Using Mixing Valve on Tank to Maximize Buffer Use and Pellet Boiler Runtime

[velvetfoot]

Something that ticks me off is when the pellet boiler is running and one or no zones are on, the flow is to the buffer tank. This means that very cool water is returning to the boiler and the protection valve comes into play, big time. Meanwhile, the boiler, which modulates down to 30%, is cranking at 100% to satisfy the the tank.

My goal is to only charge the tank with excess heat.

I am thinking that if I have a mixing valve on the tank return and set it on the high side, say 170, water will only start flowing to the tank when it gets over 170. The pellet boiler water temperature is set manually, say 160, but will continue to run to 181 or so, modulating down until it shuts itself down. I have it set up so that any time the boiler runs, it runs until it shuts itself down.

Anyway, in my plan, the boiler alone will be supplying the loads, modulating down to 30% as it goes past 160, and when the it reaches the tank mix valve temperature of 170 (or whatever), incremental amounts of cold water will be injected into the loop to keep it at 170. Gradually tank temperature will increase until the boiler shuts down, unless the modulating boiler matches the load.

My big idea requires 4 check valves and a mixing valve. I'm not sure it would work or if it'd be worth it.

Comments?

 

[__dan]

No, No, No,

Without looking at the details, the factory wiring diagram for the Windhager which would be my first step...

Somewhere on the Windhager there are two terminals that when closed give the boiler a call for heat and tell it to fire. Opening those terminals and the boiler gets told to turn off and most likely goes through a programmed cycle shutdown procedure (burning off the remaining fuel, moving the remaining boiler hot water out to the tank / loads. I know where to start, confirming these assumption first, where the boiler gets the call for heat, and how the boiler shutdown proceeds when the call for heat is satisfied.

If the boiler call for heat is not presently tied to the indoor thermostats and DHW aquastat (you are saying the boiler runs when there is no call for heat) possibly there is a change you may make (your electrician makes) that may be simple.

Somewhere there is a call for heat and if it's not tied to your zone stats, the next place it would be is the buffer tank aquastat (the buffer would be always max hot at the tank aquastat setting and the boiler runs even with no house load until the buffer tank aquastat is satisfied (this is what I read in your post).

The simple change would be to add a set / sets of contacts controlled by any load that opens the call for heat circuit when all loads are satisfied and closes the call for heat circuit when any load calls (load dry contacts in parallel).

I would check this first and if the call for heat is only the buffer tank aquastat, keeping it always hot, yes the change will result in a noticeable drop in pellets consumption.

I did not try to follow if the double checks would do anything, cannot say it's not obvious, but any plumbing change I would make would be a change to primary secondary loops with OAT reset of the injection pump and mixing down of the secondary distribution temp. If you have conventional baseboard that expect and need the higher boiler or primary loop water temp, this change would be expensive and not necessarily advised. If you had radiant heat loads, this change would be strongly advised and probably necessary for the system to work comfortably without overheating the living spaces.

Look at opening the call for heat circuit when there is no call for heat ...

 

[velvetfoot]

Unless I misinterpret, I guess our general objectives seem to differ.

My objective is minimal start/stops. Start/stopping is not efficient and increases wear. To achieve this objective I use a buffer tank to absorb excess heat, provide heat for some while without re-starting and a head start while starting, and provide dhw.

Your objective seems to be to eliminate the buffer tank and its standby losses completely, providing heat to zone and dhw loads directly, at low temperatures. I guess this would be nice, but I don't think practical for my loads

 

[__dan]

No, same objective different method.

Scenario 1. Make change

No call for heat at the zones opens the boiler call for heat terminals at the Windhager. The boiler continues to run in a programmed shutdown cycle where remaining fuels in the fuel chamber is cleanly burned off and remaining boiler heat is transferred out to the loads, the boiler circ and the burner firing, two independent variables.

Scenario 1 the boiler burns its remaining shutdown energy and deposits it into the buffer tank with the loads off. The buffer tank is still cooler and can absorb the remaining heat usefully by raising its temp and lowering the boiler's.

