Any suggestions? Installation challenge

[BrownianHeatingTech]

Brownian, I thought that the pressurized storage required no heat exchange since you were using the heated water directly. So your saying that with a pressurized heat storage tank, you still need a heat exchanger for the DHW? Right now we have a Burnham 40 gallon water tank in use with the oil boiler. Is this tank acting like a heat exchanger right now with a coil in it. I guess I assumed it was just a storage tank, but it sounds like you are saying you can't use the boiler heated water directly for DHW, is that right?

Then if that is the case, and there is a heat exchange coil inside the current 40 gallon tank, and the domestic hot water is actually cold water that has been heated through a heat exchange coil within the 40 gallon Burnham storage tank, then I can understand how temps as low as 140 degrees might have a problem getting adequately heated water using the smaller coil to transfer the heat. Am I correct in this line of thought? If that is the case, then my whole heat storage world has been turned upside down.

That's correct - you don't use the heating water directly, for domestic use.

The heat exchangers eliminated by use of the pressurized storage are those which are used to separate the atmospheric tank from the heating system.

On a related note, it is theoretically possible to install a coil into a pressurized tank, for heating domestic water. Of course, the labor is quite high, so it only makes sense if you are really bored...

Joe

 

[acavanagh]

Well that sucks.

Still learning, but that does make sense.

So, for arguments sake, how hot would the storage tank need to be in order to have sufficiently heated water for domestic use. I know with the pressurized tanks, you can get them upwards of 200 degrees. If it is just a matter of making sure the water is above say 160 degrees for comfortable DHW use, then that may be a devil I am willing to live with. If it needs to be 180 degrees or above, I may rethink pressurized storage. I wonder how fast we would go through say 200 degree of 1000 gallon water to get it down to "non-usable" temps. We are a family of 4 with a dishwasher. Obviously it may not be a problem during the winter, but spring summer and fall it could be.

In regards to adding a coil to the pressurized tank as a heat exhange, I calculated 1/2 in pipe at 100 feet in length to be about 15 sq feet. I wonder how much that would cost and if that might add an additional say 10 degrees of usable heat from the tank. For example, if the tank requirement would need to be at 150 degree minimum, adding 100 feet of pipe might get it down to 140 degrees before it is "unusable". I know this isn't exact, since it depends on how much water you need, and what is that "base" number of usable heat. How are people getting those coils in those propane tanks? I know that the propane tanks I have seen have multiple taps on top that could be used for coils I guess, but it would seem you could only go down no more than three feet. How many feet of piping could you get in a coil going down say three feet to add a significant amount of square footage to the DHW heat exchanger?

Sorry for all the questions. I feel like a can of worms just got opened.

 

[flyingcow]

I also started to go with the pressurized route, but then for the same reason of not being the best way to heat my DHW in the summer, I went back to unpressurized.

 

[steam man]

Something I am considering is doing a kind of hybid system where my dhw tank is a preheat-indirect 120g with a coil and also using a non-pressurized large volume tank, possibly with a reasonable size coil for picking some more heat for dhw and lower temp space heating. Pressurized tanks with coils get pricey fast. Expansion is also a bigger issue.

Mike

 

[BrownianHeatingTech]

So, for arguments sake, how hot would the storage tank need to be in order to have sufficiently heated water for domestic use. I know with the pressurized tanks, you can get them upwards of 200 degrees. If it is just a matter of making sure the water is above say 160 degrees for comfortable DHW use, then that may be a devil I am willing to live with. If it needs to be 180 degrees or above, I may rethink pressurized storage. I wonder how fast we would go through say 200 degree of 1000 gallon water to get it down to "non-usable" temps. We are a family of 4 with a dishwasher. Obviously it may not be a problem during the winter, but spring summer and fall it could be.

Unless you connect a water meter to the inlet of your water heater and measure usage over an average week or two, then divide out to figure daily averages, it's very hard. There are "rules of thumb" that we use to size systems, but there is no absolute way to size hot water without knowing how many gallons of water are used (and what outlet temperature is needed).

In regards to adding a coil to the pressurized tank as a heat exhange, I calculated 1/2 in pipe at 100 feet in length to be about 15 sq feet. I wonder how much that would cost and if that might add an additional say 10 degrees of usable heat from the tank. For example, if the tank requirement would need to be at 150 degree minimum, adding 100 feet of pipe might get it down to 140 degrees before it is "unusable". I know this isn't exact, since it depends on how much water you need, and what is that "base" number of usable heat. How are people getting those coils in those propane tanks? I know that the propane tanks I have seen have multiple taps on top that could be used for coils I guess, but it would seem you could only go down no more than three feet. How many feet of piping could you get in a coil going down say three feet to add a significant amount of square footage to the DHW heat exchanger?

To get a coil into a tank, I would recommend something flexible like corrugated stainless steel tube, not something stiff like copper.

As far as "how," that varies based upon the tank type. You need to figure out how to get the coil in, and how to support it in a "coil" shape once it is in.

The other alternative is to get an indirect tank with more heat exchange surface area. There are some European tanks which have impressive amounts of heat exchanger crammed in there.

I suppose you could also use a tank with a removable heat exchanger (Vaughn tanks, for example), and build your own (larger) coil onto the existing bulkhead.

Joe

 

[acavanagh]

I am leaning more and more towards the unpressurized for multiple reasons now. It would seem that adding extra heat exchangers into the tank at a later date for multiple uses would be much easier to do. Also, I am told that the liners aren't too much $ to replace and that the PVC liners can handle temps up to 200 degrees. That is comporable to pressurized. The more I learn, the more I am leaning towards the unpressurized tanks. I have been converted. It seems that if the tank is just used for heat, ie baseboard/radiant flooring, then the pressurized tank would probably be the best option as far as economics if you could get a cheap tank. When you start figuring in DHW, the unpressurized seems like the best option, something flyingcow has already figured out. Its a learning curve. I am just glad I figured this out before I almost got a 5 ton liquid nitrogen tank. I was most worried about the liners, but am told that the EPDM liners last about 15 years with temps of about 180, and the PVC liners are the same at temps of up to 200. Spending a hundred to two hundred dollars every 10 or so years doesn't scare me. Looks like I should change my signature to 1000+ gallons of unpressurized storage pending.

 

[BrownianHeatingTech]

Just remember that there is a differential with any heat exchanger. In order to heat to transfer, the fluid on one side needs to be hotter than the fluid on the other.

So, regardless of whether a particular liner can take 200-degree temps, the only way you will get the tank that hot would be by running the boiler hotter than 200 degrees.

Joe

 

[acavanagh]

Good point Brownian. Plus, I am sure there are marginal returns the less the difference in temps between the two. For instance, if the water coming out of the boiler can be no higher than 180 degrees, and the tank temp is 178, that last couple of degrees might be a pain to get. In fact you might never get there. Kind of reminds me of some calculus equations from school. God I hated calculus. How hot do the EKO's allow water to get? I am sure the controller limits the temp to a certain max. Does anyone know what this max is? Is there a safe way around it if the limit is say well under 200. Ideally I would want the higher storage temps as that 20 degrees extra (180 - 200) might enable me to have only one burn a day since we have baseboard heat that is really only useful above 150 degrees, so 180 degree storage water only allows us 30 degrees to work with. Obviously more storage is one answer, but I would prefer to stay around 1000 to 1500 gallons.