# heating water...heat transfer



## MELOC (Sep 26, 2005)

i would like to know how to calculate how fast water heats in a boiler with a coil and like that. for example, if i am heating hot water for direct usage and have no storage, is it feasible to heat water as you use it using another hot water source and heat transfer?

my wood/oil furnace produces hot water for radiator heating and it has a coil for potable hot water that i use to pre-heat water for the electric water heater. how hot would the water jacket on the boiler need to be to heat water to a usable hot water temperature? 

i have seen ways to heat water but the usage seems to require special tanks or added tanks etc. i thought of doing something using coils of copper tubing and a cooler with water to transfer the heat. i would make a solar or compost water heating coil and connect it to a coil in a cooler of water. another potable water coil would be "married" in the spiral of the other coil and sharing the water. of course i would cover the cooler with it's insulated lid. any thoughts?


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## rambler (Jan 20, 2004)

Think one needs to know what one is doing - need pressure relief, expansion air. Or you have a pretty good steam bomb if things don't curculate the way you thought they would.

Be careful.

--->Paul


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## MELOC (Sep 26, 2005)

i am being a bit vague. that is because i am not sure what i want to do. i think i am asking for two things...a way to calculate how much water i could heat as it flowed through my furnace heating coil and simply what people think of the idea of using solar and or compost pile hot water as i described.

the boiler is normally set at 145 to 165. i have an overload that kicks on the heating circuit pump to the radiators if the water exceeds a set high temperature of 185. that is a safety feature for when i burn wood. if the temp drops below 145, the oil burner kicks on. i would like to run the temp a little hotter but i like to keep the high temp overload set to no higher than 185 and i need a 20 degree difference i think.

i could tap into the water jacket of the boiler and use the non-potable water in conjunction with other hot water sources. i fear that in the winter, i would be heating my compost pile and any solar collector i build unless i regulate the flow with thermostats and valves. in the summer, i thought of using the furnace as my heat reservoir for the alternative hot water and just using the coil to heat water as i need it. i fear that 140 water will not heat the coil enough to provide usable hot water. i would also have to shut off the radiators or i would be heating the house.

so i thought of doing what i described in my original post. when i researched compost water heating, i saw no temps above 160 degrees. 140 is more like it. i have no idea of what solar collectors are capable of. i do not think i would be heating any water hot enough to have expansion problems...but i could be wrong.


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## SolarGary (Sep 8, 2005)

Hi,

I'm not sure I understand exactly what you want to do, but here are a couple thoughts:

The best way (I think) to find out how much heat you can get on a single pass through your furnace boiler coil would be to test it. Get your boiler up to temp, then run water through the coil at a measured flow rate, and see what you get. You can easily measure the flow rate by just running the water into a 5 gallon bucket and timing how long it takes to fill. Make sure you run it long enough to get a steady output temperature.
There are heat exchanger formulas to calculate this, but you have to know quite a bit about the coil and the tank its in to use them, and in the end they are not going to give as good an answer as just doing the test.

For heating water with a conventional solar collector, you really need a storage tank to hold the heated water. Water taking a single pass through a solar collector at any kind of useable flow rate will not heat up enough to be useful. The collector works by collecting heat over the full day, and storing the heated water in a tank. The batch type solar water heaters are simple, and combine the storage tank and collector into one unit:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#Batch

The little bit I have read on the web about compost pile heat is that it takes a large and healthy compost pile to generate usable heat, but that it can work.

Not sure if that helps at all?

Gary


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## MELOC (Sep 26, 2005)

i know i am being vague. i like hearing people's ideas. i am just trying to do as much as i can with as little as possible. if i had another tank to use for storage i would dedicate it to alternative hot water. 

i need a way to utilize all ways of heating water. i am big on being redundant. in really cold weather, the compost idea will not work very well. on cloudy days, the solar collector will not work well. if i run out of firewood again like i did this year then the boiler may be out as it is not efficient with oil alone (it has an open firebox).

to use all ideas, i would need many control devices to keep peter from robbing paul so to speak.


i would like to view some tanks that are made for this sort thing. i saw someone refer to them as "heat wells" ?? anyone have some links?


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## greg273 (Aug 5, 2003)

MELOC,

I think I'm trying to do just about the same thing you are, except with a wood-fired furnace/solar combination.
I just posted regarding expansion tank issues, not sure about that one. I'm going to be using check valves to keep the seperate systems self-circulating (thermosiphoning, another wonderful form of 'free energy') when they are in use. And I will be draining down the roof-mounted solar collectors in the winter, to simplify the system requirements.
I just hooked up my rainwater collection system, and we got our first thousand gallon-plus tankfull of rainwater yesterday! Filled up the tank and overflowed into the creek, just as planned. The overflowing really helped clear the water, all the minute floaties from the very first measurable rainwater harvest were washed out of the overflow! I am using a diverter valve and a mesh screen, but still some small particles got past that.
I will let you know if i come across some useful information.

greg


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## raymilosh (Jan 12, 2005)

