Time to reach operating temps from a cold start?

[Nofossil]

Here is my dilemma: I want to control the output temp of the boiler to say, 180-190F to fill storage in a "one and done" manner. Originally I planned to do this with two viridian temp set point pumps, one on the boiler loop controlling the boiler output temp, and one in the storage loop, with it set to maintain the boiler return temp at 150F. The problem I am seeing is at start-up and when the boiler heat output starts dropping to low levels at the end of the burn. The flow rates/heads at those points in the burn cycle are below the operating point of the viridian pumps. How does your controller control the pump? Is it a bang -bang type of control?

I have a variable speed drive circuit that turns a fixed-speed circulator like the 15-58 or the Taco 007 into a variable-speed unit. If you look at the chart, I have it set for a minimum speed of 30% and a maximum speed of 100%, and it varies the speed as needed to maintain the target outlet temp. For the period in the chart above, the target temp was 180. Depending on the heat loads, I'll use a target temp anywhere from 160 to 190.

The variable speed unit is the Nimbus (from Control Resources). I drive it using 4-20mA from my Vesta.

 

[Pat53]

Nofo, that is exactly what I would like to do, but it sounds kind of complicated....and expensive. I was hoping the Vesta was a simple device that would be easy to set-up and easy on the pocket book. I should have known better...LOL, but thanks for the info.

Pat

 

[kuribo]

I have a variable speed drive circuit that turns a fixed-speed circulator like the 15-58 or the Taco 007 into a variable-speed unit. If you look at the chart, I have it set for a minimum speed of 30% and a maximum speed of 100%, and it varies the speed as needed to maintain the target outlet temp. For the period in the chart above, the target temp was 180. Depending on the heat loads, I'll use a target temp anywhere from 160 to 190.

The variable speed unit is the Nimbus (from Control Resources). I drive it using 4-20mA from my Vesta.

I am not sure how this works in practice during the early and late part of a heating cycle. When I do the math, at start up and near the end of the burn, let's say when the boiler is only outputting 50,000 btu/hr or less, if I have boiler outlet at 180F, inlet at 150F, that's a flow in the boiler loop of 3.3 gpm. At that boiler output, the storage circulator pumping at 180F and returning 80F is only moving 1gpm. Here's the grundfos 15-58 pump curve. The pump isn't able to output at those flow levels at low head losses....So what is happening??? If the pump isn't able to move that heat, where is it going??? What am I not getting???

 

[Nofossil]

I am not sure how this works in practice during the early and late part of a heating cycle. When I do the math, at start up and near the end of the burn, let's say when the boiler is only outputting 50,000 btu/hr or less, if I have boiler outlet at 180F, inlet at 150F, that's a flow in the boiler loop of 3.3 gpm. At that boiler output, the storage circulator pumping at 180F and returning 80F is only moving 1gpm. Here's the grundfos 15-58 pump curve. The pump isn't able to output at those flow levels at low head losses....So what is happening??? If the pump isn't able to move that heat, where is it going??? What am I not getting???

In closed loop piping, head loss is a function of flow rate, so the slower the circulator is running, the lower the head loss presented by the plumbing. To get the flow rate for any system, you have to lay the head loss curve for the plumbing over the pump curve chart and see where the lines cross. Here's a sample head loss chart:

Varying the speed of the circulator basically shifts it's curve downward, but note the head loss approaches zero as flow rates get low enough. You will always get some flow if the circ is running, as long as you have a closed loop.
Having said that, a big air bubble or a slug of very cold water could present a problem. Some controllers (ours included) will run the circ at full speed briefly every so often to flush out bubbles or cold pockets.

 

[Nofossil]

Nofo, that is exactly what I would like to do, but it sounds kind of complicated....and expensive. I was hoping the Vesta was a simple device that would be easy to set-up and easy on the pocket book. I should have known better...LOL, but thanks for the info.
Pat

We're really trying to be all three - simple, easy to set up, and inexpensive. It's all relative, though. If you don't have a complex control problem, it's likely overkill. Not that overkill is a bad thing ;-)

It has been a journey, and a lot of folks here have helped. For that reason, customers on the forum get more support than typical. With our new controllers, we can also offer a discount to forum members.

