Fiddling with Return Temperature

[Bob Rohr]

Unwanted or ghost flow can have multiple causes. Hot goes to cold always, so if there is a clear, low pressure drop piping circuit without any check or flow protection you will get energy transfer. This is how the old buildings heated with gravity hot water systems. Hot water would rise up the large diameter piping, cold would fall.

That transfer is so powerful that tall buildings needed balancing valves on the upper floors to prevent over heating and force some flow to the lower floors. That is one of the functions of those big wheeled valves on cast iron radiators in old gravity systems.
The taller that thermocline, the faster the hot would rise up.

Another cause of un-wanted flow is caused by circulator pumps moving flow to the path of least resistance. That is where check valves or flow control valves come into play. In some cases you may need check protection on both supply and return piping. Indirect tanks are a classic example of the need for checks on both sides.

It is hard to predict how and when ghost flows are going to haunt you. Vertical piping runs encourage it, cold tanks can also. You can even experience two direction flow in a pipe, hot water rising up the center, cold falling down around it.

Best to just design the piping components with flow protection at the get go. Pumps now have check protection built in or in the box with the pump, that solves one side. Add a spring "hydronic" check on the other side of the loop.

 

[JohnDolz]

Unwanted or ghost flow can have multiple causes. Hot goes to cold always, so if there is a clear, low pressure drop piping circuit without any check or flow protection you will get energy transfer. This is how the old buildings heated with gravity hot water systems. Hot water would rise up the large diameter piping, cold would fall.

That transfer is so powerful that tall buildings needed balancing valves on the upper floors to prevent over heating and force some flow to the lower floors. That is one of the functions of those big wheeled valves on cast iron radiators in old gravity systems.
The taller that thermocline, the faster the hot would rise up.

Another cause of un-wanted flow is caused by circulator pumps moving flow to the path of least resistance. That is where check valves or flow control valves come into play. In some cases you may need check protection on both supply and return piping. Indirect tanks are a classic example of the need for checks on both sides.

It is hard to predict how and when ghost flows are going to haunt you. Vertical piping runs encourage it, cold tanks can also. You can even experience two direction flow in a pipe, hot water rising up the center, cold falling down around it.

Best to just design the piping components with flow protection at the get go. Pumps now have check protection built in or in the box with the pump, that solves one side. Add a spring "hydronic" check on the other side of the loop.

Thanks Bob. I am actually reading Pumping Away as we speak and I often think about how impressive that engineering was/is! I am sitting here shoving water and slamming it shut with devices and these guys simply used pressure and gravity, kudos to them! I did go to an engineering school but was a Management major, now have a seriously new appreciation for what a well engineered plan is worth.

Not quite sure I follow you on the Spring check valve (or is it is applicable in my situation). My Circs have the built in Check Valves so I don't get that reverse flow, not sure where I would use Spring Valves and in what position?

I did speak with my friends at Effecta and they agreed with Karl, try turning off the alpha in front of the mixing valve and let the zone circs do the work - will give it a try (thanks Karl!). I was a bystander during the initial install, have been working hard to learn stuff as the original installer is no longer with us but I have not looked into the wiring at all. I have my thermal storage tied to my propane boiler via a controller. The controller controlls when that main pump kicks in (or not) and when the propane boiler kicks in. We'll see if taking that alpha offline takes down the zone alphas as well - I might need to start working on my EE Degree!

Really appreciate all the thought and suggestions!

 

[velvetfoot]

Can you just switch off the pump at the pump? As an aside, I have a switch in each power supply going to the pumps.

 

[JohnDolz]

Can you just switch off the pump at the pump? As an aside, I have a switch in each power supply going to the pumps.

Don't think so but I can be wrong on that. Easy enough to disconnect a wire and see what happens. Will probably try it over the weekend. For now I simply opened the Zone Valve in the Bypass, turned my pumps back to very low speed and turned on the air conditioning (mid 50's here today!)

