Water heater - how hot is too hot?

[adlertom]

To keep it simple, you could use a mechanical thermostatic valve on your heat exchanger. Thermostatic Diverter or Mixing Valve this will give you the idea. I know this is a three way, you could use to bypass your heat exchanger, two way are avallable. Or go with a injection system,http://www.blueridgecompany.com/documents/Taco_RMB-1.pdf

Thanks, a two way version of this device sounds promising.

 

[adlertom]

How exactly is your flow configured? Is the boiler circuit pumped thru the heatex below the water tank, with the domestic water from the heatex thermosiponing thru the water heater tank?

I have a heat exchanger consisting of a 1/2 inch copper pipe inside a 1 inch copper pipe. It's about 5 feet long, positioned vertically, next to and at about the same level as the water heater tank.

The 1/2 inch interior pipe contains the domestic water. The surrounding 1 inch pipe contains the wood burner water. The domestic side is plumbed to the space heating connections located on the side of the water heater tank.

The domestic water in the heat exchanger pipe heats and rises, flows into the lower space heating connection of the water heater, rises inside the water heater, flows out the upper space heating connection, and back into the base of the heat exchanger, completing the loop. Thus, the water thermosiphons and rises in both in the tank AND the heat exchanger, creating a very effective flow.

 

[RonMar]

I have a heat exchanger consisting of a 1/2 inch copper pipe inside a 1 inch copper pipe. It's about 5 feet long, positioned vertically, next to and at about the same level as the water heater tank.

The 1/2 inch interior pipe contains the domestic water. The surrounding 1 inch pipe contains the wood burner water. The domestic side is plumbed to the space heating connections located on the side of the water heater tank.

The domestic water in the heat exchanger pipe heats and rises, flows into the lower space heating connection of the water heater, rises inside the water heater, flows out the upper space heating connection, and back into the base of the heat exchanger, completing the loop. Thus, the water thermosiphons and rises in both in the tank AND the heat exchanger, creating a very effective flow.

That is interesting, what you describe sounds like a figure 8 and I have never seen a thermosiphon that works like that. Part of the force that powers thermosiphon is the cooler/denser water in the tank pressing back into the bottom of the heatexchanger I was guessing something more like a typical "Side-Arm" heatexchanger. But if it transfers heat, it transfers heat.

I am doing something kind of similar using a heatexchanger on my backup generator to recover the waste heat to maintain my HW tank at 120F, then dissipate any additional heat after that. Since I must remove the heat from the generator, my needs are a little different than yours, but it is a similar problem. The primary loop on mine is thermosiphon up thru the heatexchanger and the secondary loop uses the pump to circulate the domestic water thru the heatex and back to the tank. I will have a fan coil unit on the output from the bottom of the HW tank to dissipate any excess heat once the tank is full of hot water. I will dissipate this heat into the house when I need heat, or outside when I don't. There is a 120F thermostat from a mercury outboard motor on the output of the heatexchanger to the tank that keeps the temp being supplied out of the heatex to the HW tank near 120F.

In your case, I think a solenoid/electrically operated on/off valve on the boiler input to the heatex would probably work well. This valve could be controlled by a thermal switch on the tank wall(where the normal HW tank temp control sensor is). When the tank gets cool, the thermal switch opens the valve and 180F water enters the heatex from the boiler, heats the secondary(domestic) water in the heatex and starts up the thermosiphon. When the tank gets hot, the thermal switch closes the valve and the thermosiphon stops. Basically an on/off heating control just like the one already on the hot water tank. This second circuit would be set at a temp above the normal HW heat control so the normal tank heater never comes on unless the boiler heat is unavailable in which case the HW tank would work like normal. Since you are using a tempering valve on the output, you only need to maintain the tank temp within a certain range just like the existing HW tank control system does. Add a second switch set just a little higher to activate a high temp alarm so you know the valve or switch is stuck open and you need to investigate.