 

[maple1]

Wondering what the advantage is to heating storage in one pass?

 

[Nofossil]

Wondering what the advantage is to heating storage in one pass?

For me, it's delta T. With one-pass heating, you're getting relatively cool water to the boiler the whole time, so the boiler can be running flat out. Otherwise, the storage bottom temp is rising during charging, and eventually the water going to the boiler is hot enough that the boiler output goes too high and the boiler has to idle.

 

[Kristen]

you're getting relatively cool water to the boiler the whole time, so the boiler can be running flat out

Hadn't really assessed the benefit properly before, but my plumbing has return to boiler from both the middle and the bottom of the store, via a mixing valve (which itself then goes through a laddomat valve which recirculates water back to boiler during early, cold, stages of the burn).

I presume this means that the top half of the store heats first and then, once the temperature is high enough that that becomes inefficient, the return-to-boiler is switched to the bottom, colder, half of the store.

 

[maple1]

For me, it's delta T. With one-pass heating, you're getting relatively cool water to the boiler the whole time, so the boiler can be running flat out.

I suppose that is true, yes. My boiler only has min. temp (140) water for the first lap of storage. But that first lap takes about 2/3 of my burn time, as there is quite a bit of flow around the bypass to compensate for the fully depleted water coming from storage - but quite a bit going to loads.

My thought though was that if the boiler was putting out max hot water for the whole burn, you would be losing some efficiency on the other end by a bit more heat being lost to the flue since the boiler was max hot for the whole burn, rather than only the last lap - I think cooler boilers are more efficient boilers? (Roughly speaking).

 

[kuribo]

In closed loop piping, head loss is a function of flow rate, so the slower the circulator is running, the lower the head loss presented by the plumbing. To get the flow rate for any system, you have to lay the head loss curve for the plumbing over the pump curve chart and see where the lines cross. Here's a sample head loss chart:
View attachment 172038
Varying the speed of the circulator basically shifts it's curve downward, but note the head loss approaches zero as flow rates get low enough. You will always get some flow if the circ is running, as long as you have a closed loop.
Having said that, a big air bubble or a slug of very cold water could present a problem. Some controllers (ours included) will run the circ at full speed briefly every so often to flush out bubbles or cold pockets.

Yes, I am familiar with head loss and it's relationship with flow rate. The issue I am trying to articulate is the low flow/low head regime which the pump would need to run at in the first and last stages of the burn. If you lay the system curve over the pump curve, you find you need to operate the pump at points below the pump curve in an area where the pump is not able to produce flow.

 

[Nofossil]

My thought though was that if the boiler was putting out max hot water for the whole burn, you would be losing some efficiency on the other end by a bit more heat being lost to the flue since the boiler was max hot for the whole burn, rather than only the last lap - I think cooler boilers are more efficient boilers? (Roughly speaking).

Cooler is slightly more efficient, but idling is considerably less efficient. In my case, bypass circulation reduces effective boiler output flow rate *and* I vary the speed of the boiler circ to get the output temp that I want. Where I have a problem is that I don't have pressurized storage so I'm heating through a HX coil in the tank. As the tank heats, the return temp rises (just like unstratified pressurized storage would). At some point, the highest possible flow rate through the boiler isn't enough, and I have to throttle back the burn to prevent overheating. See the chart below - as inlet temp rises, the outlet temp gets too hot and I have to drop the combustion temp target from 1200 to 700.

 

[maple1]

Hmm, OK - I'm pressurized, and my boiler doesn't even have the ability to idle.

One thing seems certain in this stuff - one size does not fit all.

 

[Nofossil]

Yes, I am familiar with head loss and it's relationship with flow rate. The issue I am trying to articulate is the low flow/low head regime which the pump would need to run at in the first and last stages of the burn. If you lay the system curve over the pump curve, you find you need to operate the pump at points below the pump curve in an area where the pump is not able to produce flow.

You'll always get flow if the impeller is spinning (thermal lock aside for the moment).