Scenario 2. Current

No call for heat, the boiler continues to fire as long as the buffer tank aquastat is closed and it may be set high. 175 or 180. The buffer tank aquastat opens at 180 and dumps the boiler call for heat circuit. The boiler continues to run while it burns clean the remaining chamber fuel and dumps the remaining hot water out to the loads.

But now the loads are off and the buffer tank is at 180. It cannot store any remaining 180 boiler water, only boiler water above that temp, which will be created as the boiler continues to run after receiving the shutdown signal.

So the boiler has run longer every cycle but the heat produced has only gone up the stack or to standby losses, with very little of the scavengeable off cycle energy quantity being stored or used.

 

[velvetfoot]

boiler continues to fire as long as the buffer tank aquastat is closed

No, once the boiler fires based on a buffer tank aquastat, the boiler is latched and continues to fire until its internal control turns itself off, gracefully, with a 13 minute burnout pump run, discharging its heat to the buffer.

I am trying extract only EXCESS heat from the boiler, while extending its runtime.

 

[__dan]

If I recall the buffer tank also has the DHW coil in it, the Tarm USA Energy Tank? Then for DHW you would want the buffer tank hot all the time and the thing to look at is the tank aquastat setpoint, lowering that. The tank aquastat gives the boiler fire / shutdown signal. The burner and boiler circ each independently have there own shutdown sequence, continuing to run for a few more minutes purging heat / flue gas.

 

[velvetfoot]

If I recall the buffer tank also has the DHW coil in it, the Tarm USA Energy Tank?

Yes.

Then for DHW you would want the buffer tank hot all the time and the thing to look at is the tank aquastat setpoint, lowering that.

Yes. I try to get away with as cool a launch temperature as possible to stretch out the buffer tank and still provide enough dhw while the boiler is warming up. I also turn off the zones if the top of tank temperature gets too low-sort of priority.

The tank aquastat gives the boiler fire / shutdown signal.

The tank give the boiler fire signal. Shutdown signal is internal to the boiler. I want to stretch out run times.

The burner and boiler circ each independently have there own shutdown sequence, continuing to run for a few more minutes purging heat / flue gas.

Right.

Again, my goal in this thought exercise is to stretch out run times of the boiler, using the buffer to store ONLY minimal excess heat produced by the boiler.

 

[Kristen]

Just curious: why not charge Buffer fully? Is this because you are burning pellets (in this scenario) rather than, more commonly, logs? so you want to conserve what is, presumably, a more expensive fuel and is probably being used because you are away / not able to load the Log boiler? (But I'm guessing !!)

Here I aim to charge the buffer fully on each burn, and to have the boiler go out just as the buffer is fully charged. To achieve that I dumped excess heat into the house (if the buffer will be charged before boiler has finished its burn AND house thermostat is already satisfied). but we only burn logs ... I'm considering how we might add a second, smaller, boiler for "when we are away" but we don't have main's gas and adding a tank for Gas or Oil would be a huge job - it would have to be sited far from the house, dig up drive to lay pipe, etc.

 

[velvetfoot]

Just curious: why not charge Buffer fully?

That is indeed my aim, but not at 100% power-more like 30%. (The pellet boiler modulates down to 30%.) Currently, if one or all of the zones are not operating, the tank is being charged at 100% power and, to begin with, a fairly long time at a little more than than the return temperature of 130F provided by the boiler protection valve, and the resultant lower flow to the zones. If I recirculate around the tank with a mixing valve, I can get high temperature faster because there is no water going to the tank-just to the load. When the boiler temperature gets to a certain point, it starts modulating down. It would be after that point that I would begin to open flow to the tank. It would be slow, so that the return temperature wouldn't change drastically, and the tank would charge slowly, only with excess heat, and the pellet boiler operating at 30%. Unless there is more big load demand: then the boiler temperature would drop and flow to the tank would be cut off, boiler could modulate up to match the load, until things get hot again, and then flow is again diverted to the tank.

