Staging A Pellet Boiler With An Oil Boiler With An Ecobee SmartSi Thermostat

[velvetfoot]

Running a pellet boiler at a lower output level is supposed to be good. They say there is less wear and tear from startup and better economy. Thing is, it probably won't have the oomph for those cold days, or if a deep setback is chosen.

I have a couple Ecobee SmartSi thermostats that have staging capability for 2 heat stages (cooling too-but I have no A/C). It turns on the second stage when the first stage can't keep up with keeping the area warm. My thought is to assign the first stage to the zone pumps and the second stage to the oil boiler (they are attached in parallel).

I don't have that much understanding of hvac equipment control, so I measured what happens on the first stage and second stages terminals. It does appear that the first stage remains energized after the second stage comes on. So, stage 1, the zone pumps would run and if they can't satisfy the requirement within a certain amount of time (2 hours max) and at a minimum temp delta (10 deg max) before stage 2, the boiler, is turned on. Or reverse staging could be used which starts stage 1 (the zone pumps) and when the minimum temp delta is exceeded (again 10 deg max), stage 2, the boiler, would fire up and stay fired up until the room temp is again within the stage 2 temp delta, at which time stage 2 turns off and stage 1 brings it to setpoint.

I'm not sure if reverse staging would be best for my setup. As a boost from a deep setback, I can see it, because heat loss through the house envelope would be exceeded by the capacity of the pellet boiler. If it's real cold out I wonder if this would lead to excessive cycling of the oil boiler, with the pellet boiler running flat out as well.

Of course, I could arrange it so that any time it is started up, the oil boiler would run until its aquastat is satisfied and it, along with the pellet boiler, charges up the buffer tank. This would assure no short cycling for the oil boiler. The oil boiler's aquastat would have to be lowered so that the pellet boiler doesn't shut down by its own built-in aquastat.

Another control plan offered by the thermostat is to run stage 1 for up to two hours and if the temp requirement isn't satisfied to some minimum delta, as above, stage 2 would start and would run until the setpoint is reached.

Ecobee's online app would allow analysis of stage 2 run times for tuning.

This is also a solution for unattended switchover to oil if the pellet store is emptied or the pellet boiler is inoperative.

It also has the advantage of eliminating some of the controls initially installed.

I'll update this thread when/if I get something working.

Ecobee thermostat manual: https://ecobee.zendesk.com/attachme...?name=EB-SmartSiIM-01-rev1_april3_general.pdf

 

[cityboy172]

I'm driving right now so I can't look at the ecobee manual but typically there is a there's options for staging between differential temps and a time delay to enable second stage.

 

[cityboy172]

Page 27. Stage two delta and stage one max run time. Heat minimum on time also.

 

[velvetfoot]

Thanks. I'm intrigued by the reverse staging, page 26. It comes down to tweaking things so that the aux heat doesn't come on too much while still maintaining a good comfort level.

Unfortunately, the minimum heat on time probably won't work for minimizing short cycling the second stage oil boiler since it's applicable to stage 1 as well.

Reverse staging excerpt from page 26:
Reverse Staging If enabled, the thermostat
will cycle down from the higher stages so
that as it approaches set point it will only be
running in stage 1. The HVAC equipment will
26
start in stage 1. As the stage 1 temperature
delta is exceeded, the second stage will
engage. Once the equipment has brought
the current temperature back to within the
heat or cool differential setting, stage 2 will
disengage and stage 1 will remain running
until the set point is meet.

 

[velvetfoot]

Interesting articles:
http://www.pmengineer.com/articles/...are-sized-differently-from-conventional-units
(broken link removed to http://www.radiantandhydronics.com/articles/86918-biomass-boiler-cycling)

Mr. Siegenthaler is into the small biomass/aux boiler concept.