Let's assume that you have plumbing that gives you 10' of head loss at 6gpm. That curve just happens to coincide with the 'speed 2' line on the Grundfos chart, so at speed 2 you'll get 6gpm.

That curve for your plumbing extends to 0' at 0 gpm - it's not a horizontal line. It probably crosses the 'speed 1' line at a little over 4 gpm, and crosses the 'speed 3' line at around 7gpm - those are the flow rates you could expect to get at those settings.

If you take the grundfos speed lines and shift them downwards, you'll intersect your plumbing curve at lower and lower flow rates, but always above zero until you shift them right off the page.

 

[Nofossil]

Hmm, OK - I'm pressurized, and my boiler doesn't even have the ability to idle.

One thing seems certain in this stuff - one size does not fit all.

Boy, is that the truth!

What happens in your system if the output temps get too high?

 

[maple1]

Boy, is that the truth!

What happens in your system if the output temps get too high?

My NO zone valve on the top of the boiler opens (controlled by an aquastat setting - currently set to 200f, I believe), and heat dumps to my upstairs bedroom zones. This is my heat dump circuit. So also opens on power outage.

 

[kuribo]

You'll always get flow if the impeller is spinning (thermal lock aside for the moment).

Let's assume that you have plumbing that gives you 10' of head loss at 6gpm. That curve just happens to coincide with the 'speed 2' line on the Grundfos chart, so at speed 2 you'll get 6gpm.

That curve for your plumbing extends to 0' at 0 gpm - it's not a horizontal line. It probably crosses the 'speed 1' line at a little over 4 gpm, and crosses the 'speed 3' line at around 7gpm - those are the flow rates you could expect to get at those settings.

If you take the grundfos speed lines and shift them downwards, you'll intersect your plumbing curve at lower and lower flow rates, but always above zero until you shift them right off the page.

Here is the operating regime of the Taco Viridian VT2218, which you are using. My required circulator flow at low boiler output of 50,000btu/hr are about 1 gpm at 0.55 ft of head. My boiler circ flow would be 3.3 gpm at a even less head. Both of these points are below the lower limits of the operating regime of the Viridian pump, which is to say, they are below line A in the pump curve. Taco tells me the pump does not run at speeds below line A.

You mentioned a 30% minimum speed setting- wouldn't this be overpumping at the beginning and end portions of the burn when you would need flows below 30% speed? Assuming of course your boiler output is still at a set 180F (or whatever)...Perhaps your controller does indeed continue to reduce speed of the Grundfos down to very low levels...???

 

[Kristen]

At some point, the highest possible flow rate through the boiler isn't enough, and I have to throttle back the burn to prevent overheating

Could you dump heat into the house at that point (well, presumably slightly before that point as I doubt you will "instantly" loose enough heat). Probably only worthwhile if your house has good thermal mass.

We do that, deliberately overheating the house by a couple of degrees in the evening, when the boiler is burning, and I think! that the net effect is that the following morning the house temperature is, say, 1-degree warmer as a result and therefore need 1-degree less heating to get it back up to morning temperature.

When my boiler throttle's back it just smoulders the wood, so in effect the fuel is wasted. Yours might be different?

 

[Nofossil]

Viridians do have a lower limit. In our experience, we can run a 15-58 slower and still have flow.

I use 30% as a minimum because lower speeds don't seem to help, and I suspect the relationship between percentage drive and flow rate is not linear - 30% seems pretty close to zero.

I do use a higher room temperature setpoint when heating from wood (and a *much* lower one when heating from fossil fuels). I also dump heat into the basement zone if the outlet temp gets really high.

When I throttle back, I try for a combustion temp that's high enough to sustain gasification. The factory controller just shut off the fan, with occasional purge cycles to prevent explosions.

 

[goosegunner]

Hey gg. I know you weigh your wood. Does the 100 pounds sound right to you for a full fire box?

Probably, I usually put about 60-70lbs in at start and set the timer for 2 hours. If I left for work my wife makes sure the the fan turned off and closes the window.