There are big difference between your setup, and all the other the wood boiler people, is that the pellet boiler is continuously fed with fuel and is not batch burn, can modulate to a fairly high degree, and that your storage is huge compared to mine. For 'you guys', it's mostly a matter of managing heat flow from the large tank, where I do have a small 109 gal tank, I also have to coordinate the pellet boiler. Actually, it could be looked at that the tank is coordinated with the pellet boiler, which is I guess where I'm at.

Oh, and my oil tank is in the basement.

 

[maple1]

I'm considering how we might add a second, smaller, boiler for "when we are away" but we don't have main's gas and adding a tank for Gas or Oil would be a huge job - it would have to be sited far from the house, dig up drive to lay pipe, etc.

My solution was an electric boiler. Takes up next to no space, no chimneys or fuel supply piping needed. It does need a hefty power supply - think mine is on a 100a. breaker. But mine is oversized a bit at 18kw - I could get by with a 12kw, which could be fed by a 60a breaker.

Easy & cheap to install - but expensive to run. So might not be the best solution if it will be needed frequently. Ours is typically used only 2 days a year. It feels very nice to not have an oil tank in the house anymore.

 

[Kristen]

That is indeed my aim, but not at 100% power

Thanks, I've got it now - I think . Priority is for heat to the radiators; once the return temperature from them is approaching that when the boiler will modulate then start mixing "cold" water from the buffer. This keeps the boiler at best-temperature for longest and, presumably, means that the boiler is burning for the length of time that you want / need the radiators to be hot (or, failing that, a "decently long time") which will be more efficient than stop/start cycles. As you rightly say, with a large buffer things change; as soon as the top of my buffer is at a reasonable temperature I can heat the house ... its then many hours until the buffer is fully hot, so my boiler is running efficiently whether I'm heating the house as well, or not (up until the point where the buffer is fully charged, but the boiler still has logs that are not completely burnt).

I may have misremembered but I thought you had a twin-boiler which allowed burning either Pellets or Logs?

my oil tank is in the basement

No longer allowed in the UK. My tank used to be indoors too, we removed it when we put the log boiler in and I suspect if we had not then by now the oil delivery people would have, finally, told me they couldn't fill it any more. But that would have made for an easy standby oil boiler - I could have had a much lower kW boiler; the original boiler had to have enough kW to heat the water going around the radiator loop, whereas with my buffer tanks a smaller boiler could run 24/7 to get the tanks up to temperature, and the tanks would provide the "surge" power when the radiator circuit came on and all the water in the radiators and pipes was cold.

My solution was an electric boiler

I do have some immersion heaters in my storage tanks, but I hadn't considered them as a heat source, only a "limp along" if the boiler broke. Perhaps I should re-think that ...

mine is oversized a bit at 18kw

I have 2 x 18kW heaters in my tank. That is close to the total power that the house has available, so I would have to turn off all significant power consumption equipment when using them. Actually I've never used them, as they were for emergencies only, perhaps I should try that. I can do a cost-benefit calculation of the capital cost of installing an additional boiler, to use only once-in-a-while, and the cost of electricity to run the immersions. I have Solar PV here, so it might be that I am currently exporting some electricity on sunny winter days which I could, instead, dump through my immersions. My 60kW Log boiler runs for about 6 - 8 hours, so I assume that is 360 - 480 kWH. If I use 2 x 18kW immersions presumably they will have to run for 10 - 13 hours. Overnight our electricity is GBP 0.06 per kWH so I suppose that would be GBP 21 - 27 per day, but I don't know how to guesstimate how much Oil would be needed in a nice, shiny, new boiler to generate 360 - 480 kWH of heat? Then I could work out how many days it would take to get a payback on the electricity.

 

[velvetfoot]

Thanks, I've got it now - I think

Yes, you have it. Whether it really makes enough difference to make it worth all the piping changes is another matter entirely.

 

[velvetfoot]

I may have misremembered but I thought you had a twin-boiler which allowed burning either Pellets or Logs?

Just pellets. I do have a wood burning insert which heats a part of the house and isn't always on, requiring flexibility on the pellet boiler's part.