 

[JohnDolz]

Can you just switch off the pump at the pump? As an aside, I have a switch in each power supply going to the pumps.

Karl - I grabbed a minute and disconnected to main alpha, zone alphas still working. All my zones were off because it was so hot today but supposed to cool down tonight, we'll give it a whirl and see what we gt in the next 48 hours or so. Appreciate the suggestion!

 

[velvetfoot]

Hey, it's not costing anything. That's a plus. Won't be this warm for long, I'm sure.

 

[JohnDolz]

Can you just switch off the pump at the pump? As an aside, I have a switch in each power supply going to the pumps.

Appreciate the help so I thought I would give you an update. Things worked fine when I took that alpha off line but it looked like my mixing valve was open a bit more then usual. I figured the return side had a little more pressure that the supply side, thinking worst case would be that I had a much narrower window for the mixing valve to work. I knew that once it got a little colder I would get a better sense of what was happening. It is colder today and sure enough my mixing valve was almost wide open and he flow temp was 45 degrees less than my tank temp. I closed the valve 100 % to see what would happen and I was still 35 degrees under tank temp. Scratched my head for a few minutes and then looked for where else my friend "Mr. water could be coming from. It is obvious now but my return loop runs through my propane boiler (in via the rerun pipe and out via the supply pipe. Once it comes out I have a 45 degree split that takes rerun water to my mixing valve and/or storage tank. The other split goes to the supply manifold (a zone valve closes off the return/tank loop when propane boiler fires). Seems another zone valve is in my future to close of the propane supply pipe when heating from wood/storage. I would like to say that this will do it but Mr. Water seems to be outsmarting me at every turn so I will hold judgment for now.

 

[velvetfoot]

Ha. Good luck with that! "Nothing's easy" is my motto. Very rarely, anyway, for me.

 

[JohnDolz]

Ha. Good luck with that! "Nothing's easy" is my motto. Very rarely, anyway, for me.

I am going to go move some wood. At least I know if I move it from point A to point B, it will still be at B next time I go look for it.

 

[velvetfoot]

Just started the pellet boiler. Turned down to 10kw (from 15kw) and turned up boiler temp to 176 (max). Will see how the 165 return makes it react. Will probably modulate, which would be good for letting the boiler run a little longer (desireable), but not so much if max output is desired. Will probably turn it down. First observe a cycle.

 

[JohnDolz]

Just started the pellet boiler. Turned down to 10kw (from 15kw) and turned up boiler temp to 176 (max). Will see how the 165 return makes it react. Will probably modulate, which would be good for letting the boiler run a little longer (desireable), but not so much if max output is desired. Will probably turn it down. First observe a cycle.

My Supply is 128 and my house is 72, I am tempted to say make the max boiler temp 135 and let it flow but honestly I have no idea how to best run a pellet boiler. Out of curiosity I am going to send my friend at Effecta an email and ask his opinion on the most efficient way to run a pellet boiler (with buffer but not big storage). I'll let you know what his thoughts are, will be interesting to hear what he thinks vs. how you are running.

 

[velvetfoot]

Back up from the basement. The boiler initially turned off on high temp. I believe there was an air bubble-things got better after I turned on a zone. Maybe that's what's up with that guy on the other thread-I'll have to tell him.

I relearned that at 10kw, both zones running is a bit much for it. Would only get up to 154. That's when the wood stove goes on, hah.

Just went down and looked again: no zones on, 165 return, 174 boiler, 80% modulation, 235 exhaust temp.

I'm sure the best way to run the pellet boiler is low temp as possible without condensation, constant load.

Runtime: 3:45

 

[JohnDolz]

Back up from the basement. The boiler initially turned off on high temp. I believe there was an air bubble-things got better after I turned on a zone. Maybe that's what's up with that guy on the other thread-I'll have to tell him.

I relearned that at 10kw, both zones running is a bit much for it. Would only get up to 154. That's when the wood stove goes on, hah.

