Here is my summary. I have been doing lots of reading and essentially settled on the simplest pressurized storage setup with one minor modification in that I have two different temp areas. My thought was I wanted to keep pumps to a minimum so I have mixing valves for temps and a pump for each temp area, with zone valves in each of these zones. My concern of late is the piping that essentially is between the zones/wood boiler/storage/backup boiler and if I need to follow primary/secondary piping, and if so do I need another pump? Also note that It is a rough drawing and not everything down to the ball valve is represented fully.
Well...I'll swallow my pride and ask for help (piping critique).
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Nice job on the drawing, easy to understand.
I don't know but I suspect if you sharpened your pencil you might find that you only need the lower of the two mix temperatures you're contemplating. So it looks like you can do away with one pump and one mixing valve assuming the lowest temperature is high enough. (We assume you'll be using a Grundfos Alpha or Wilo Stratos for any multi-zone circuits.)
DHW needs its own (tiny) pump, mixed temperature won't do the job. If storage is far away from DHW HX you might want to consider a separate 1/2" PEX circuit all the way from storage to the DHW HX.
[Edit:] Or just run 3/4" PEX from the mixing valve and DHW draw points all the way to storage. This will mean you won't be filling fat pipes with hot water in the summer for DHW, and it helps guarantee there will be no parallel flow through either boiler when the loads are drawing and the boilers are idle.
Oh, and as always, all of the storage tank volume above the upper ports and below the lower ports is lost for heat storage.
I don't know but I suspect if you sharpened your pencil you might find that you only need the lower of the two mix temperatures you're contemplating. So it looks like you can do away with one pump and one mixing valve assuming the lowest temperature is high enough. (We assume you'll be using a Grundfos Alpha or Wilo Stratos for any multi-zone circuits.)
DHW needs its own (tiny) pump, mixed temperature won't do the job. If storage is far away from DHW HX you might want to consider a separate 1/2" PEX circuit all the way from storage to the DHW HX.
[Edit:] Or just run 3/4" PEX from the mixing valve and DHW draw points all the way to storage. This will mean you won't be filling fat pipes with hot water in the summer for DHW, and it helps guarantee there will be no parallel flow through either boiler when the loads are drawing and the boilers are idle.
Oh, and as always, all of the storage tank volume above the upper ports and below the lower ports is lost for heat storage.
Well, the lower zones are in a concrete slab and the upper zones are staple up pex. The temperatures I potentially need are too far apart to try and combine them. That's why I show the DHW off of the upper zones. I was going to put Tee's in so I can add a pump if needed and feed it prior to the mixing valves with it's own pump, but I thought I would try it here first to see how it did. I found an amazing deal on a Crown Stainless steel indirect water heater that I am using for this. I also have a standard electric water heater for backup/summer. The pumps are Grundfos Alphas.
I'm trying to picture this but I apologize I'm not quite following what you mean by this?
I'll have dip tubes and diffusers off of double tapered bushings to hopefully mitigate this (thanks to long evenings reading this site).
Thanks!
Since you've got a plan B for DHW should be worth a try.
I was assuming you'd be using storage for summer DHW, so the suggestion was to run a small pipes to the top and bottom of storage instead of tapping into the big lines from and to the boilers. This way you wouldn't be filling the big boiler lines with hot water every time DHW calls, which would save a little heat. But since you're going electric in the summer then you're good to go on that score.
As for parallel flow, if you look at the supply flow into either of the mixers, you can see that water could come from the top of storage, or from the top of the wood boiler, or from the top of the propane boiler. You may want it to all come from storage, but water has the obedience of a cat. Hot water rises and all that, but if the distances don't work out in your favor then you could be getting more flow than you wanted through the boilers. (Although Danfoss should totally prevent flow through the wood boiler when it is not active.) If you supply water to the loads with a separate pipes that go directly to the top and bottom of storage you can minimize parallel flow through the boilers. (But with low temperature radiant loads probably no big deal in the first place.)
Didn't see the risers and dip tubes for what they were, looks like you're squared-away there.
So your plan looks A-OK the way it is. You could look into strategies for using the return water from the hot zones as supply water for the lower temperature zones so as to increase storage capacity if it seemed important enough to you.
I was assuming you'd be using storage for summer DHW, so the suggestion was to run a small pipes to the top and bottom of storage instead of tapping into the big lines from and to the boilers. This way you wouldn't be filling the big boiler lines with hot water every time DHW calls, which would save a little heat. But since you're going electric in the summer then you're good to go on that score.