 

[__dan]

Just pellets? I do have a wood burning insert which heats a part of the house and isn't always on, requiring flexibility on the pellet boiler's part.

Buffer/Energy/DHW tank aquastat, lower the setpoint to the point where you still get sufficient hot water production. 140 to 150 F may work and if not, the problem will be insufficient DHW production under load.

Setback, during setback, open the buffer tank aquastat so it will not call for heat during setback but does call for heat when the setback house load calls. When the house load is satisfied it should dump the buffer tank call for heat in setback.

Setback, adjust the setback time earlier so it has recovered from setback at the time when you want heat, ie, begin setback prior to the time you want heat adding time,starting earlier, so the house has recovered at the time your demand for heat returns to normal. ~ 1.5 to 2 hours earlier.

Windhager HWS setpoint. You have previously mentioned reducing this. The caveat is that as you reduce this you may run the boiler in its condensing range where it will make a mess. Take a picture of your clean shiny boiler as it is the last time it will be this clean if you accidentally run the boiler in condensing range.

Ideally this would be adjusted in auto with OAT reset with a low limit to prevent condensing. If the boiler does not have this built in, you may manually try to lower it. Flue gasses will condense in the boiler and flue if the boiler operating temp falls below ~ 140 F. My guess is that 160 F or above would be OK but if you tried to adjust HWS setpoint to the 150 to 160F range, flue gas condensation is possible and will make a mess.

No changes to the boiler's auto burner shutdown and boiler circ shutdown sequence. These continue to run when the heat call has been dumped to purge remaining heat/flue gas from the boiler.

 

[velvetfoot]

Buffer/Energy/DHW tank aquastat, lower the setpoint to the point where you still get sufficient hot water production. 140 to 150 F may work and if not, the problem will be insufficient DHW production under load.

I have aquastat sensors at a point 1/3 of the the way from the top as well as the top of the tank. I start the boiler when 1/3 is at 129. I stop zone circs when top of tank is 125. The Energy Tank is efficient. So far, so good. No complaints yet.

Setback, during setback, open the buffer tank aquastat so it will not call for heat during setback but does call for heat when the setback house load calls. When the house load is satisfied it should dump the buffer tank call for heat in setback.

This might a short duration. My goal is long run times for the pellet boiler.

Setback, adjust the setback time earlier so it has recovered from setback at the time when you want heat, ie, begin setback prior to the time you want heat adding time,starting earlier, so the house has recovered at the time your demand for heat returns to normal. ~ 1.5 to 2 hours earlier.

A quicker setback recovery would be enabled by my plan of closing off the buffer tank as much as possible while the pellet boiler is running. Heating up the tank's cold bottom water is a deterrent to quick recovery.

Windhager HWS setpoint. You have previously mentioned reducing this. The caveat is that as you reduce this you may run the boiler in its condensing range where it will make a mess. Take a picture of your clean shiny boiler as it is the last time it will be this clean if you accidentally run the boiler in condensing range.

Reducing the external temperature supply temperature helps a lot to stretch things out because the boiler modulates down. I currently have it set at 156. I have a Caleffi Thermix mixing valve supplying 130F minimum to boiler return. Note that 130 is when the valve starts opening, and that it is fully open at 148. Please note this is a pellet boiler. No mess seen as yet.

No changes to the boiler's auto burner shutdown and boiler circ shutdown sequence. These continue to run when the heat call has been dumped to purge remaining heat/flue gas from the boiler

Yes.


I'm wondering how my piping scheme would work.

 

[velvetfoot]

Looking at some charts I made on https://www.hearth.com/talk/threads/pellet-boiler-cycle-monitoring.152539/ , it looks like the boiler goes about a hour and a half from modulate start to 30%, even on low or no load. I think taking the tank out of the mix for as long as possible could shorten this up. To that end, I bought a mixing valve and controller. Now I have to figure out how to best install it. The valve and actuator is by Johnson Controls, bought on ebay.