 

[velvetfoot]

I am thinking of setting up something like this:

Set point: 69
Differential: .5
2nd stage delta: 2
Smart recovery: On
Reverse staging: On

So, it might go like this on a cold night:
-Room temp drops to 68.5
-Zone circ (stage 1) comes on
-Buffer tank temp drops to sensor 1/3 from top setting of 135 **
-Pellet boiler starts up
-Room temp drops to 67
-Oil boiler (stage 2) comes on
-Room temp climbs to 68.5
-Oil boiler (stage 2) shuts off
-Room temp climbs to 69.5
-Zone circ (stage 1) shuts off
- Pellet boiler continues to run until its aquastat is satisfied (176)

** Perhaps increase this during cold snaps with outdoor reset of some sort in future?

I am concerned about possible short cycling the oil boiler. I don't think it will be a problem in the above cold weather example.
I am hoping the Smart Recovery feature, which starts up the heat at a time determined by the stat to have the room at the set temperature at the set time, does it gently enough so as not to trigger Stage 2.
An alternative is to set up the oil boiler to run until its aquastat is satisfied after stage 2 is triggered.

Another alternative, for now, is to only run oil.

Here is that crude diagram again:

 

[velvetfoot]

Well, it doesn't look like it's going to work out.
When both boilers were running the pellet boiler heated up and shut down prematurely, so it doesn't look like the Ecobee with oil as stage two is going to work out.
I'm making some electrical changes so that the oil and pellet boilers won't be running together.
Maybe the tank temperature can be used to kick off the oil boiler in cold weather, and for sure the 'out of pellets' scenario.
Or, I can just turn on the oil when it gets real cold and it needs it.
Oh well.

 

[cityboy172]

Well, it doesn't look like it's going to work out.
When both boilers were running the pellet boiler heated up and shut down prematurely, so it doesn't look like the Ecobee with oil as stage two is going to work out.
I'm making some electrical changes so that the oil and pellet boilers won't be running together.
Maybe the tank temperature can be used to kick off the oil boiler in cold weather, and for sure the 'out of pellets' scenario.
Or, I can just turn on the oil when it gets real cold and it needs it.
Oh well.

It might respond differently if there's actually a load on it. It will be interesting to see what ends up working.

 

[velvetfoot]

Thanks. There was a load-the 119 gal. buffer tank was down to 130 or so. I'm thinking the thermomix valve might have been involved. I might try fiddling with the pump flows, but I'm thinking a major piping change would be required.

 

[cityboy172]

What are the boilers set at for high limit? Could it be as simple as making sure the fuel oil boiler is set under the pellet boiler?

 

[velvetfoot]

I HAVE A SOLUTION. Hopefully it will be repeatable.

It just seems to have been that the pb and ob pumps weren't set at the same speed. They are B&G Ecocircs, and were originally set at 4 (out of 7) for the ob, and for the pb. I set them both to 2 and got a good thermo fill of the buffer tank. The ob dropped out first, which is desirable, after a 33 minute run time at a tank temp of 174. The pb took it the rest of the way to 176, modulating down over 23 minutes. The timer on the pb circ pump, which runs for 13 minutes, brought the tank to 178. Memo to me: put a timer on the ob circ pump as well.

A funny thing happened on boiler shutdown: the display went blank and I had to hit the on/off button to get it displayed again-I might have accidentally touched the on/off button. We'll see how it works on the next test. Next test might be a while since it's still pretty warm.


The pellet and oil boilers share the same supply and return

 

[velvetfoot]

It is repeatable! Getting good buffer tank fills-hitting 180. Oil boiler drops out and pellet boiler takes it the rest of the way. If there is big load at the time, pellet boiler will just continue to run until ecobee calls for stage 2 again. Will check at some point how well the oil boiler alone fills the tank, but it should be okay.

I have both boilers set so that once either, or both start, they run until their aquastats are satisfied. I think I will keep the oil boiler set that way, since it is not amenable to potential stage 2 short cycling as, say, electric strip heaters.

No problem with the display-I must have touched the on/off switch by accident.

Now, I have to adjust the Ecobee's settings.
I am thinking of starting with this:
Set point: 69
Differential: .5
2nd stage delta: 2
Smart recovery: Off
Reverse staging: On

Note that I am not really using the Smart Recovery action since the boiler will run until its aquastat is satisfied, but didn't want to set a max run time for stage 1 at this point.

I am not sure how the ecobee acts as far as stage 2 heat goes. What setback settings will trigger it, would Smart Recovery avoid triggering it, would a timer setting along with the delta T setting be better, etc, etc.

