTU/hr.

The amount of heat being added to your home per hour has NOTHING to do with how much water you have in your boiler. Nothing at all.

If you really want to know how much heat per hour is being added to your home during a heating cycle, shut off all other gas-consuming devices (hot water heater, stove, etc.) and go sit by your gas meter with a stopwatch. Check the gas meter reading before the cycle, check it after the cycle, record the amount of time the cycle took, then multiply the cubic feet of gas by a thousand (close enough), divide it by the number of minutes the cycle took, multiply that by 60 and you have Btu/hr.

If you really want to know how to make your heating system more efficient and cut down on your heating bills, due respect that is the question you should ask - that we can help you with!

Sounds like your boiler is short cycling. This is generally bad for efficiency. Either your boiler is very much oversized, or you are not pumping enough water for the size boiler. Either way it sounds like you are cycling the burner on the water high temperature limit.

But that's just a guess.

Suggest you start a new thread with all the data you know - boiler size, amount of radiation, home heat loss, burner cycle time, etc. Then add what you feel might be happening and comment on why you think your system is inefficient, and sit back and watch the fun!

Ron Schroeder

Hi Ross,

With 23,000 Btu stored in the boiler and 55,000 Btu stored in the buffer tank, almost 1/3 of those Btu's (the 23,000 in the boiler) can be lost up the chimney possably before the next call for heat. The buffer tank will retain it's energy much longer and also will lose it into the basement rather than to the outside. That is why a bigger buffer and/or a lower mass boiler uses less fuel.

Ron