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Post in 'The Boiler Room - Wood Boilers and Furnaces' started by kuribo, Mar 5, 2013.
Got cha....Curious if anyone has actually done it this way? It makes sense....
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Seems like the guys that are having trouble getting storage up to temp turn the three position pump down to lowest setting and then all is well
Thats exactly what I did. I played with pump speed settings for a while (cause you have to play with EVERYTHING when its new to you), but now just leave it on speed 1, unless I overfill the boiler with wood or something, then Ill crank it up to try and help keep the delta T down and keep the boiler from limiting out.
Yeah, mine is set up this way, so I'm confident it works correctly and reliably. There's some details in this recent thread:
(Note that the schematic shows a second injection pump for 'finishing off storage', since that's what I thought I needed when I designed the system. It turns out that with good supply temperature control there's no finishing off to do, when storage is up to temp I'm done, so just turn off the combustion fan. The second pump will independently kick in if the primary pump fails, so it's not a total waste.)
In your case I gather you already have a mixing valve, so you don't need two pumps. (One of the goals of the two pump solution is to avoid the expense of a mixing valve. I couldn't find one for less than $200 at the time, but now they're getting down around $100, so that weighs against the two pump solution.)
To meet Econoburn's 20 degF deltaT requirement you would need to get 160 degF guts for the valve so you would have approximately the deltaT they require. (Or you could conclude that there's no good reason for Ecnoburn's 20 degF deltaT requirement and stick with a 140 degF mixing valve if that's more convenient.) Then a single pump with continuous or bang-bang PID control to maintain supply temperature high enough.
Ideally you'd use a variable speed setpoint controlled ECM pump to send water to storage at the exact temperature desired. The mixing valve will maintain return temperature and the pump will slow down or speed up to maintain supply setpoint.
The Bumblebee would do the trick but as far as I can tell it does not offer reverse acting control. If there is a small, economical, setpoint control ECM pump with reverse action temperature control mode out there it would be just the thing.
What might work well would be a small Grundfos Alpha, Taco 007, or Grundfos 15-58 with a PID controller to maintain supply temperature while the mixing valve controls return temperature.
The underlying point is that if supply temperature is correctly controlled to be high enough then flow to storage will only be about 5 gpm or less (in your particular case). By taking advantage of the low return temperatures of in-floor radiant you can avoid ever needing to pump 20 gpm through your boiler-to-storage circuit.
EWD is usually right on with his advice, but I am missing something here. What I see is that this works as described with low temp return water from storage, but as storage temperature rises gpm flow to storage has to increase, that is, high flow, to deliver boiler output without idling. At storage return temp of 160F and above, essentially all boiler output has to move to storage, which is the 20 gpm at delta-T of 20F for 200,000 btu boiler output.
Also, the operation of my Termovar mixing valve is nearly the same as described with the pumps, controller and relay. With 100F water from bottom of storage, return seen by the boiler is about 150F and output to top of storage is about 170+F. As temp of return from storage rises, supply from boiler to storage also will rise, up to about 190F maximum. I know others have experienced some issues with mixing valve return water protection. I have not had any, and this is now my 6th heating season. In my case the only circulator is a 15-58, about 70 watts total. The 15-58 delivers about 12-14 gpm total flow, divided between return to boiler and supply to storage, as needed to provide return water protection.
That's what I thought originally until it was pointed out that the return from storage is always at 100F, assuming your entire tank is at a uniform 100F to start, a well stratified tank upon filling, and that your flow rate to and from storage is such, given a 185F supply to storage temp, that you are only taking the volume of water out of the tank one time.
We need to put X number of btu's into the tank. We can do it at lower temps and higher flows, or at higher temps and lower flows. The advantage here to high temps/low flows is smaller piping and pumps, if I understand correctly.
With this method, It might be best to match your burn time to the time it takes to empty storage once???
How about this idea? Single speed 20 gpm pump for boiler circulator. Comes on when the boiler is started, TCV limits return temps at start-up. Variable speed pump to storage kicks on when return temps hit 170F and maintains this temp. Draws 4.5gpm at 190F at full boiler output and reduces flow as boiler output falls to keep boiler output at 190F and return temp at 170F. WIll this work? I think I can use a taco 007 for the boiler circulator and a bumblebee for the storage pump using the temp set point. Any comments?
The key there is not to load too much if your storage temps are getting up there. Then the boiler output will be lowering as the load burns out just as storage is approaching capacity - then things just nicely coast to hot storage & cool boiler with constant flow.
Timing, as they say, can be everything.
It looks like the mixing valve wouldn't be doing anything in this case, one way or another all the boiler recirculation flow will come back to the boiler whether or not all, some, or none of the flow short circuits through the mixing valve bypass leg.
Using return temperature to control the injection flow works pretty well and is the simplest. If you control return temperature to a constant 170 degF then supply temperature will start out at 170 degF as the boiler comes on line, then supply temperature will max out at 185 degF or 190 degF as the boiler puts out full capacity, then it will fall back to 170 degF as boiler output declines. Not ideal, but still quite good for a simple economical control.