 

[charlz]

Have you done any tests to see how much that heat exchanger helps? Usually they have many feet coiled inside a tube. I'd turn the water heater off, take a nice long shower and then see what the recovery time is.

 

[adlertom]

Have you done any tests to see how much that heat exchanger helps?

It works very well. Too well in fact, which is the reason for my post asking "how hot is too hot"

Usually they have many feet coiled inside a tube. I'd turn the water heater off, take a nice long shower and then see what the recovery time is.

My heat exchanger is not like that. It is simply a 1/2 inch copper pipe, about 5 feet long, inside a 1 inch copper pipe pipe.

 

[DavidVT]

This type of heat exchanger (tube in tube vertical next to HW tank) is very effective indeed.

I have not yet installed one but have been doing research regarding adding this to my outside wood boiler loop. My hot water is currently being generated by the oil boiler, it's only real task any more aside from being back up heat.

How do you control run away situations with your wood boiler? Most of us have become aware of the house too hot or our circulator groaning when the boiler has run away and exceeded the 180 set point. Mine can do it in very heavy winds or if the control damper gets hung up.

Some folks put in a dumping zone that can chew up the extra heat so as to protect the system components that should not see over 180.

If your water heater needs to stay under 180, you need to consider the run away situation and include dealing with it in your plans.

My friend was going to hook one of these up for a customer and was agonizing about how to control it. I believe he used an aquastat on the domestic hot water as it left the tank,near the feed to the bottom of the vertical heat exchanger. He was then going to use a normally closed valve off that aquastat to allow the boiler water to enter the heat exchanger as needed. Last we talked, he was still trying to avoid making extra zones on the boiler loop. I will try to seek him out and find out how he managed.

FYI: In my rental trailer the previous owner had a small plate heat exchanger that preheated the water entering the electric hot water heater. That had no thermal protection and always complained about the showers being way too hot.

 

[charlz]

It works very well. Too well in fact, which is the reason for my post asking "how hot is too hot"

My heat exchanger is not like that. It is simply a 1/2 inch copper pipe, about 5 feet long, inside a 1 inch copper pipe pipe.

Right my question was more about 'how fast'. I realize that if you let it sit the tank will eventually reach the same or close to temp of the boiler hot water loop. But how long does it take to bring up a cold tank? Can you turn your hot water heater off altogether or at least way down?

 

[RonMar]

Right my question was more about 'how fast'. I realize that if you let it sit the tank will eventually reach the same or close to temp of the boiler hot water loop. But how long does it take to bring up a cold tank? Can you turn your hot water heater off altogether or at least way down?

Well with a bit more information, you can calculate this pretty accurately, but making some assumptions, I can get a pretty good idea of performance. 1/2' copper pipe has a OD of .625", so 5' of it will have a surface area of 117.81 SQ/IN. A copper water to water heat exchanger has a heat transfer co-efficient of between 60 and 80. That is 60-80 Btu per SQ/FT per hour per degree F of temp difference. So with a heatex with .818 SQ/Ft of surface area, say enough 180F water to have very little drop on the primary side and a stone cold 50 gallon water tank(55F), the exchange would start out at around 8180 BTU/HR at the best case coefficient of 80(80 X .818 X 125F temp difference/per hour). 50 gallons is 400LB of water. 1 BTU will raise the temperature of 1 pound of water 1 degree F. So 8180 BTU in the first hour divided by the 400 pounds of water equals a temp rise of 20F or from 55F to 75F. This of course is not strictly accurate as you raise the water temp in the tank, you lower the temp difference and decrease the thermal transfer rate.

