Stretching Out The Buffer Tank

[velvetfoot]

I find it too bad that after a zone or two drop out, the buffer tank is charged with the boiler at 100% power. I'd like to modulate the boiler down in these situations. One possible way, as Beagle Dad and nofossil have noted with a Froling wood boiler, is to control the return temperature. When the return temperature gets close to the boiler set point, my boiler modulates down to a minimum of 30%. It'd be nice to charge the buffer up nice and slow while keeping the boiler running when other zones kick on.

It would also be nice if the buffer could be 'emptied' while the boiler was still running, instead of it being a one way street-only charge the buffer, never discharge while the boiler is running.

I'm trying to come up with some ideas, but thought I'd throw this out there in case anybody has ideas. I will post further when I do.

 

[leon]

I find it too bad that after a zone or two drop out, the buffer tank is charged with the boiler at 100% power. I'd like to modulate the boiler down in these situations. One possible way, as Beagle Dad and nofossil have noted with a Froling wood boiler, is to control the return temperature. When the return temperature gets close to the boiler set point, my boiler modulates down to a minimum of 30%. It'd be nice to charge the buffer up nice and slow while keeping the boiler running when other zones kick on.

It would also be nice if the buffer could be 'emptied' while the boiler was still running, instead of it being a one way street-only charge the buffer, never discharge while the boiler is running.

I'm trying to come up with some ideas, but thought I'd throw this out there in case anybody has ideas. I will post further when I do.

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I would like you to invest in a copy of Pumping Away by Dan Holohan from Amazon or Dan Holohan books.

This paperback is a huge help with heating with hydronic heat and will show you how both a boiler bypass loop and system bypass loop work and save energy,

Continuing:

Option one would be using a B+G circuit setter or a Taco circuit setter for a boiler bypass loop that would let you push cooler hot water into the storage without extra electrical parts by placing the circuit setter on the return side and using the tapping into the storage for the tee coming from the B+G Circuit Setter to a tee that would be plumbed into the return line before the boilers water enters the sump of the boiler at that tapping. This slows down the boilers burn rate and reuses a portion of the cooler return water at all times keeping the boiler hotter at all times.

Option Two would be a system bypass using a B+G circuit setter to push X amount of hot water coming from the hot water circulator and then going back into the return line tapping to the boiler sump to be reheated and then slowing down the burn rate even further.

Both loops are easy to do and not difficult as long as you understand how its done and how easy it is to do once you see how its done.

Neither method requires a new circulator, you can also do it without a bypass valve to meter the flow by using a ball valve.

 

[Nofossil]

I control return temperature just to protect the boiler. I modulate the boiler circulator itself to achieve my desired outlet temperature.

Ideally, if you can maintain good stratification in storage, the return temp to the boiler will be very cool until storage is fully charged. I have a customer who is an absolute fanatic on stratification and has achieved very impressive results. I can post some charts if that would help.

 

[__dan]

after a zone or two drop out,

This is all the information right there. The ideal control strategy (imo) is to keep the zones on as long as possible which means lowering the zone distribution water temp independent of the boiler loop water temp via primary secondary pumping and outside air temp reset of the secondary, distribution loop, water temp. The secondary loop water temp is mixed down with either a three way valve or variable speed injection pump.

IMO this should be a sort of reference design for every application and the only reason it is not is the added cost.

This presents a lower instantaneous running load to the boiler which allows it to come up to temp faster, and then to modulate down the firing rate as setpoint HWS is achieved. The boiler has time to match the load as the load stays on longer, extending the burn time cyle.

How the buffer tank loads the boiler may be more of a plumbing issue which is outside my expertise. Usually the buffer tank and return protection valve are working together. Tank is loaded and drawn from at the top of the tank are the tank hot zone travels from the top down. There are also ways to try to intercept the heat for the loads immediately before it enters the tank, which I tried to do in my design, but I would say that's non standard.

In any cold start event, the buffer tank presents immediate max load to the boiler which means max firing rate and the necessity of the return protection valve functioning properly to avoid thermal shock/flue gas condensation to the boiler. I see no (easy, standard) way of modulating the load presented by the buffer tank to the boiler which is why the return protection valve is there, and it will still present max load to the boiler until the buffer tank is hot at the bottom.

However there is an easy, standard way of modulating the house load so that it presents a lower load for a longer time and that is P/S pumping, OAT reset, and mixing down of the secondary loop distribution water temp.

For a new system I would consider this an essential design element. But for an installed system that is otherwise functioning properly, I don't know if the gains would out weigh the cost of the change.