If you try to control supply temperature with injection flow then there's nothing to stop the injection pump from flooding the boiler with cool water, which the sensor can't 'see' until it is way too late. You could probably find a Tekmar control that would take care of both return temperature and supply temperature with a variable speed injection control for four figures.
Unfortunately the Taco HEC-2 (Bumble Bee) does not appear to offer reverse-acting temperature control, so it won't work for return temperature control in an injection configuration. (I've got a request in to Taco seeking clarification about lack of reverse-acting mode.) But you can do it very nicely with a PID bang-bang control.
But if you're using a mixing valve to control return temperature you could control supply temperature directly using a single pump as described in my post above.
Thanks for the reply....
I see what you mean about the TCV. Have to rethink that.
I thought the taco bumblebee would control to a set temperature? If so, and you set it to control a 170F boiler return, wouldn't it simply vary the injection flow to keep the return temp constant? It seems this is the easiest way to control this system.
I am not following the one pump suggestion: don't you need one pump to circulate the water from the boiler around a loop at 20gpm and another to inject to storage?
However, if you move the P2 sensor to sense the temperature going to storage then you could control temperature to storage and let the mixing valve worry about return temperature, plus P1 would give you plenty of flow through the boiler to keep deltaT through the boiler down around 20 degF. So yeah, if you move the sensor for P2 I think your drawing satisfies all your requirements nicely.
A temperature controlled pump can work in two opposite modes, direct-acting and reverse-acting, a.k.a., heating and cooling. In heating mode more flow makes the sensor see increasing temperature, in cooling mode more flow makes the sensor see decreasing temperature. As far as I can tell the darn HEC-2 only offers heating/direct-acting mode. But you need reverse-acting since the more P2 pumps the lower the temperature to storage.
If you had one pump like a Taco 007 it would provide close to 20 gpm total flow in a conventional one-pump mixing valve storage loop. The trick would be to limit the flow to storage so that supply temperature is hot enough. A single small pump would provide enough recirculation flow through the boiler as long as the flow through the storage loop is low, since the resistance through the mixing valve is low for the three quarters of the flow. But in your situation with distant storage, if you needed to draw the majority of the flow from storage on the second or third lap, then a small pump couldn't sustain the flow needed.
If I move the sensor for P2 to downstream of P2, then if the boiler starts to output too cool of water, P2 will pump less and return temp at the boiler will rise, causing the boiler out to increase and the sensor after P2 sees increasing temp. Doesn't this also require a decreasing flow, increasing temp control?
It seems that indeed, I need a pump that modulates flow to meet a temperature, pumping less to increase temp and pumping more to lower temps.
As for the one pump solution with a mixing valve, that is what I was planning at the onset, but I would need a large pump and the water would make several laps. The injection method makes sense to me as you have explained the advantages, as long as I can get the hardware to control it....Otherwise, I may have to just go back to the one pump with multiple laps strategy...
Thank you for your invaluable help.
Exactly. Bumblebee would be perfect if it did reverse-acting.
But I was wrong again about the P2 sensor placement. It would need to sense supply temperature anywhere on the boiler recirculation loop before the point where P2 draws off. If is was downstream of P2 it would never see what's going on with supply temperature unless it was running, and it would never run as long as downstream temperature was below setpoint since it's reverse acting.
But a PID bang-bang would do reverse acting and would work perfectly since you've retained the return temperature protection mixing valve.
A PID bang-bang control would use a constant timed cycle of a minute or two, I'll use 100 seconds for an example. The controller senses supply temperature and it also senses the rate of change of supply temperature. So as supply temperature rises near setpoint, at some point the controller will turn the pump on for a certain percentage of the 100 second cycle, e.g., 10% means 10 seconds on. Then according to what supply temperature is doing it adjusts the amount of time for each cycle, taking into account historical information. The beauty of a PID is that it can react correctly in the short term while keeping a steady hand in the long term.
You could use a small steep curve pump like a Grundfos 15-58 set on high speed or a Taco 008. The PID would normally run less than half the time while maintaining supply temperature withing a tight range of a couple degF.
Thanks, I will look into PID controllers...Do you have a commercial product in your system you could recommend?
I've got a couple of this type in service:
One for stack temperature control, one for boiler return temperature control, and one for hot tub temperature control. What appears to be the same controller appears with different model numbers as the months go by, all with the same pin-out and compatible instruction sheets.
There's a few others that are less expensive. Just search on e-bay for "PID controller" or "PID temperature controller", sort by price plus shipping and look in the neighborhood of $15 to $40. Need to be sure that the power input voltage is OK (most will accept 100 - 220 VAC, 50 Hz or 60 HZ, but some are 220 VAC 50 Hz only). Also some have SSR DC output only, presumably you need a PID controlled relay contact. It can be hard to tell unless they give the exact model number and then you have to google for the data sheet separately to find out what they're selling. This one looks right:
You need the controller, plus a temperature sensor. Many of them come with a sensor or you can order one separate for a couple bucks. Don't know much about the advantages and disadvantages of different types of sensors, I've been using K-type thermocouple and have found them to be accurate and inexpensive. Just make sure the lead length and physical probe type will work for your design. The ones I got had those threaded dohickies on the end that I had to grind down a little with a dremel to get them to fit in a well.