Controller:
http://www.supplyhouse.com/Honeywel...ure-Inputs-2-Analog-Outputs-1-Sensor-Included

Valve/Actuator:
(broken link removed to http://www.ebay.com/itm/Johnson-Controls-VG1841DR-906GGA-M9106-GGA-2-1-1-4-Valve-Actuator-AC-24V-/272055797954)

 

[velvetfoot]

They came in the mail. The valve looks pretty well put together for $99 on eBay. cv=29
I'm really not sure I'll ever install them because of the uncertain benefits, hassle and risk, but at least I have them.

 

[velvetfoot]

I thought I'd throw in a chart from that other thread. My idea is for a faster rise to temp and associated decrease in power output-pushing the 30% load start to the left, if you will, and stretching out the time on.

 

[Sparky978]

I like that Honeywell controller. Is it backlit? Thats what I don't like about the Ranco sometimes hard to read without backlight.

 

[velvetfoot]

I believe it is backlit; I haven't powered it up yet. The same unit comes in various configurations. Mine has provision for two sensors and two analog controls. Some others include relays, or even outdoor reset, all for more money, natch. I'm still trying to puzzle out the piping diagram.

 

[velvetfoot]

Report from the basement: One or two zones on and the buffer tank valved off, the boiler modulated beautifully to approximately the set temperature of 156. Maybe 46% for one zone at steady state. With two zones on it was 90% plus or so, and modulating nicely. When the one zone dropped out the temp went up and output was modulating down. Anyway, with one zone on and the buffer tank isolated, it modulated beautifully - no buffer tank required.

When both zones went off, the cold water from the bottom of the buffer tank came into play again, the water temperature went down 10 149 or so and the modulation went up to the high 90's%. It's going up just to heat the darn buffer tank. Although it's a two-pipe setup, and the zones have first crack at the boiler water, the cold water still has a big impact on the return temperature especially with only one zone on.

Anyway, I'm still trying to talk myself into actually doing this (in the summer, of course.)

 

[TCaldwell]

Shouldn't be too much work to cut the 3 way valve into your return, draw it out first, get the fittings, pipe, I'll bet you'll find yourself doing it I no time!

 

[velvetfoot]

Of course, now that I actually have it in my hands, things might move along faster. Still not this winter...too many things to go wrong.

 

[velvetfoot]

Setback, during setback, open the buffer tank aquastat so it will not call for heat during setback but does call for heat when the setback house load calls. When the house load is satisfied it should dump the buffer tank call for heat in setback.

You gave me an idea, although it's probably the opposite.
To recover from an overnight setback, a fully charged buffer tank, rather than a nearly depleted one, at start of recovery would be helpful. So, maybe, if I had more control over things, the boiler could be made to charge the tank during certain time intervals with a tank temp was somewhat more than usual, for faster recovery.

 

[velvetfoot]

Another recovery thought. I isolated the tank this morning while one zone recovered from a 6 degree setback. Without the massive cold water returning from the tank, (the bottom was at 110), the supply temperature got hot quick and heated the zone quicker than if the tank was involved.

I turned up the thermostats and had both zones running for a while. It's great to see things working nicely with a load, delivering hot water to the baseboards, modulating nicely, all without the tank involved. Direct from boiler to zones!

Delivery of hot water quickly (relatively) for reset, and then modulate down and divert hot water to the tank while maintaining the hot water in the loop for delivery to a calling baseboard. I think I like it.

Because of the low distribution system and boiler water volume, I'm supposed to be able to do away with the boiler protection valve, keeping the boiler pump off until a suitable temperature (135). Not sure of what the benefit would be to remove, except for less head loss and maybe natural convective flow during a power outage.

The issues for me are setback recovery, as I've said, and another building source - my wood insert. So, it has to have the 'oomph' to power back from setback, and the ability to heat only one zone, gently. I think keeping the tank out of the equation for 'power' and using the tank for 'gentle' might help.

Still talking myself into an off-season project.

edit: Just would like to say that at high supply temps, response times to the baseboard are almost gazelle like.
edit2: At the high end, 176, the loss of one zone led to boiler burnout. The margins aren't much at that set temp, with a high temp of 176, so it'd be better to set it (external) at something less.