Adding a pellet boiler and buffer tank

skalor · Sep 24, 2016

Bought the house in 2014 and the propane boiler had been replaced in August of 2013 IIRC. It's a 140k Crown Aruba 4(CI 83%) and the very first winter I noticed is was short cycling a lot. I immediately started researching about boilers as I'm not the type that hires anyone to do anything. I found that the boiler was massively oversized for my house, 2500 sq ft ranch. The house has about 60k btu of baseboards with three zones. I've been tracking the cycle time for the past two winters and found that on a day with a 17°F high/0° low that the boiler duty cycle was 37%. Besides being massively oversized there were quite a few things wrong with the setup. They had a 007 installed horizontally but the motor was on the top and the air scoop is after the circulator. The zone feed seems too small as well as it's 1" feeding three 3/4" zones. One point I feel lucky about though is that we moved in after the winter of the propane shortage but that has always weighed heavily on my mind. I decided that I was going to add a buffer tank and a pellet boiler so that I wasn't at the mercy of one type of fuel. This past spring/summer I picked up a used Harman Hydroflex 60 and a 120 gallon buffer tank. I've been reading a lot on the forums and have picked up a lot of useful information but there is always more to learn as this is not my field.

Basic plan layout is the propane boiler and pellet boiler run in parallel feeding the buffer tank. I'm going to leave the propane boiler controls intact and just use an aquastat in the buffer as a backup for when the pellet boiler runs out of pellets or I turn it off for cleaning. Just going to run 1 1/4" and a 007 for the propane boiler. I'm not 100% on the HF60s controls but from what I understand it will modulate in Auto mode to maintain a temp between the mix and max settings on the front. I'm going to run 1" and a Taco VT2218 in boiler protection mode so as to not cause creosote buildup. The circulator feeding the zones from the buffer is going to be an Alpha 15-55. I want to use a TACO ZVC-406 for control of the zone valves and house circulator. I have drawn up some rough schematic of the layout but I'm not sure anyone here wants to see my chicken scratch.

Just started with locating and thinking about how I'd like the layout to be. My basement has a garage door so I can bring in full pallets of pellets which is convenient.

maple1 · Sep 24, 2016

I have an Alpha, but I don't run it this way so can't say for sure - but you might not need a zone controller for it. The lit implies that it can sense a zone valve opening, so as long as there is power to the Alpha, it will ramp itself up & down as it senses a change in pressure differentials caused by zone valves opening & closing. They have a wiring option for just simply plugging it into an outlet.

It is one sweet circulator. IMO any system with multiple zones & varying loads served by one pump, should be using an Alpha.

PassionForFire&Water · Sep 24, 2016

Control schematic from Siegenthaler

Bob Rohr · Sep 24, 2016

I'm not convinced a ∆T circ just piped to the boiler is sufficient to assure return temperature protection. A large cold load like a concrete slab or high volume buffer will overpower that circ as shown in the manufactrers literature.

This is a better way, yes more piping. It does to assure 100% return protection IF you want to use a ∆T pump.

Read the entire journal to get some ideas and pros and cons of each method.

A two pipe boiler buffer tank answers most all piping and control challenges.

http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_17_na.pdf

velvetfoot · Sep 24, 2016

Couple diagram issues? S2=165? S2 is outdoor temp, no? Then on the outdoor reset controller, what's going on with S1?

Caveat: I might not know what I'm doing.

To summarize the concept, during pellet boiler operation the lower tank temp is varied based on outside temp. With no pellet boiler operating the mod/con backup keeps the top of the tank only warm, and when there is a call for heat it fires to supply the demand, maybe heating the buffer tank (only to the extent that there is more heat produced than used when the backup boiler is modulating all the way down?). It looks like the use of the backup boiler is similar to when no buffer tank is used.

The idea seems to me is to keep the amount of heat stored in the buffer tank as low as possible, while only using the auxiliary boiler to heat zones, and not the tank, when the pellet boiler is unavailable.