As for parallel flow, if you look at the supply flow into either of the mixers, you can see that water could come from the top of storage, or from the top of the wood boiler, or from the top of the propane boiler. You may want it to all come from storage, but water has the obedience of a cat. Hot water rises and all that, but if the distances don't work out in your favor then you could be getting more flow than you wanted through the boilers. (Although Danfoss should totally prevent flow through the wood boiler when it is not active.) If you supply water to the loads with a separate pipes that go directly to the top and bottom of storage you can minimize parallel flow through the boilers. (But with low temperature radiant loads probably no big deal in the first place.)
Didn't see the risers and dip tubes for what they were, looks like you're squared-away there.
So your plan looks A-OK the way it is. You could look into strategies for using the return water from the hot zones as supply water for the lower temperature zones so as to increase storage capacity if it seemed important enough to you.
Your diagram is based on a fairly standard scheme. This is part comment and part question. I am a volunteer to an organization which has a mostly similar plumbing scheme which should work "on paper" but doesn't work well in practice. My thought is that the devil is in the details of flow rates and pump head, the action of parallel and series circulators, and then also to the type and diameter of the piping. I think what you don't want are at least four situations:
1) when the boiler is operating and where the system demand flow rate results in all flow going through the wood boiler. This may happen due to the action of the parallel and series circulator flow rates, pump head, etc. The result may be that any excess btu output capacity of the boiler will not go to storage, excessively warm water will return to the boiler, and the boiler may cycle/idle even though storage is not fully charged and can accept boiler output.
2) similar to #1, but system demand does not result in all flow going through the wood boiler. The return water from the mixing valves/bottom of storage may be warm to the extent that again excessively warm water will return to the boiler, and the boiler may cycle/idle even though storage is not fully charged and can accept boiler output.
3) there is no system demand but system flow/head is such that boiler output being moved by the boiler circulator is divided between flow straight through the mixing valves and flow to storage. This may result again in excessively warm water returning to the boiler, and the boiler may cycle/idle even though storage is not fully charged and can accept boiler output.
4) there is no system demand but the flow/head capacity of the wood boiler circulator is not sufficient to move the entire boiler output to storage, particularly as storage is charging and return from storage rises. Pay attention to the flow/pump head capacity of the boiler circulator to make sure that flow will be sufficient to handle boiler output. Some loading units use relatively small capacity circulators that may work fine to move boiler output (depending on boiler output and boiler/storage pump head) at large delta-T's but not at low delta-T's.
All of the above, while stated as comments, also are questions to which ewd or others may have useful comment.
When the system is operating, you also will want water at as low a temperature as possible returning to storage (and the boiler). Any warm return water to storage will mix storage to that minimum temperature. You might want to consider, based on return water temperature from the system, injecting return water at a storage midpoint.
To amplify ewd's allusion to cat, water will always take the path of least resistance, and flow through your system in various scenarios may not be what you expect. This is complicated by the action of parallel and series circulators.
Lastly, because of all of the above, I would recommend consideration of using the wood boiler to solely feed storage, having storage act as a large hydraulic separator, and then drawing solely from storage to meet system demand. In this way you can better assure that all of boiler output is best used: all always goes to top of storage and coldest return water always comes from bottom of storage. Based on system return water temperature, you also may wish to consider injecting warm system return water to a storage midpoint rather than to the bottom of storage to minimize warm return water to the boiler.
1) when the boiler is operating and where the system demand flow rate results in all flow going through the wood boiler. This may happen due to the action of the parallel and series circulator flow rates, pump head, etc. The result may be that any excess btu output capacity of the boiler will not go to storage, excessively warm water will return to the boiler, and the boiler may cycle/idle even though storage is not fully charged and can accept boiler output.
2) similar to #1, but system demand does not result in all flow going through the wood boiler. The return water from the mixing valves/bottom of storage may be warm to the extent that again excessively warm water will return to the boiler, and the boiler may cycle/idle even though storage is not fully charged and can accept boiler output.
3) there is no system demand but system flow/head is such that boiler output being moved by the boiler circulator is divided between flow straight through the mixing valves and flow to storage. This may result again in excessively warm water returning to the boiler, and the boiler may cycle/idle even though storage is not fully charged and can accept boiler output.
4) there is no system demand but the flow/head capacity of the wood boiler circulator is not sufficient to move the entire boiler output to storage, particularly as storage is charging and return from storage rises. Pay attention to the flow/pump head capacity of the boiler circulator to make sure that flow will be sufficient to handle boiler output. Some loading units use relatively small capacity circulators that may work fine to move boiler output (depending on boiler output and boiler/storage pump head) at large delta-T's but not at low delta-T's.