When the Ecobee is set up for a heat pump, there's an option for auxiliary heating which can be set to only come on at a certain temperature. I believe I have an extra Ranco temperature controller, so I could route its sensor to the outside. If the aux boiler comes on too often, I could and set the Ranco so that the oil boiler won't come on above 10 degrees, or whatever. I might do that now, while the weather is warm.

 

[woodgeek]

Its been awhile since I set my Ecobee Si up for staging....but I think you could def lock out the second stage based on outdoor temp (based not on a sensor, but an internet source). I also recall you have a ton of control....you can also lock it out until after x mins of first stage, y degrees of droop, etc.
But I also recall the manual being a little lean....I ended up calling tech support, which was very helpful. I also seem to remember that I had to call my system a two-stage electric furnace, or something else counterintuitive.

 

[velvetfoot]

Thanks for commenting.

but I think you could def lock out the second stage based on outdoor temp (based not on a sensor, but an internet source).

Only true for heat pump setting, not two stage boiler.

Which specific settings did you use for differential, time, delta T, etc? Is it still comfortable? Have you tracked how much your aux heat comes on? etc, etc, etc.

 

[woodgeek]

It seems to me you can tell it you have a HP+aux if you want....and it will just do two stage heating.

I think I went to small diff with a decent long time lockout. That is, as it gets colder, it 'hovers' on the first stage and cools below the first stage setpoint really slowly. If it is <1°F below for an hour, I want it to fire the second stage...its not going to get better. Similarly, when it is really cold, it will drop below setpoint rapidly on the first stage, so i want it to fire right away.

I did track it....and you set the diffTemp to the minimum to avoid unnecessary calls to the second stage.

 

[velvetfoot]

I did track it....and you set the diffTemp to the minimum to avoid unnecessary calls to the second stage.

That's good info. How about returning from setback?

 

[woodgeek]

Honestly, re setback, I don't really remember. I recall the Nest sucked at it, but I think the Ecobee had better options.

I do remember being impressed. It tries to figure out how early to fire up the first stage to reach the new higher setpoint before the setpoint steps up. In my experience, it usually tried to be a little early, and then when it got close it would take a little break, and then fire again to hit the new setpoint/time point on the nose.

That's during mild weather with first stage recovery. I think the recovery that needs second stage is a bit dumber.

 

[velvetfoot]

I do remember being impressed.

You don't have it anymore?

It tries to figure out how early to fire up the first stage to reach the new higher setpoint before the setpoint steps up

I think they call it "smart recovery".

I think the recovery that needs second stage is a bit dumber.

Which recovery is that? Is that the same "smart recovery" mentioned above only in cooler weather? It sounds like smart recovery might use stage 2 less because of a more gentle return to setpoint.

The idea of locking out auxiliary heat (oil) by outside temperature might have a place at some point as well.

 

[woodgeek]

You don't have it anymore?

I think they call it "smart recovery".

Which recovery is that? Is that the same "smart recovery" mentioned above only in cooler weather? It sounds like smart recovery might use stage 2 less because of a more gentle return to setpoint.

The idea of locking out auxiliary heat (oil) by outside temperature might have a place at some point as well.

I DO still have it, but stopped obsessing and got back to life (after a couple years)

I think it was the default recovery...maybe I set it to smart.

It is possible that the recovery without the second stage works because I figure out myself the balance point (where second stage is needed) and locked out the second stage at outdoor temps much higher than that.

 

[velvetfoot]

I was just thinking about using the outside temperature to lock out aux heat, also known as stage 2 for me. Unlike a heat pump setup, the derated pellet boiler would carry the load way below freezing. If there's a problem with the pb, or it runs out of pellets, the house could freeze.

I guess I could wire it so that outside temp lockout would only interrupt the stage 2 (W2) wire. If there was a failure with the PB, the zone pumps would bring down the buffer tank temp so that the OB would fire.

When it gets cool outside, I could observe the calls for stage 2 heat on Home IQ, fiddle with settings, especially Smart Recovery, all the while not kicking in the aux heat because of the outside temp lockout.

Sorry to bring you back from your happy place.

 

[woodgeek]

Sorry to bring you back from your happy place.

No worries.