This concept might not be best in this case since the backup boiler seems to be a cast iron non-mod/con boiler.

In my case, with a cast iron oil-fired boiler, I figured it might be a good thing as far as soot and (maybe standby losses from the oil boiler jacket and piping) to have it fire fewer times but for a longer duration. It's generally 45 minutes to heat up my 120 gallon tank with the oil boiler. I also heat up the tank all the way (no reset) with the pellet boiler, figuring long runs are good for it as well.

As I recall the HF60 does reset.

You're thinking about where to put the HF60 exhaust, right? There are rules, like clearance to windows and doors, and I guess you want a little height too for draft in case the power goes out-all the usual pellet things I guess that the manual has in it.

I'm not clear on the Taco VT2218 pump's use for boiler protection-maybe you could talk about that. It would still circulate the pellet water, right? I used a callefi boiler protection valve.

It's too bad you couldn't rig up some kind of automatic fill for the pellet boiler. I built a 4x4x4 box for mine, but the boiler came with automatic vacuum fill capability. Maybe you could think about where you'd put a box like that in your layout, or not. Plus, it's probably best to have components as close as possible, with room for maintenance. Yeah, don't forget about maintenance access. And pellet storage. Since you have to feed manually, you could get a pallet jack and move one pallet close to the boiler, so you'd want to have some room for that.

I did (and still do, lol) a lot of agonizing about things, as you can see in the install link in my signature.

There was a guy on the forums who installed an HF60 and was helpful to me: VT_Bubba , I believe.

skalor · Sep 24, 2016

maple1 said:
I have an Alpha, but I don't run it this way so can't say for sure - but you might not need a zone controller for it. The lit implies that it can sense a zone valve opening, so as long as there is power to the Alpha, it will ramp itself up & down as it senses a change in pressure differentials caused by zone valves opening & closing. They have a wiring option for just simply plugging it into an outlet.

It is one sweet circulator. IMO any system with multiple zones & varying loads served by one pump, should be using an Alpha.

Definitely have to look into that and maybe even try it out before purchasing the zone controller. Trying to keep costs in check so that would be a good savings.

skalor · Sep 24, 2016

Bob Rohr said:
I'm not convinced a ∆T circ just piped to the boiler is sufficient to assure return temperature protection. A large cold load like a concrete slab or high volume buffer will overpower that circ as shown in the manufactrers literature.

This is a better way, yes more piping. It does to assure 100% return protection IF you want to use a ∆T pump.

Read the entire journal to get some ideas and pros and cons of each method.

A two pipe boiler buffer tank answers most all piping and control challenges.

http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_17_na.pdf

I was actually planning on setting the aquastat, that drives the propane boiler, for 170° with a 30° differential. That way the buffer tank should never drop below 140° and the pellet boiler shouldn't see incoming water colder than that. I've thought about adding a mixing/protection valve on the pellet in case the propane boiler is ever offline though.

skalor · Sep 24, 2016

velvetfoot said:
Couple diagram issues? S2=165? S2 is outdoor temp, no? Then on the outdoor reset controller, what's going on with S1?

Caveat: I might not know what I'm doing.

To summarize the concept, during pellet boiler operation the lower tank temp is varied based on outside temp. With no pellet boiler operating the mod/con backup keeps the top of the tank only warm, and when there is a call for heat it fires to supply the demand, maybe heating the buffer tank (only to the extent that there is more heat produced than used when the backup boiler is modulating all the way down?). It looks like the use of the backup boiler is similar to when no buffer tank is used.

The idea seems to me is to keep the amount of heat stored in the buffer tank as low as possible, while only using the auxiliary boiler to heat zones, and not the tank, when the pellet boiler is unavailable.

This concept might not be best in this case since the backup boiler seems to be a cast iron non-mod/con boiler.

