lrrowe wrote:Socal Tom wrote:lrrowe wrote:Same here. My pump is lower then the hot water tank exit. No priming needed. I used it this week for a 4 day outing at VA Beach with a TearJerker event and it worked perfectly. I turned the heat down to 64 for the night and all was great.
My next modification will be to put a rheostat on the fan to slow it down and maybe one on the pump so the hot water will flow through the heater more slowly thus maybe reducing the propane used.
Also my air flow has slowed down. Seems like I broke one of the fan blades last fall and had to purposely break off one on the opposing side to balance things out. That for sure reduced the air flow.
The best way to slow the water flow would be a needle valve upstream of the pump. This will also reduce the power used by the pump. On the other hand a rheostat will probably increase the power used.
FWIW, slowing the water flow is unlikely going to reduce the propane load. The propane is being used because you have taken BTUs out of the water, and the amount of BTUs needed won't change based on the flow rate of the pump. However, if you lower the speed of the pump, then you might reduce the rate at which the BTUs are removed ( the BTU transfer is most effeient when the air temp and water temp are farthest apart) and potentially use more propane, since you would spend more time sending cold water into the tank, and causing the thermostat to stay engaged longer or more often.
Tom
Interesting. I think you are saying that if I run the pump slower, I could remove more of the water's heat and thus send colder water back to the heater. If so, then more propane would be used. If I have this right, the cost of DC current used is far less then propane. Maybe you just talked me into not slowing the pump down.
No, but yes
Here is what i "think" would happen, I'm going to make up numbers because it will be easier to explain so bear with me.
Let say that if 1 gallon of water drops 1 degree that is 1 Tom Thermal unit ( like a BTU but with gray hair).
So it it takes 300 TTU/hr to warm your cabin, then either you need 300 gallon/ degrees to keep it warm.
if you have 120 degree water going into the HC ( Heater core) at 2 gallons per minute and it drops 50 degrees then you transfer 100 TTU per minute so you have to run the pump for 3 minutes per hour to warm the cabin. ( This assumes that the water heater can produce a consistent 120 degree water)
Now if you cut the water flow in half, then you pump 1 gallon per minute, but you only get an 80 degree drop in the water so now it takes closer to 4 minutes to warm the cabin. ( and I assume now that the water heater has to run for 4 minutes instead of 3).
Now if the water heater has to run 15 minutes for every 3 minutes the pump runs, then you are still better off with it at full speed because the greater the difference between the water and the air, the more efficient the heat transfer ( This is why cars don't over heat when its freezing outside, but they do when its a 100 degree day)