Also since the internal relays are pretty dinky you may want to add an external relay like a RIB to help insure against relay contact failure inside the controller.
There's a lot of info on home brewery and barbecue enthusiast sites that show what they can do. For background on PID control the widipedia article is pretty good.
So basically, one of these controllers would be wired to the pump through a relay and set to turn the pump on and off to control a temp sensed by the thermocouple located at the control point?
Isn't turning the pump on and off hard on the pump?
Thanks again for having the patience to answer my rookie questions.
Right, that's all it amounts to. In fact in this particular application maybe all you need is a digital aquastat like the Ranco ETC-111000-000 Digital Temperature Controller. You could play with the hysteresis to get a reasonably long cycle and be done with it for $60. Pump comes on at 185 degF and goes off at 182 degF, for instance.
Pump on, pump off, all through the burn cycle.
You might think so, but circulators have low starting torque and small angular moments. They can be turned on and off once a minute for years on end. However relay contacts are supposedly only good for a 100,000 cycles at rated amperage, though they typically do much better.
Sure, look at the Taco 00 variable speed stuff for instance, just more expensive.
Which is why the Bumble Bee would be so nice if it had reverse-acting mode, $190 for pump and control, no PID controller, no relay, no extra junction box and wiring.
Yeah, those 00 taco pumps with variable speed and set point would be the ticket but really expensive...
So, do I understand correctly how this would be set up??:
I would use this to control the pump flow to storage to vary the boiler return temp to get a set 190F boiler out, sensing on the boiler out. As my delta T dropped through the boiler from say 20F at full output (170F return) to say, 5F (185F return at 50,000 btu/hr out) as the burn was near the end, the pump to storage would be cycled by the pid to pump from 4.5 gpm down to 1 gpm.
If I had it set up so that the boiler pump turned on at boiler start up and the storage pump turned on when the boiler return hit 170F, would I even need the TCV?
I don't use a TCV on mine, but I have to control return temperature instead of supply temperature. Without a TVC, if you control supply temperature with injection flow then you can flood the boiler with cool water and the supply sensor won't see it until it's too late. So I control return temperature in lieu of having a TVC, and I get pretty good supply temperature control as a byproduct since when the boiler is working in steady state supply temperature is pretty constant relative to return temperature. But at the end of the burn as boiler output falls off supply temperature falls off.
With your two pump design with TVC you can control supply temperature directly because the TVC takes care of return temperature. I suppose you could add an aquastat that would disable the storage circuit pump if return temperature got too low and do away with the TVC, (edit) which I see now is what you already were suggesting.
I will talk with Taco tomorrow about the bumblebee. If that is a no go, I will end up going with the pid. I did notice in the instructions for one of the pid controllers that it if you connect the sensor leads the wrong way, it will act in reverse....Wonder if something that simple might make the bumblebee do the same???
My other problem is seeing if I can return or exchange the 100 feet of 1-1/2" pex I have already!
I guess if I can't, I can bury it next to some 3/4" or 1" for back up...
Thanks again for all of your help!
page two shows both injection and protection diagrams. let us know what taco says.
What flow rates to storage are you seeing? How far is your boiler from the storage tank and what size pex do you use?
Seems I can return the 1-1/2" pex I already bought. Seems for 5gpm or so, I could get by with 3/4" pex to and from storage....
Ours is a conventional boiler-adjacent-to-storage setup, tanks are four feet away piped with 1.5" black iron. The piping is over-sized to allow gravity-flow power-failure heat dump protection. System deltaT runs about 54 degF and it takes about five hours to take 750 gallon tanks from 107 degF to 161 degF since the boiler only puts out about 67500 btu per hour. So average flow for one lap is about 2.5 gpm, which jibes with my stopwatch on the 15-58 on low speed that normally runs about 27 seconds out of a 80 second cycle, or one third of 7.5 gpm.
IIRC you have something on the order of 200 ft equivalent round-trip taking into account elbows and whatnot. Assuming a Grunfos Alpha 15-55, Grundfos 15-58, or a Taco 008 it looks like a little over 3.5 gpm for nominal 3/4" PEX (0.671" actual) and maybe 6 gpm for nominal 1" PEX (0.863" actual).
Thanks for the info....Yes, around 250 feet equivalent....I'm thinking the 1" pex should do it.
I talked to Taco today and they said the bumblebee is only direct acting. I asked if it might be possible to hack it in some way to get reverse acting and they said they would look into it and get back to me...
I did find a decent deal on a 007 variable speed with set point control that would work, I may go with that.
I also want to call honeywell to see if their AQ252 controller might work with a standard variable speed pump. I need a controller for the system and if that has what I need already on board, it might be an option....
Be careful, it looks to me like the 007 curve is all wrong for that circuit.