In my case, with a cast iron oil-fired boiler, I figured it might be a good thing as far as soot and (maybe standby losses from the oil boiler jacket and piping) to have it fire fewer times but for a longer duration. It's generally 45 minutes to heat up my 120 gallon tank with the oil boiler. I also heat up the tank all the way (no reset) with the pellet boiler, figuring long runs are good for it as well.

As I recall the HF60 does reset.

You're thinking about where to put the HF60 exhaust, right? There are rules, like clearance to windows and doors, and I guess you want a little height too for draft in case the power goes out-all the usual pellet things I guess that the manual has in it.

I'm not clear on the Taco VT2218 pump's use for boiler protection-maybe you could talk about that. It would still circulate the pellet water, right? I used a callefi boiler protection valve.

It's too bad you couldn't rig up some kind of automatic fill for the pellet boiler. I built a 4x4x4 box for mine, but the boiler came with automatic vacuum fill capability. Maybe you could think about where you'd put a box like that in your layout, or not. Plus, it's probably best to have components as close as possible, with room for maintenance. Yeah, don't forget about maintenance access. And pellet storage. Since you have to feed manually, you could get a pallet jack and move one pallet close to the boiler, so you'd want to have some room for that.

I did (and still do, lol) a lot of agonizing about things, as you can see in the install link in my signature.

There was a guy on the forums who installed an HF60 and was helpful to me: VT_Bubba , I believe.

Yes, I figured I would keep the buffer fully charged temp wise all the time during heating season. Maybe in the future I can add a reset controlled mixing valve that feeds the house zones.

At some point I do want to add a large hopper for the HF60 but this is step one to see if I like it and whether it's worth continuing to upgrade. I have plenty of room and there are no upstairs windows within 10' if I run the flue through the wall on the right. There is a basement window about 6' away but the flue exit should be above that and I can lock it permanently closed. There is also cedar out there currently but that's coming down now that the weather is getting cooler. I'm an Electro-mechanical Tech and I'm the controls guy at my work so setting up a couple sensors, PLC/microcontroller, and a motor feeding the HF60 is no big deal. The only caveat would be that it should be air tight like the HF60 hopper. Right now if I move some shelving that is on the right of the HF60 I can fit three pallets.

Bob Rohr · Sep 24, 2016

This drawing is trying to address all the potential of the components. Back up boiler only heats a small portion of the tank to maintain DHW, really no reason to enable the entire buffer volume with a modulating output boiler.

Also included is a NC valve for power outage.

Sorry to jump in mid stream, use the download and start on page 51 to get a better grasp on this fairly complicated piping setup.

velvetfoot · Sep 25, 2016

Here is a thread I actually started on the HF60. As I said, VT_Bubba was very helpful.
https://www.hearth.com/talk/threads/harmon-hydroflex-60-pellet-boiler.81612/

Another thing I can say is think about how you want the flow to go and use check valves if there's a chance it'll go the wrong way.

skalor · Sep 26, 2016

velvetfoot said:
Here is a thread I actually started on the HF60. As I said, VT_Bubba was very helpful.
https://www.hearth.com/talk/threads/harmon-hydroflex-60-pellet-boiler.81612/

Another thing I can say is think about how you want the flow to go and use check valves if there's a chance it'll go the wrong way.

That thread is probably what sold me on picking up the HF60 and subsequently signing up for the forum. It was installed by a stove place and I don't think they had any idea what they were doing and the guy I bought it off of didn't seem too knowledgeable about it. He had a lot of trouble with it, only ran it for one winter, and I paid $1200 for it.

I was going to run a check valve on the output of each boiler near the buffer tank. The VT2218 has an integrated flow check, do you think that it adequate or run that and the check valve. I was just going to run swing check valves or is there something better?