All of the above, while stated as comments, also are questions to which ewd or others may have useful comment.
When the system is operating, you also will want water at as low a temperature as possible returning to storage (and the boiler). Any warm return water to storage will mix storage to that minimum temperature. You might want to consider, based on return water temperature from the system, injecting return water at a storage midpoint.
To amplify ewd's allusion to cat, water will always take the path of least resistance, and flow through your system in various scenarios may not be what you expect. This is complicated by the action of parallel and series circulators.
Lastly, because of all of the above, I would recommend consideration of using the wood boiler to solely feed storage, having storage act as a large hydraulic separator, and then drawing solely from storage to meet system demand. In this way you can better assure that all of boiler output is best used: all always goes to top of storage and coldest return water always comes from bottom of storage. Based on system return water temperature, you also may wish to consider injecting warm system return water to a storage midpoint rather than to the bottom of storage to minimize warm return water to the boiler.
1 & 2. With radiant the mixing and flow strategy are adjusted to deliver the heat low and slow so return temperature is nearly minimized automatically. So excess flow with high return temperature is not possible.
3. I'm not sure I'm following this one.
4. I think this is what I call the problem of needing multiple passes through the boiler to heat storage completely. First lap return temperature is limited to 140 degF, supply temperature is 160 degF (or whatever). Second lap return temperature is 160 degF, supply temperature is 180 degF. Third lap boiler idles on high supply temperature. I advocate the one lap solution where flow is limited on the first and only lap and water doesn't go to storage until it is hot enough the first time. I overlooked this problem in the OP's drawing since storage is not adjacent to the boiler. So yes, it would be best to make sure there is a strategy in place to guarantee the full output of the boiler can make it storage.
Again in this particular type of installation, parallel flow and stratification shouldn't be a problem because the net flow from storage will normally only be a gallon or two per minute and the buoyancy of the hot water from storage should completely overcome the tendency for water to draw through the boilers.
3. I'm not sure I'm following this one.
4. I think this is what I call the problem of needing multiple passes through the boiler to heat storage completely. First lap return temperature is limited to 140 degF, supply temperature is 160 degF (or whatever). Second lap return temperature is 160 degF, supply temperature is 180 degF. Third lap boiler idles on high supply temperature. I advocate the one lap solution where flow is limited on the first and only lap and water doesn't go to storage until it is hot enough the first time. I overlooked this problem in the OP's drawing since storage is not adjacent to the boiler. So yes, it would be best to make sure there is a strategy in place to guarantee the full output of the boiler can make it storage.
Again in this particular type of installation, parallel flow and stratification shouldn't be a problem because the net flow from storage will normally only be a gallon or two per minute and the buoyancy of the hot water from storage should completely overcome the tendency for water to draw through the boilers.
#1 and 2 - I understand your point and I agree, my mistake.
#3 - on 2nd look I don't follow either. Brain failing to operate correctly.
The differences I'm now seeing between the the OP's design and the design issues I was dealing with in my comment is 1) the presence of mixing valves in the OP's design (in my case no mixing valves, hot water supplied at 160F+, return from coils at roughly -20F from supply, resulting in hot return water to boiler and boiler idling; and 2) insufficient flow from the boiler to move entire boiler output as delta-T rises to less than 30F.
#3 - on 2nd look I don't follow either. Brain failing to operate correctly.
The differences I'm now seeing between the the OP's design and the design issues I was dealing with in my comment is 1) the presence of mixing valves in the OP's design (in my case no mixing valves, hot water supplied at 160F+, return from coils at roughly -20F from supply, resulting in hot return water to boiler and boiler idling; and 2) insufficient flow from the boiler to move entire boiler output as delta-T rises to less than 30F.
Thanks to both of you for your input. I'm not sure I fully follow this, but do you mean I need someway to insure that the boiler is feeding only storage and not the loads (if needed)? I assumed that once the loads were satisfied (house is warm), the extra heat would naturally go into storage.
Some, all, or none of the boiler output can go to storage or to load, boiler doesn't care and that is as it should be.
The problem you need to be aware of is the boiler idling because it can't move water fast enough to storage to take away all the heat the boiler is putting out. This can happen if the return temperature from storage gets too hot because storage has been filled 'the first time around' with water that is not all the way up to the desired storage temperature.
ewd is correct. An example might help. Assume a boiler rated at 170,000 btuH, the boiler is equipped with a loading unit and the specs on the loading unit state maximum flow rate is 12.3 gpm. Assume boiler output at 170,000 btuH. At delta-T = 30, maximum btu's that the loading unit will move are 12.3 x 500 x 30 = 184,500, and the entire boiler output will be moved to load/storage/both. But at delta-T = 20, the loading unit will move only 123,000 btuH, and if the boiler output is greater than that, the boiler will cycle through idling periods until the load/storage/both catch up.
