Woodfire pool heating solution

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BARBERTONHACK

0
Jun 7, 2016
8
South Africa
Hi all

After reading tons of why not, I have the following request. I am attempting to use a woodfire solution to heat a school pool for no other reason but cost of electrical heaters and electricity being just above school budget.

The rectangular pool size is:
Length : 25m or 82 feet
Width: 10m or 33 feet
Volume: 345000 liters or 91 140 gallons

We have an old woodfire stove from days gone by. I have no idea as to the BTU'S. As a start, we have pool blankets covering whole pool.

Preliminary results at flow rate of 1200 liters per hour or 317 gallons per hour
Current pool temperature: 18°C or 64.4°F
Water Temp after heating out of 1" HDPE CLASS 6 pipe: (Evening) 32°C or 89.6°F
Water Temp after heating out of 1" HDPE CLASS 6 pipe: (Daytime) 45°C or 113°F

Preliminary results at flow rate of 2400 liters per hour or 634 gallons per hour
Current pool temperature: 18°C or 64.4°F
Water Temp after heating out of 1" HDPE CLASS 6 pipe: (Evening) 24°C or 75.2°F
Water Temp after heating out of 1" HDPE CLASS 6 pipe: (Daytime) 30°C or 86°F

My question is how can I improve the time it will take to heat the pool to 28°C or 82°F maximum.

Should I increase the flow to 1902 gallons per hour ( I realise there will be a temperature drop)

Any assistance will be appreciated.


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Heating water is all about how many BTU's you can transfer to the water. for example: if you have a pool that has 100,000 Lbs of water( 12,050 gallons) it will take 100,000 BTU's to raise the temp 1 deg. At this point it is not time dependent. Heaters are usually rated in BTU's input per hour. This is a measure of energy consumed by the heater to transfer to the water as heat.

Since heat transfer is not 100% efficient in conventional heaters the output is lower than the input to the heater. So, for the example above of a pool that has 100,000 lbs of water, it would take a heater with an output of 100,000 BTU's to raise the temp of the pool 1 deg per hour. In order to raise it faster, the BTU output will need to be raised. for example, to get 2 deg per hour of temp rise the heater would need to have an output of 200,000 BTU's. Of course this is all theoretical as it does not account for heat loss due to evaporation.


I order for your wood fire to raise the temp faster, you either need to increase the available BTU transfer rate (bigger heat exchanger) or find a way to increase the available BTU load faster such as a more efficient heat exchanger or bigger fire.
 
You have a very large pool and it will take a long time to heat it. Keep that fire burning and the pump running, keeping the pool covered will help too. As long as you are getting 1-2 degree temp rise out of your heat exchanger you are doing good on flow rate. Based on your numbers you will need to input 750,000 btus into your pool just to get 1 degree of temp increase.
 
Mr Jadamec is correct in his figure of 750000 BTU'S to get 1°F in actual fact my calculation gets 760 140 BTU'S . Thus about 7 to 8 hours to lift the current temperature by 1°F excluding heat loss from the pipe etc. My question remains will a higher flow rate assist in speeding up the heating from a distribution point of view into the pool. I have to assume it would. What gets me is when measuring the temperature on the chimney the metal of the chimney measures in excess of 200°C or 392°F. It begs for a better heat exchanger unit. I am looking at using a 3 gallon stainless steel unit, utilising the chimney as part of this secondary unit. I do think I would need to maintain the same high flow rate through both units. Any suggestions?

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In a set up like this flow as much water as you can as long as you are getting some temperature rise on the outlet of the heater like 1-3 degrees. The cooler you keep the water in the heat exchanger the faster you will remove heat from the fire. You can use the chimney to grab more heat for sure. Just remember to keep the chimney clean as you will get a lot of ash and soot build up if you take all the heat out of the exhaust gas.
 
Since we know the weight of the water mass (756,462 Lbs) we should be able to calculate the BTU output of the heater (roughly). All we need are two more factors to use in the equation. We need a specific amount of time. This can be any time span but, more in this case is better. 5 hours would be a good starting point. The last thing we need is the temperature differential from the start of the time to the end.

Lets assume for example we have a 10 deg F temp rise over the 5 hour time. This will equate to a 2 deg F temp rise/hr. Knowing that the weight of the water mass is 756,462 lbs and it takes 1 BTU to raise a lb of water 1 deg we can extrapolate the BTU output of the heater in this case to be 1,512,924 BTU/hr.

However, a lot of factors come into play here as to an accurate BTU rating as water flow, ambient air temp, etc can affect the temp transfer.


In the case of your pool and the weight of water, I doubt you will be able to raise the temp more than 1/2 -1 deg/hr. 750,000 BTU is a lot of energy to transfer and even the largest residential heaters are limited to roughly half of that BTU size. The other problem is you can not maintain a constant temperature with a wood fire. As I mentioned in an earlier post, the best method to do this faster is a bigger heat exchanger and a bigger fire. as was mentioned, move the water as fast as you can thru the exchanger. Adding a chimney heat collector might work. however, keep in mind that for a fire to burn properly you need a proper draft. If you cool the chimney too much, the draft will slow down and smother the fire.
 
Thank you very much all for your assistance. I was actually thinking of doing the following. Take cylinder with chimney on the inside running straight through. Water in at the bottom from the main unit already partly heated out at the top. Seal the cylinder at the top with the chimney keeping it standard flow. It will therefor use the metal of the chimney to heat the water further. Additionally I was thinking of drilling a few holes through the chimney and soldering copper pipes through the chimney (in one side out the other)which should conduct more heat into the water.any water moving through the pipes will heat more. This should have a very limited effect on the flow of air and even airflow at 300 degree plus will not transfer more than 120°F due to the cooling effect of the flowing cooler water. Any suggestions on this idea?

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That idea works but reverse the flow. Have the water enter the chimney heat exchanger first then go into the firebox heat exchanger. This way you keep the temperature differences the highest and get the best heat transfer. Using copper pipe and solder is a bad idea in the chimney.It could get hot enough to melt the solder out of the joint if there is no water in the pipe.
 
OK now I'm confused. As far as my knowledge goes (limited as you may have gathered by the posts :-( ) hot water rises. That means I will have to force hot water down when the natural tendency of hot water is to rise. I understand the principle of keeping the temperature difference to a minimum to improve heat absorbtion. ( That sounds terrible but hopefully you get my layman's translation.

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I know you didn't ask but the chances of it working are around nil. You will not have enough btu's out of that stove to keep up.

My stove was rated at 450,000 btu's and it COULD NOT heat my pool.
 
What size was your pool sir? Please bear in mind that I am running a solar system in conjunction with the wood fire. We are fortunate with high winter temperature in the daytime; roughly averaged at 25°C or 77°F. Although you did not supply further information I would assume you had solar covers on your pool and a strong flow rate. I acknowledge your comment though and will keep all informed regarding success or failure.

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