I'm probably going to be ordering most the of parts this week. I was busy this weekend closing the pool and with family functions. There is far too much to do than I possibly have time for but that is mostly my own fault. haha

velvetfoot · Sep 26, 2016

I used spring check valves, but I'm no expert!
Another thing I thought of is to use more equipment isolation valves rather than less.
Do you have to drill through concrete to get the vent outside? I drilled a couple of 4" holes for the oil fill/vent pipes when I relocated the tank, and I'm not sure what diameter hole you'd, need, but it went pretty smooth for me with a water cooled diamond core drill I rented; I did a thread on that.

skalor · Oct 15, 2016

Starting piping the primary side. It's 1.25", on the propane boiler side, with the extra 1' lines for the pellet boiler, that I have but don't plan on running this winter. It's 1.5" on the buffer tank side of the tee.

skalor · Oct 18, 2016

This is the closest diagram for how I'm plumbing it.

Screen-Shot-2016-01-12-at-12.36.03-PM.png

Would it make a difference efficiency wise to run the buffer at a lower temp? Would it be better to run a reset controlled mixing valve for the zones or run a thermostatic mixing valve on the boiler side and a reset controlled aquastat on the buffer?

velvetfoot · Oct 19, 2016

Again...what do I know?

You have a 140btu/hr oil boiler that's like a bull in a china shop. I would think you'd want to use more of your buffer tank rather than less. (Can you run it at a lower firing rate? I was able to go from 105 to 75 on my oil boiler). Again, why not use your buffer tank to extend its run time?

I told you my philosophy earlier on the oil. Last night it took an hour and 9 minutes to charge up my well insulated buffer tank starting at a top tank temp of 127, with my 75k (net) oil boiler, with no zones running. I figure it's good to let the oil boiler run a little and the buffer tank is insulated better than the oil boiler as far as standby loss. Remember, you oil boiler is on or off...there's no mod/conning going on.

On the hydroflex pellet boiler, you should look, but I think it's got outdoor reset built in. If you wanted to experiment, you could maybe use that to change the buffer tank maximum temperature. My understanding though. is that you want longish run times for you pellet boiler, like a couple of hours average, so that may argue against it.

You've got baseboard zones, so it likes 180 for its rated output. The way you have it now, the zones will run for longer and longer cycles as the buffer tank temp goes down. That's the way mine is. If you mix the water for the zones, you can run the zones, maybe, nearly constantly, with no or minimal cycling of the zones. There'd be less pipe creaking, at a minimum, and less slugs of cool water (from the zone pipes) moving around. It would use more electric energy though, I imagine, but my feeling is that it wouldn't be more efficient from a heat point of view. Remember also, that it's only a 120 gallon buffer, not a 1000 gal storage tank.

You didn't mention how you heat your domestic hot water. That could be an issue.

skalor · Oct 27, 2016

Finished everything up last weekend, pressure tested and filled Monday, and then wired in the new controls because it was supposed to drop down to freezing Tuesday night. When I took it apart two weekends prior we had a couple 80° days so I thought I had more time. haha

It didn't turn out to bad. Nothing is wired in yet in these pictures. I have two aquastat wells on the buffer just to see if there was going to be a difference in temp but I didn't find much difference. I only checked it immediately after a boiler cycle to make sure I wasn't getting really high temps at the top of the buffer. I'm using the lower well with a cheap $16 digital temp controller from Amazon that calls the boiler to run through the TT contacts. It's set for 175° with a 20° differential just to be safe with supply temps to the propane boiler for the time being. Temp difference for the Propane boiler to the buffer is 9° when it's running. So I have the boiler set to a high limit of 185° but it's usually at 183°/184° when the buffer temp controller is satisfied. I have the boiler overrun set for 7 minutes and the temps between the two equalize to the 180°/181°. I wired the zone circulator with a 24VAC Omron relay that is tied to the end switches of the zone valves. Working really good so far with run time right at 10 minutes just charging the buffer alone and that was when I had the buffer differential at 15°(170°/155°). Much longer runs than this boiler has ever seen. The boiler has been in operation for three winters and has 2,100 hours and 31,000 cycles which equates to 4 min average run time. Next up is getting the Pellet boiler plumbed in but I'm not in a hurry because I'd like to collect some more data to see what difference the buffer makes in the propane boiler efficiency wise.

