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[higgins11]

1)Everyone always talks about measuring return heat why is it important for the temp returning not to be to low?

2) why use a heat exchanger in domestic hot water........why not just use the water out of the OWB tank and plumb the local well up to refill the OWB when water is used out of the OWB tank? My domestic hot water tank is only 80 gallons ........the OWB is 750 gallons

Thank for entertaining the silly questions of a newbie

[Ridgekid]

1.  So you can see what your heat load is.

2.  Really?  Don't you treat your OWB?  How would you control your ph and nitrates if your drawing water off your OWB?  Do you really want to use treated water in your house? I don't think so.

[RSI]

It depends on the boiler if a low return temperature is bad or not. Most it doesn't matter on. Especially if they have a pipe inside the boiler for the return that runs to the front.

There are several reasons to use a heat exchanger for domestic water. Most outdoor boilers are not pressurized so it would just run over the top constantly.
If you have a pressurized system you wouldn't want to run to run the pressure that high either.

The water usually has chemicals in it to stop corrosion.

Even if you did have a pressurized boiler that cold run at the well pressure and didn't use chemicals it is still going to be dirty compared to normal domestic water.

[martyinmi]

What some of the manufacturers are finding out is that when water in the boiler gets below a certain temperature, the inside of the burn chamber forms condensation, or starts "sweating". This condensate in and of itself is corrosive, but when it mixes with ash, it becomes even more corrosive. I believe that that magic number is around the 140* mark. That's why a few of the bigger manufacturers are even including a tempering(mixing) valve with the OWB purchase. The valve helps maintain at least 150*. I think there have been many boilers that corroded through prematurely because of too low temperatures.   

[RSI]

What some of the manufacturers are finding out is that when water in the boiler gets below a certain temperature, the inside of the burn chamber forms condensation, or starts "sweating". This condensate in and of itself is corrosive, but when it mixes with ash, it becomes even more corrosive. I believe that that magic number is around the 140* mark. That's why a few of the bigger manufacturers are even including a tempering(mixing) valve with the OWB purchase. The valve helps maintain at least 150*. I think there have been many boilers that corroded through prematurely because of too low temperatures.

That is the water temperature in the boiler you want to keep up. If the return is colder than then it is pretty much instantly heated as it mixes with the hot water and won't cause any condensation. (as I said above this depends on the boiler design)

[martyinmi]

His question is "why is it important for the temp returning not to be to low". The reason is to prevent condensation in the event that your heat load is too large and your boiler temp gets too low. I am not sure about the different types of stainless out there, as their metallurgy is much different, but cold formed and hot rolled carbon steel are effected by low boiler temps, as well as the so called "boilerplate". They will fail prematurely, no matter what OWB brand, if boiler temps get below 140* on a regular basis. That is why it is important to make sure your return temperature is not too low. There are many posts on this forum about too high of heat load vs too low boiler temps.

[Ridgekid]

Yeah we had a post on this somewhere. Once you get above 180* it cooks out the oxygen in the water. Fall below 140*  you have to start all over again to cook out the oxygen.

Marty is right too about the condensation. That's why us cb owners have thermostatic valves.

[MattyNH]

 You don't drink the the water that is in your oil furnace.. My hot water 40 gal holding tank has a heat exchanger inside the tank..Def don't want to drink the water that is in your owb or in your existing  oil furnace..

[RSI]

His question is "why is it important for the temp returning not to be to low". The reason is to prevent condensation in the event that your heat load is too large and your boiler temp gets too low. I am not sure about the different types of stainless out there, as their metallurgy is much different, but cold formed and hot rolled carbon steel are effected by low boiler temps, as well as the so called "boilerplate". They will fail prematurely, no matter what OWB brand, if boiler temps get below 140* on a regular basis. That is why it is important to make sure your return temperature is not too low. There are many posts on this forum about too high of heat load vs too low boiler temps.

I think we are talking about two different things.
If you are pumping say 2 gpm you will have the return water going back to the boiler at a very low temperature. Because it is such a small amount of water it will mix with the hot water and not cause any condensation. If you are pumping say 20 gpm and have the same heat load you will have a much higher return temperature. The higher gpm will allow for a much larger heat load / boiler temperature loss.

[yoderheating]

 I believe the valves are used with mild steel furnaces that have no grates and use natural draft to fire the furnace. This system has a slower recovery time and so proper furnace sizing is very important. If a heat demand is too great the furnace may drop below 140 unless they use a valve to not allow the heat to leave. This isn't a problem with furnaces with a quick recovery time so you don't see valves sold with furnaces brands that have grates and fan drafts.

[beeman]

not to get off suject but would,nt be great if you put a valve to fill your coffee cup in the morning

[mcarter]

There is some correlation to efficiency if the return heat is not lower than 20 degrees from the supply heat.  This is an observation I have read about concerning industrial applications.  I think each heat exchanger will drop your return heat by 2+ degrees, I doubt in a residential application that you would have more than 3 heat exchangers, so I doubt you would be out of that 20 degree range.  I know there are more factors that effect return heat temp but that should give you a rough estimate with which to stay in range.  Monitoring return heat could help indicate if there was a problem with your lines too.

I'm curious about those that do monitor the return heat, what differential are you measuring between supply heat and return heat and how many heat exchangers on your circuit?  What kind of drop do you see with a radiant heat setup?

Michael

[Ridgekid]

I get a 2* drop just running through the exchangers. I get 15* drops on each exchanger when they are in use.

[willieG]

drops in temp through heat exchanges are because (mostly) the size (rating of the echanger) and GPM.  if you are moving only (for example and not true figures) 2 GPM through an exchanger rated at 50,000 btu per hour and you drop the temp from 180 in and 140  then you increase the GPM to say 10 your actual drop might now only be 5 degrees. also the low heat return could indicate that when theoutside temps are severly low that the 2GPM may not be able to keep up with the heat demand due to heat loss of the home in the extreme cold.

you must remember that though your furnace may be rated at 500,000 btu per hour it is in fact your GPM and how big your exchanger is rated for. it is said that 1GPM can deliver 10,000 btu per hour to your home  so to in fact get the 500,000 btu that your furnace may be capable of producing you would need to deliver the water to the home at 180 degrees and moving at about 50 GPM and of course have enough exchangers to extract that heat. on average with the small pumps that we use on these stoves we may move anywhere from say 8 to 12 GPM (in most cases)  so a low return temp on an average winter day is a warningn that perhaps on the few coldest days of the year that you may not be able to keep your house at your desired temp. this on top of all the info allready provided about condensate and such.

[church]

For my clarification regarding sweating.
Are you talking about actual condensation being caused by such water temp diiferentials, cold water returning to a warm/hot surface and not the condensation of the moisture being released from the wood during burns? I have been getting that type of condition in the burn chamber, even worst during this mild weather, longer idle times. I have my temp set @140 with 5 degree on/off. Would your recommendations be to raise temp or widen on/off to help prevent this? This temp setting has been heating the home ok. I haven't taken a return water temp reading so I do not know what it is. The temp gauge on supply @ hx stays within that range. I have a open system boiler. I can understand the water jacket sweating due colder temp exposure.
Thanks,
Church