Meloc,
I have some thoughts...if you put a "coil" in the waterjacket of your boiler, I am thinking the longer the path, the better. I have considered using several of the "radiator" looking devices out of dead air conditioners. They can hold lots of pressure, and can be hooked in series or parallel using copper tubing and epoxy. Of course, getting them into the waterjacket and geting it sealed back up again is the hard part.
In your case, I'd suggest running a pipe out of a dead water heater tank through the firebox and back into the tank. If the tank is mounted above the firebox, it will thermosiphon. If you're really interested in specifics, ask me and I'll go into details. Then plumb a cold water into that same tank and a hot line out of it to the fixtures in the house. Whenever the firebox is hot, it'll be heating the water in the tank. 
I'm thinking that heating the water in the boiler's waterjacket with solar or compost would add a lot of challenges and cost. Instead, I would recommend using solar or compost to heat the dead water heater tank using an external heat exchanger. If you mount that tank rather high, it may be possible that heat generated by solar or compost could thermosiphon up to it, too. This website has a simple heat exchanger that hooks to a standard dead water heater tank for about $300. http://www.solarhero.com/HXdetail.htm.

For any of this work, please do your homework carefully to avoid steam bombing your house. Get Lehman's book on "Heating water with Wood". It has lots of safety and practical stuff. It also has stories about people sending waterheaters through the roof and flipping over cast iron woodstoves with steam explosions. 

One last idea...If you decide you need several separate systems for heating water, run all the hot water lines to somewhere that is easily accessible in the house and expose them all and install a valve on each one and then plumb them together and send the one pipe to the faucets. You can then manually open the one valve that has the hottest water. You can even run it through an instant water heater so it boosts the temp on the days that none of the preheated water is as hot as you want it.


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## mink (Feb 10, 2005)

how about ising the same heat exchangers they use on the outside wood furnaces? mine sits on top of the hot water tank and the house water runs threw it once then the tank just stores it. i can leave the hot water running all day and never run out of hot water as long as the fires going............mink


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## Ross (May 9, 2002)

So its a wood/oil boiler not a furnace as such. Your low limit is 145 (which is pretty normal for a boiler) and the high limit is likely set for the oil at 180, and at 185 you have a safety switch that cranks up the household radiator system to try and cool the boiler if the wood gets it too hot. Your T+P releif valve is set to blow off at 200 degrees and anything above 15 PSI if its a normal boiler releif valve and it likely is. Your electric water heater has a similar T+P releif valve set to blow at a higher pressure. What you want to do is replace the boiler with a new source of heated water from solar or compost. Currently your potable coil in the boiler is set to work at 180 degrees so if you plan to tap into the boilers black water or glycol system and heat that outside the boiler that's what that coil needs. Now I need to know how potable water is heated in your boiler, either just on the inlet water or is it circulated to hold temp? If the coil is just taking the water heater's inlet water and heating it on demand leaving the electric to hold the temp, consider adding a circ pump to keep the electric completely off the water heater. If the oil burner and wood are not going to be used in summer then you can slow the potable water circ pumps GPM to slow down the flow through the boiler's potable water coil and give the water more time to pick up heat. If it's on the inlet water pipe only (no potable water pump) you'll either need a larger seperate heat exchanger coil or an auxiliary one with the boiler's black water or glycol. Still at 140 it's probably still going to kick on the electric water heater elements. Personally I'd leave the oil burner on (if the potable water is circulated through the coil not just, as used through the inlet) and add a seperate heat exchanger on the water heater inlet water side for the outside alt energy system to preheat the potable water.


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## joken (Dec 25, 2005)

MELOC, Assuming that I understand correctly. Your cold water supply to the elect htr goes thru a coil in the boiler and is pre heated when the boiler is on? Here is what I would do. Configure the piping so that the inlet of the boiler heat exchanger is from the bottom of the water heater. Tee at drain valve. The discharge of the boiler heat exchanger is piped to a tee at the top of the elect wtr hrt hot water outlet. Install a small Grundfos type pump that pumps from the tank bottom to the heat exchanger. The cold water supply needs to go back to the tank cw connection. As far as pump control goes there are many ways to do it. Probably the easiest is boiler on-pump on. It would save a few bucks to turn the pump off when the tank water is hot enough so I suppose you could put a surface mounted aquastat on the tank in series with the pump. Tank hot pump off. Someone tell me if I am off target. Ken in Oregon


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## Ross (May 9, 2002)

Sounds better when you say it Ken. Configured with an aquastat on the circ pump the oil fired boiler could do 100% of the water heating cheaper than electric if there's moderate to heavy use (4 people +) 3 or less people and electric is often cheaper. Adding on an alt energy heat source is the challenge so if it did the preheating of new water and the oil did the holding with a pump so the electric was eliminated, there should be substantial savings IF they use a lot of hot water. Shutting the boiler off and just plumping into it as a collector/ heat transfer through its coil is OK but going to be less efficient as it would work best at 180degrees


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## joken (Dec 25, 2005)

Ross, I read yours after my post. HONEST


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## Ross (May 9, 2002)

Never doubted it for a second!


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## MELOC (Sep 26, 2005)

lol, thanks guys.


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