If you split the temp difference between 180F and a cold 55F tank(125F) and 180F and a hot 140F tank(40F), you get a mean average temp difference of 82.5F. At that figure, best case your transfer will be 80 X .818 X 82.5F delta or around 5400 BTU/HR. 400LB of water raised 85F(55f to 140F) requires 34,000 BTU. At a rate of 5400 BTU/HR average, it will take this heatex around 6.3 hours to get that HW tank from 55F to 140F best case, and assuming basically unlimited BTU input from the boiler(no temp drop across the primary side). I would probably rate this smooth single wall heatex on the low side toward 60 BTU/SQFT/F/HR(add 25% to the time to temp figure or around 8 HR) as it has no flow control to break up the laminar flow that will develop along the smooth pipe wall.

Someone said it earlier, these things usually have coils and coils. To transfer heat, you need surface area and temp difference. If you don't have much temp difference, you need more surface area to compensate and vis-versa. To transfer heat efficiently you need turbulence to break up the laminar flow along the transfer surface so the actual temp of the fluid along the transfer surface is always at it's maximum difference. With laminar flow, it actually insulates the transfer surface the same way a layer of stagnant water insulates a diver in a wet suit. This thing obviously transfers heat and can maintain the tank, but it will have one heck of a recovery time if any significant ammount of hot water is used from the tank.

If you want to optimize this system and get more use out of the boiler heat for domestic water, you need many times the surface area you have now and something to turbulate the water such as bare copper wire wrapped around the inner pipe and a strand or two set down the middle of the inner pipe to force the fluids to tumble. Or you could go to E-bay and search for "brazed flat plate heat exchanger" and probably get exactly what you need for around $100-200. These exchangers are small yet have a huge internal surface area. The plates have a herringbone pattern stamped into them that makes for very turbulent flow with low restriction. You will still be able to thermosiphon thru it, mine thermosiphons off my generator great. I transfer 16,000+ BTU/HR from my generator to domestic water with a 70F final temp difference with no problems. I am using a 10 plate 5" X 12" flat plate heatex that cost me $98.

Goto FlatPlate Heat Exchangers and they have some descriptions of how they are put together and a free online calculator. Just type in your temps and flows and it will make size reccomendations for you, then look for that size on E-bay or thru your boiler supplier.

 

[charlz]

Well with a bit more information, you can calculate this pretty accurately, but making some assumptions, {snip}

Fantastic explanation, thanks! Obviously how much this type of system helps the water heater sorta dictates how much time and money you want to put into it.

I didn't use that calculator as you have to give an email to do so. I guess in theory, given enough 180F boiler water and a big enough heat exchanger you could go 'tankless' at least in the winter months. I believe Aldertom said the water heater is propane so I would be studying this real hard as well... that stuff is expensive. (was my primary heat source till I moved to pellets). One tank fill avoided might pay for all the heat exchangers, control valves etc.

Charles

 

[RonMar]

Fantastic explanation, thanks! Obviously how much this type of system helps the water heater sorta dictates how much time and money you want to put into it.

I didn't use that calculator as you have to give an email to do so. I guess in theory, given enough 180F boiler water and a big enough heat exchanger you could go 'tankless' at least in the winter months. I believe Aldertom said the water heater is propane so I would be studying this real hard as well... that stuff is expensive. (was my primary heat source till I moved to pellets). One tank fill avoided might pay for all the heat exchangers, control valves etc.

Charles

I bet he has a lot of available heat from the boiler, and with the right exchanger it could have an acceptable recovery time. and I agree, the heatex, fittings and a on/off thermostatic control valve could probably be had for much less than a tank of fuel. Or he could build 3 or more of what he has(with turbulence devices added) and put them in parallel with the one that is there now(quadruple his surface area) and drop his recovery time to 2 hours or less. I only have 16,000+ BTU maximum available from my generator at full load, so with my 600 SQ/IN heatex, my tank will have about a 2 hour recovery time. But since this is a backup generator(3KW), and it is not big enough to power the electric hot water tank element, but just the bare necessities(lights, pellet stove, wellpump and reefer/freezers), a ready tank of hot water is just gravy without burning any additional fuel to get it. You can do a lot with a full tank of hot water.