 

[velvetfoot]

For reference, I include a sketch of my current system.


I sure have to read and think some more.

My pumps are continuously (manually) variable and I've done tweaking with flows. I don't have a zone setter (yet).

I currently have flow set where, when two zones pumps are running, like after a setback, the boiler circulates only through the zones and not the buffer tank. The buffer tank has about 15 minutes or so of run time on it, at current settings, before things get too cool and I shut off the zones while the pellet boiler continues to warm up. There is some cycling when the thermomix protection valve is partially opened, I believe from 130-147 and the zones suck up some of the water while flow from the boiler is lowered by the protection valve, until it is fully opened.

Of course, as I said, even though keeping the buffer tank cool as long as possible is a good thing, when a zone drops out, cold water from the tank goes to the return of boiler. I noticed that boiler supply temps went down from setpoint of 165 to maybe 154, with modulation going back to 100%.

I originally thought that if the ThermoMix were to be replaced with an adjustable electric mixing valve, that valve could be set to throttle the cold temperatures, and maybe keep it high enough where modulation still occurs, or adjust it so. Or maybe tap into the boiler temp sensor wire and spoof the readings so that it's forced into modulation.

The other component is tapping into a hot buffer tank while the pellet boiler is still running. Say, modulating at 30% and a zone comes on. Why not draw it from the tank while the boiler stays modulating at 30%? I'm not sure how that could be achieved.

I have a lot of reading and thinking to do.

Happy New Year!

 

[leon]

For reference, I include a sketch of my current system.
View attachment 170613

I sure have to read and think some more.

My pumps are continuously (manually) variable and I've done tweaking with flows. I don't have a zone setter (yet).

I currently have flow set where, when two zones pumps are running, like after a setback, the boiler circulates only through the zones and not the buffer tank. The buffer tank has about 15 minutes or so of run time on it, at current settings, before things get too cool and I shut off the zones while the pellet boiler continues to warm up. There is some cycling when the thermomix protection valve is partially opened, I believe from 130-147 and the zones suck up some of the water while flow from the boiler is lowered by the protection valve, until it is fully opened.

Of course, as I said, even though keeping the buffer tank cool as long as possible is a good thing, when a zone drops out, cold water from the tank goes to the return of boiler. I noticed that boiler supply temps went down from setpoint of 165 to maybe 154, with modulation going back to 100%.

I originally thought that if the ThermoMix were to be replaced with an adjustable electric mixing valve, that valve could be set to throttle the cold temperatures, and maybe keep it high enough where modulation still occurs, or adjust it so. Or maybe tap into the boiler temp sensor wire and spoof the readings so that it's forced into modulation.

The other component is tapping into a hot buffer tank while the pellet boiler is still running. Say, modulating at 30% and a zone comes on. Why not draw it from the tank while the boiler stays modulating at 30%? I'm not sure how that could be achieved.

I have a lot of reading and thinking to do.

Happy New Year!

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Ah not quite as crazy as my rats nest.
I would install a pair of system bypass loops on both boilers to save on heating water
if it were me for sure.

Even diverting two gallons per minute back to the sumps of both boilers would save heat and fuel.

anyway If I were you I would change the
location of your circulators and draw off the top of the boilers which allows the
cooler return water to enter the sump of both boilers which is what is referred to
as "pumping away" and described very well in Dan Holohans book titled
Pumping Away" and other cool piping options for hydronic systems.


I am having my hand fed wood and coal boiler and oil fired boiler removed and the plumbers are installing a dual fuel Keystoker KAA-4-1 on Monday so I can get away from wood and oil.

I am having a 15 gallon steel expansion tank reinstalled and I am reusing one B+G 3 speed circulator for my single heating loop and eliminating the second circulator as a temp balance circulator.

I am going to save on my electric bill as the balance circulator runs continuously while the hand fired boiler is working to maintain the temps in the oil fired boiler for the heating loop-no more of that

I will also have 51 one gallons of water counting the water in the heating loop so I have plenty of water that will work at 140-160 degrees and I dont expect to butn more than 12 pounds an hour if that when heating with rice coal.

 

[velvetfoot]

I lowered the pellet boiler supply temp to 156F. I'm getting almost an hour more runtime, to 3 hours, with very light to no load-basically just to buffer. The boiler starts modulating some even when the tank is relatively cool, and at 30% for a long time until it cruises to shutoff at about 181. Easily reversible if more heat is required.

 

[velvetfoot]

I ran it on oil tonight. It takes about 45 minutes for that 79000 btu/hr Burnham oil boiler to charge the tank and a run a zone. Nothing modulating about that puppy!