Still a couple small things to do like add a length of pipe to the buffer relief valve and I'd like to insulate the lines as well. And yes I flipped the zone pump around so it's right side up. haha

velvetfoot · Oct 27, 2016

I can't say I understand it all, but where is your lower sensor? Having it turn on with the lower sensor at 155 seems too much to me. My sensor on the bottom of the tank gets WAY low before the top sensor gets cool enough where I want to start up the boiler. I think you could get way longer run times on the propane boiler.

skalor · Oct 27, 2016

velvetfoot said:
I can't say I understand it all, but where is your lower sensor? Having it turn on with the lower sensor at 155 seems too much to me. My sensor on the bottom of the tank gets WAY low before the top sensor gets cool enough where I want to start up the boiler. I think you could get way longer run times on the propane boiler.

The lower well is even with the boiler input on the buffer. I've seen the temp controller read 155° and the boiler read in the mid 140° range. I just have the temp controller probe sitting in the well at the moment so it's probably not 100% accurate. Definitely something I'll continue to tweak to get the run time as long as possible. It should be about 15 minutes with the 20° differential I haven't timed it again.

velvetfoot · Oct 27, 2016

I'm just curious.

So, the boiler heats the buffer tank with the connections on the bottom of the tank that are pretty close to one another? Are these just connections to the tank, or is there a coil or riser pipe inside the tank?

I still think you could get more out of the tank by having the 'turn on' sensor closer to the top of the buffer tank. If no pump, or even just the boiler pump, is not running, the water in the tank gets stratified, and it's way more hot at the upper regions. Is there a well a little way down from the top? Even if you put the 'turn on' sensor right at the top and set it to 155, I bet you'd get enough hot water until the boiler caught up.

skalor · Oct 28, 2016

velvetfoot said:
I'm just curious.

So, the boiler heats the buffer tank with the connections on the bottom of the tank that are pretty close to one another? Are these just connections to the tank, or is there a coil or riser pipe inside the tank?

I still think you could get more out of the tank by having the 'turn on' sensor closer to the top of the buffer tank. If no pump, or even just the boiler pump, is not running, the water in the tank gets stratified, and it's way more hot at the upper regions. Is there a well a little way down from the top? Even if you put the 'turn on' sensor right at the top and set it to 155, I bet you'd get enough hot water until the boiler caught up.

Yes, the connections near the bottom are the boiler feed and return and are only about 6" apart, on center. This is the tank I'm running, no coil and no riser is listed in the literature.
http://www.bradfordwhite.com/sites/default/files/product_literature/407-B.pdf

I removed one of the anode rods in the top and that's where the top aquastat well is located. Like I said, I have not compared the temperature difference top and bottom after it sat for a while, only after a boiler cycle to make sure I was not getting excessive high temps in the top of the tank. I'm sure it was well mixed at the time. I'm planning on adding another temperature controller in the top well in series with the bottom controller and set to 185° or so, just as a safety precaution on max tank temperature. The bottom controller is still going to be what ultimately controls the boiler as I don't want the boiler to condense. Maybe I'll add a mixing valve next year but I still think this is big improvement over the old setup. Going to plumb in the pellet boiler next week but I still need to figure out my venting setup and drill the hole through the foundation.

velvetfoot · Oct 28, 2016

We have different setups so it's hard for me to relate, and it's not like I do this for a living. With mine, the heated boiler water has the option of being pumped through the zones if heat is called for, or it goes to the top of the tank and returns from the bottom of the tank. I do have a Caleffi boiler protection valve that's in front of both boilers for protection, but I've been told that's unnecessary when the pellet boiler has a pump that is set to start up at X degrees (for me X=140).