Just went down and looked again: no zones on, 165 return, 174 boiler, 80% modulation, 235 exhaust temp.

I'm sure the best way to run the pellet boiler is low temp as possible without condensation, constant load.

Runtime: 3:45

What would happen if you left the woodstove off and just let the boiler run a 154 and the circs just kept pumping? Like I said I am not familliar with how a pellet set up runs. What would things look like if you targeted like a 50% modulation (is that even possible) and just went with continuous burn (is that even possible)? In talking to the Engineers at Triangle Tubes (my propane boiler manufacturer) they loved the thought of the propane boiler firing at an extremely low rate for a long period of time, slowly warming up my 1000 gallon storage, turning of at some point, using storage for as long as it lasted and then starting over again (leveraging the storage was to help avoid the short cycle firing that I get because that boiler also is driven by an outdoor reset and I set the parameters the same as my woodboiler).

 

[velvetfoot]

What would happen if you left the woodstove off and just let the boiler run a 154 and the circs just kept pumping?

If both zones were running, it would take a longer time to recover from setback, which might not be a big thing if it wasn't super cold outside.

What would things look like if you targeted like a 50% modulation (is that even possible) and just went with continuous burn (is that even possible)?

If it runs for 6 hours it stops and rests, adds pellets or whatever, for a little while. I'm not sure that constantly burning at lower modulation is that good for things. The longer the burn, the better-supposedly 2+hours/start is best.

Those Triangle people might like it, but this is propane, right, and they don't have to pay for the standby losses of your 1000 gallon tank. That being said, when I run on oil I charge my 120 gallon tank. It takes an hour and a quarter with no zones on, but I feel the long run is good for the boiler.

Running on the 10kW setting is a lot less energetic than at 15kW. I'm sure run times will be longer. Setback recovery is a choice I've currently made, so that might ultimately prompt a return to the higher setting.

To make this a little thread-relevant, by fiddling with the return temp, I've ensured that only 'excess' heat is shunted to the tank, and the weak output can go towards heating the house.

 

[JohnDolz]

If both zones were running, it would take a longer time to recover from setback, which might not be a big thing if it wasn't super cold outside.


If it runs for 6 hours it stops and rests, adds pellets or whatever, for a little while. I'm not sure that constantly burning at lower modulation is that good for things. The longer the burn, the better-supposedly 2+hours/start is best.

Those Triangle people might like it, but this is propane, right, and they don't have to pay for the standby losses of your 1000 gallon tank. That being said, when I run on oil I charge my 120 gallon tank. It takes an hour and a quarter with no zones on, but I feel the long run is good for the boiler.

Running on the 10kW setting is a lot less energetic than at 15kW. I'm sure run times will be longer. Setback recovery is a choice I've currently made, so that might ultimately prompt a return to the higher setting.

To make this a little thread-relevant, by fiddling with the return temp, I've ensured that only 'excess' heat is shunted to the tank, and the weak output can go towards heating the house.

Thanks, interesting, Next time I talk to my friend in Alaska that runs pellet and wood (he has the same boiler as me but added the pellet burner option) I'm going to pick his brain just out of curiosity.

The setback recovery definitely is a challenge at lower temps. I do it because my boiler has the option but if I had any sense I would stop it, doubt I am saving much of anything. To give you an idea I start Recovery at 3:00 AM (setback 6 degrees at 9:00 PM). It is a rare day that I am over 150, even during recovery when my controller sends hotter flow as it wants to hit the Recovery target.

I guess I never think of it from a return temp perspective. I try to find the lowest flow temp combined with the lowest flow rate that gets me to the desired temp.

 

[velvetfoot]

I just looked while the pellet boiler is charging the tank (only). 174 supply, 231 flue, 78% power (modulation).
The 78% power probably helps for a longer run.

 

[velvetfoot]

It's averaging around a 3 hour run to charge the tank with no zones.