O C Diver
Guru
- Joined
- Dec 16, 2010
- Messages
- 13,197
- Location
- Fort Myers, Florida
- Vessel Name
- End Of The Line
- Vessel Make
- Trinka 10 Dinghy
Follow me into the weeds for a minute.
Let me throw some numbers and calculations out to help people understand using engine coolant to heat cabin air. The heaters I installed are rated at 28,000 BTUs. Most of us can relate to it takes so many BTUs of heat or air conditioning to make our boat comfortable. For instance, my 45' boat has 3 optimized 12,000 BTU reverse cycle heat and air conditioners. While they won't heat or cool the boat quickly, they can easily maintain temperature. So, I need atleast 36,000 BTUs from my heaters. More would be better to raise the temperature more quickly. So I have 3 28,000 BTU units that aren't putting out there rated capacity. So let's look at some numbers to understand what might cause that.
A BTU (British Thermal Unit) is a measurement of of heat energy. It takes 1 BTU to raise one pound of water 1 degree Fahrenheit.
There are 8.33 pounds of water to a gallon. So it takes 8.33 BTUs to raise a gallon of water 1 degree.
When talking about heating air from engine coolant, we talk about extracting the heat from the coolant in a heater. The hotter the coolant is, the more heat that can be removed, to a point. If the engine coolant was 70 degrees and our saloon was 60 degrees, we're not going to be able to extract much heat by blowing 60 degree air over 70 degree coolant filled pipes. But if the coolant filled pipes are 170 degrees, we can get a lot of heat (maybe 10 times as much) by blowing 60 degree air over the pipes. For these types of heaters you need a substantial difference between the coolant and the air to transfer heat. For the sake of discussion, let's say heat output tapers off quickly below 120 degrees. So we want to understand how much heat can be extracted from coolant starting at 170 degrees and ending at 120 degrees. The difference is 50 degree.
Going back to there are 8.33 BTUs in a 1 degree difference in water temperature. If we have a gallon of water at 170 degrees and we extract the heat out of it over 1 hour until it reaches 120 degrees, we can say that we removed 416.5 BTUs of heat (8.33 × 50 = 416.5). So a heater using 1 GPH of water (170 degrees in, 120 degrees out) produces 416.5 BTUs of heat.
One GPH is pretty low. How about 1 GPM? Since there's 60 minutes in an hour, we could multiply the 1 GPH BTUs times 60 to to get BTUs at 1 GPM. 416.5 × 60 = 24,990 BTU. So a 1 GPM flow rate with a temperature drop of 50 degrees can generate approximately 25,000 BTUs.
These numbers look similar to my heating numbers. So if I want 3 heaters to put out 25,000 BTUs, I'll need 3 times as much coolant flow. You should be able to imagine if the starting coolant were hotter I could use less GPM. If I could draw the return temperature down further, I would need less GPM.
With 3 heaters in series, you should expect the first one to produce lots of heat with a large volume of hot coolant. The second should be about average as there is lots of flow but reduced temperature. The third one would be below average as the coolant temperature is substantially cooler.
There's more to figuring out how many heaters and how many GPH of flow you may need, but this gives you a starting point.
Ted
Let me throw some numbers and calculations out to help people understand using engine coolant to heat cabin air. The heaters I installed are rated at 28,000 BTUs. Most of us can relate to it takes so many BTUs of heat or air conditioning to make our boat comfortable. For instance, my 45' boat has 3 optimized 12,000 BTU reverse cycle heat and air conditioners. While they won't heat or cool the boat quickly, they can easily maintain temperature. So, I need atleast 36,000 BTUs from my heaters. More would be better to raise the temperature more quickly. So I have 3 28,000 BTU units that aren't putting out there rated capacity. So let's look at some numbers to understand what might cause that.
A BTU (British Thermal Unit) is a measurement of of heat energy. It takes 1 BTU to raise one pound of water 1 degree Fahrenheit.
There are 8.33 pounds of water to a gallon. So it takes 8.33 BTUs to raise a gallon of water 1 degree.
When talking about heating air from engine coolant, we talk about extracting the heat from the coolant in a heater. The hotter the coolant is, the more heat that can be removed, to a point. If the engine coolant was 70 degrees and our saloon was 60 degrees, we're not going to be able to extract much heat by blowing 60 degree air over 70 degree coolant filled pipes. But if the coolant filled pipes are 170 degrees, we can get a lot of heat (maybe 10 times as much) by blowing 60 degree air over the pipes. For these types of heaters you need a substantial difference between the coolant and the air to transfer heat. For the sake of discussion, let's say heat output tapers off quickly below 120 degrees. So we want to understand how much heat can be extracted from coolant starting at 170 degrees and ending at 120 degrees. The difference is 50 degree.
Going back to there are 8.33 BTUs in a 1 degree difference in water temperature. If we have a gallon of water at 170 degrees and we extract the heat out of it over 1 hour until it reaches 120 degrees, we can say that we removed 416.5 BTUs of heat (8.33 × 50 = 416.5). So a heater using 1 GPH of water (170 degrees in, 120 degrees out) produces 416.5 BTUs of heat.
One GPH is pretty low. How about 1 GPM? Since there's 60 minutes in an hour, we could multiply the 1 GPH BTUs times 60 to to get BTUs at 1 GPM. 416.5 × 60 = 24,990 BTU. So a 1 GPM flow rate with a temperature drop of 50 degrees can generate approximately 25,000 BTUs.
These numbers look similar to my heating numbers. So if I want 3 heaters to put out 25,000 BTUs, I'll need 3 times as much coolant flow. You should be able to imagine if the starting coolant were hotter I could use less GPM. If I could draw the return temperature down further, I would need less GPM.
With 3 heaters in series, you should expect the first one to produce lots of heat with a large volume of hot coolant. The second should be about average as there is lots of flow but reduced temperature. The third one would be below average as the coolant temperature is substantially cooler.
There's more to figuring out how many heaters and how many GPH of flow you may need, but this gives you a starting point.
Ted