As an added wrinkle, and probably not applicable to you, I put in another electric three way mixing valve that mixes the tank water so that return is at 155. To tell the truth, I don't know if it makes that much difference with only a 120 gallon tank, but my idea was the higher boiler temp might help with a faster recovery from thermostat setback. When the tank is cool, even with the boiler protection, there's more heat being sent to the zones rather than recirculating until the boiler comes up to temp.

You have to do what you have to do with your setup, but my point earlier is that the tank really stratifies when no pumps are running. I just looked and the top of my tank is 157 and the bottom is 84. I have it set so that the boiler comes on when the tank top is 130. Catching up to a heat load at that point might take longer, especially with the pellet boiler since it takes a little while to crank up. I get my domestic hot water from a coil in the tank so my zone circulators turn off at 120. So far, so good. You have a gas water heater so you have no concerns there.

How are you tying in the pellet boiler?

On the venting, I don't know if I already said this, but I drilled a couple of 4" holes through a concrete foundation when I relocated my oil tank. It was one of the smoother operations I've performed. I rented a Huskvarna wet core drill from an equipment rental place. It was light, especially compared to others I saw at hardware store rentals. You have to bolt the thing to the wall and it would've taken two people to lift and mount the ones from the hardware store. I also covered the area with a sheet of plastic hung from the ceiling and put the bottom of the sheet into a big plastic tote I had. The sprayed watery slurry hit the curtain and ran down into the tote.
I'll try to paste a link: My Windhager BioWin 100/150 Install Thread

skalor · Dec 26, 2018

Just wanted to post an update that I finally finished the HF60 install. Been running it for a week now and it's been working great. I was concerned because the guy I bought it from said he had trouble heating his house with it and I have a bigger house. I think he was running it as a regular boiler, so when the thermostat called for heat it would fire up. I'm running it in manual mode at max 160° and min 150° with the deltaT circulator set at a 15° differential, which is always at 9 watts. With constant circulation between the 120 gallon buffer and pellet boiler, the pellet boiler is very responsive to changes in buffer tank temperature at the bottom of the tank. The outside temperatures have been in the 40° range during the day and 20° at night. It has had no problems so far and every time I check on the buffer temps they are at least in the 150° range if a zone is running and the boiler is back to idling about 10-15 minutes after the call ends. The box above the buffer tank are the temperature probes for it at the top and bottom. The top controller cool contacts controls an overheat zone, the largest zone in my house at ~90', and turns on at 185° so the pellet boiler doesn't overheat. The top and bottom controllers are in series on the heat contacts and calls the propane boiler should the temperature drop below 140°, which hasn't happened yet. Just have to run a dedicated outlet for the pellet boiler and maybe permanently affix the temperature probe I have in it. I know it hasn't been that cold but I have confidence in this little HF60 due to the fact that I'm only running it at 160° and it's been keeping up just fine. I know I might have to turn it up when it gets colder outside and I wish I had the OAT sensor for it so I won't have to fiddle with the max temperature setting all the time. Definitely looking to pick one up soon.

velvetfoot · Dec 26, 2018

Your project took longer than mine, and looks a bunch better.

At this point, I have a hard time understanding what I wrote and did, never mind you!

I look at the buffer tank as a way to absorb excess heat from the pellet boiler and to let it run longer, so that the ignitor lasts longer.
I also charge the buffer tank with my oil boiler when that runs, figuring the longer run times are good for it as well.

skalor · Dec 26, 2018

Propane was fairly cheap the last two seasons and it hardly seemed worth it to even bother. I actually had it all plumbed in for last winter but didn't have the flue done. I've calculated only saving ~$125 a month so it's not a whole lot for the extra work put in. Going to run it for a month straight to see pellet usage and cost. Propane cost has jumped for me because the company that I was using got bought out and I don't own the underground tank, which was installed by the previous home owner. This is the first time winter I've paid over $2.00/gallon

velvetfoot · Dec 26, 2018

LOL, I've been burning oil the last two seasons. Probably should look at the numbers again.

PS: The wood insert is running all the time as well, so there is that.

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