Night time during cold weather, 60lbs to start. Go back out after 1 to 1.5 hours. stir and add more wood. Output stays much better with a stir over nozzle. I have weighed wood for a long time. I use 5000 btus/lb and even sometimes that is too much. I doubt that the econoburn can get much more output to tank than 5000 btu/lb of wood. The fan volume sends too much heat out the stack. If the speed could be controlled it would help. That is more complex. sometimes simple is better for the masses.

gg

 

[kuribo]

Viridians do have a lower limit. In our experience, we can run a 15-58 slower and still have flow.

I use 30% as a minimum because lower speeds don't seem to help, and I suspect the relationship between percentage drive and flow rate is not linear - 30% seems pretty close to zero.

I do use a higher room temperature setpoint when heating from wood (and a *much* lower one when heating from fossil fuels). I also dump heat into the basement zone if the outlet temp gets really high.

When I throttle back, I try for a combustion temp that's high enough to sustain gasification. The factory controller just shut off the fan, with occasional purge cycles to prevent explosions.

Perhaps the ability of the grundfos to pump down to very small flows is the difference that allows your system to function at low output....

Thanks for the info.

 

[kuribo]

Probably, I usually put about 60-70lbs in at start and set the timer for 2 hours. If I left for work my wife makes sure the the fan turned off and closes the window.

Night time during cold weather, 60lbs to start. Go back out after 1 to 1.5 hours. stir and add more wood. Output stays much better with a stir over nozzle. I have weighed wood for a long time. I use 5000 btus/lb and even sometimes that is too much. I doubt that the econoburn can get much more output to tank than 5000 btu/lb of wood. The fan volume sends too much heat out the stack. If the speed could be controlled it would help. That is more complex. sometimes simple is better for the masses.

gg

Just for clarification, I calculated by theoretical values as follows: from Econoburn, 8 ft3 firebox, from Siggy, 15lb of wood/ft3, gives 120lb. Used 100 as a conservative figure.

I see 6000 btu/lb quoted for dry firewood. I used this and then used 75% efficiency for the Econoburn. Maybe generous, but it was just a ballpark estimate...

Also, Econoburn people told me they get 4-5 hour burn times...

Seems it wouldn't be too difficult to set the fan up with a speed controller tied to exhaust temps...

 

[Pat53]

We're really trying to be all three - simple, easy to set up, and inexpensive. It's all relative, though. If you don't have a complex control problem, it's likely overkill. Not that overkill is a bad thing ;-)

It has been a journey, and a lot of folks here have helped. For that reason, customers on the forum get more support than typical. With our new controllers, we can also offer a discount to forum members.

Will the Vesta system work with any Grundfos or Taco circulator?

 

[Bob Rohr]

Keep in mind when slowing down a pump, once the flow goes laminar, very little heat is exchanged. Either out of the boiler or into the HX or distribution system. That is why you see a truncated pump curve on those VS pumps.

It is a debateable point where flow turns laminar, typically below 2 FPS (feet per second) is what the industry agrees on.

Next you would look at pipe size in the hx or distribution to see at what gpm you drop below 2 fps.

Also as Nofossil discovered, air removal becomes difficult below 2 FPS, as the water and air tend to separate, especially in vertical piping. Fluid may be flowing down, but air can rise up inside the fluid. Always have a float type air vent at the high points in the system.

Speeding the pump up occasionally is also a great idea to assure air is shoved back to the air seperator.

Here is a cheat sheet to show required GPM to assure adequate velocity in various pipe sizes.

Remember also, air can form in systems even after a complete and 100% purge. Restrictive piping devices or at the hot surfaces of the boiler wall, for example.
If you have a chronic air problem, snoop around for the causes.

Always use a top quality air removal device in the proper location, the hottest point in the piping, typically close to the supply off the boiler.

 

[Nofossil]

Will the Vesta system work with any Grundfos or Taco circulator?

It won't work with ECM circulators, and it's limited to 200 Watts. It works with any of the smaller 'standard' circulators like the Taco 007 and the Grundfos 15-58. It also works with most fans / blowers.

 

[Pat53]

I've got a Taco 010 3-speed on my boiler loop