The bottom line is that the ecobee can do anything you want if you fiddle with it enough. You could do a lockout below freezing (my lockout is 25°F), and have it send you an alarm by email or text if the house gets to say, 50°F due to a malfunction.

Believe or not...the Nest couldn't due any of these things, that's why I ditched it. Who you gonna get your stat from, some techies in Palo Alto, or hardcore nerds in Toronto?

I'm not sure if the lockout is adjustable over the internet though...many of the setting have to be changed at the unit, I think to stop internet meddling. In this case your fail safe stat, or a friendly neighbor with a key (and a bag of pellets) would take care of things.

 

[velvetfoot]

Aux heat doesn't exist in the non-heat-pump scenario-I am stuck with staging unless I set it to run as a heat pump, and there could be unanticipated results from that. Without aux heat there is no aux heat lock out, hence my consideration of using a Ranco controller with it's sensor outside to switch the W2 wire to lock out stage 2.

Also in my scenario, if stage 2 doesn't fire the oil boiler because it's locked out, the zone pumps will continue to draw down the tank temp until the pellet boiler fires, and if that doesn't happen and the tank drops a little more, then the oil boiler fires. In summary, freeze protection is not dependent on the W2 wire.

I tried bumping up the set point 3 degrees to see what would happen with the 2 degree stage2 delta, and yes, (virtual) stage2 kicked in. So, I think it's a good assumption that it will kick in if the standard recovery option is chosen. So, the deltaT could be bumped up past the setback delta, and also, manual adjusting fingers.

With a deep setback, say during a winter vacation, where one could actually use the internet control, stage2 would come on immediately and heat things up quick, which might not be necessary unless it's pretty cold.

I originally thought of adding supplemental heat by monitoring the tank temp, but I don't know it reacts during cold temps. I'll keep an eye on it.

I might have some history how the upstairs thermostat behaved last winter. Did I mention I run a wood insert on the first floor, which screws up any kind of extrapolation of central heating performance?

Of course, Tekmar, or the like, would probably sell me a digital unit that does all the work of my various and sundry relays, for a fabulous price. Or, someone here will chime in about variable speed pumps, outdoor reset, etc., that would require boocoo piping changes, maybe installing radiant heat, etc, etc. The fact is oil (here) might be cheaper than pellets, so the whole exercise could be moot. But it's "fun", so there's that.

 

[velvetfoot]

I just a nice fill with only the oil running, so that's a good thing if I was to run on oil only.
I set the stage2 deltaT to 6 degrees, to take into account a 4 degree setback (69-65).
Next step: see if my spare Ranco works with an abandoned outside temp sensor that I forgot I had installed.

 

[velvetfoot]

If I have several setback periods, like 64 night/66 day/68 or 69 evening, the biggest set back would be 3 degrees, so I could get by with a 4 degree stage 2 delta T, unless someone manually adjusts to a setpoint which is 4 degrees greater than current. Then, there still is the vacation thing. I'm liking the maximum temp for stage 2 concept.

In the real cold weather scenario, the pellet boiler would be running and the house temperature would keep dropping until the stage2 delta T is exceded, ie, 64 or 65, and if it's cold enough outside, the oil boiler fires and both run until the oil boiler drops out before the pellet boiler because of its aquastat setting, and the pellet boiler continues to run. That's the current idea anyway.

I'm not sure what to do about the winter vacation scenario. If the house was sitting at 60 (cat), if the outside temperature was below the setpoint, the oil boiler would come on and it'd be like the above scenario. Big deal. That's fine.

If I was home and I (or another) was annoyed with a long recovery time, although still above the DeltaT, I could change the DeltaT.

If the house was losing temperature or not gaining temperature at a sufficient rate, I could change the outside temperature lockout for stage2.

Both of those last two scenarios involve time, so maybe I should use that as well as deltaT to kick off stage2.

I wish Siegenthaler would have touched upon control issues in his articles.

 

[velvetfoot]

Well, here is what I have now:
I added a Ranco control with a sensor outside, and tapped into an unused stage on a Ranco controller that senses pellet boiler temperature. So, two hours after call for heat starts, if the pellet boiler temperature is under 145 and the outside temperature is under 5 deg F, the oil boiler will fire. These can be tweaked, but at least the setup is flexible. I don't know how things will actually go when it gets real cold out.