This may not be a problem in a load to storage only situation if the wood load is burning down as delta-T (boiler output temp - bottom of tank return temp) falls below 30F because boiler output also will be falling. To avoid idling periods you will not want to put a full wood load in your boiler as storage begins to top off. Also, a boiler rated at 170,000 btuH output will not output at that rate continuously. The rating more likely is a peak rating which will gradually fall as the boiler moves beyond high burn.
Another complication. Even though the loading unit is rated at 12.3 gpm maximum, that gpm rating is based on a particular system pump head. If system pump head is higher, then flow rate will be less. You will want to calculate pump head between the boiler and load/storage and determine as best you can the actual flow rate based on the pump curve for the circulator.
And another complication and this can get difficult to understand and apply. Your system design incorporates circulators in series and parallel with the boiler circulator. Two identical circulators in series will move the same volume of water at double the pump head at each point on the pump curve for the single circulator. In other words, if the pump curve show 8 gpm at 5 feet of head, two identical series circulators will move 8 gpm at 10 feet of pump head. Series circulators can overcome high pump head situations, and often two small circulators will take less electricity than one large circulator.
Also, two identical circulators in parallel will move double the flow at each point on the pump curve for the single circulator. If the pump curve show 8 gpm at 5 feet of head, two identical series circulators will move 16 gpm at 5 feet of pump head.This does not mean that flow will double in a particular system, because as flow goes up, so does pump head by about an exponent of 1.7 of the increase in flow (or by the square to do a quick calcuation), so you will have to determine actual flow based on increased pump head and the pump curve.
And when non-identical circulators are involved, the calculation is beyond my current knowledge.
I guess the main point is that a plumbing schematic is just that, and until pump head also is moved into the schematic and calculations, you really don't know very well what the outcome of the schematic will be when applied to the installed system. I know this only too well, as I have gone through 3 re-plumbings of my system to finally get it about right. I think that a pump head calculation and sizing of piping and circulators to meet the intended load are essential to make sure a system will actually perform to meet the desired need.
This may not be a problem in a load to storage only situation if the wood load is burning down as delta-T (boiler output temp - bottom of tank return temp) falls below 30F because boiler output also will be falling. To avoid idling periods you will not want to put a full wood load in your boiler as storage begins to top off. Also, a boiler rated at 170,000 btuH output will not output at that rate continuously. The rating more likely is a peak rating which will gradually fall as the boiler moves beyond high burn.
Another complication. Even though the loading unit is rated at 12.3 gpm maximum, that gpm rating is based on a particular system pump head. If system pump head is higher, then flow rate will be less. You will want to calculate pump head between the boiler and load/storage and determine as best you can the actual flow rate based on the pump curve for the circulator.
And another complication and this can get difficult to understand and apply. Your system design incorporates circulators in series and parallel with the boiler circulator. Two identical circulators in series will move the same volume of water at double the pump head at each point on the pump curve for the single circulator. In other words, if the pump curve show 8 gpm at 5 feet of head, two identical series circulators will move 8 gpm at 10 feet of pump head. Series circulators can overcome high pump head situations, and often two small circulators will take less electricity than one large circulator.
Also, two identical circulators in parallel will move double the flow at each point on the pump curve for the single circulator. If the pump curve show 8 gpm at 5 feet of head, two identical series circulators will move 16 gpm at 5 feet of pump head.This does not mean that flow will double in a particular system, because as flow goes up, so does pump head by about an exponent of 1.7 of the increase in flow (or by the square to do a quick calcuation), so you will have to determine actual flow based on increased pump head and the pump curve.
And when non-identical circulators are involved, the calculation is beyond my current knowledge.
I guess the main point is that a plumbing schematic is just that, and until pump head also is moved into the schematic and calculations, you really don't know very well what the outcome of the schematic will be when applied to the installed system. I know this only too well, as I have gone through 3 re-plumbings of my system to finally get it about right. I think that a pump head calculation and sizing of piping and circulators to meet the intended load are essential to make sure a system will actually perform to meet the desired need.
Well I thought I would bump/update this thread to post a question I now have since my schematic is still in the first post. About the only changes are that I wound up installing the boiler in the basement, as I ran out of time to build the shed. So, everything including the tanks are in close proximity in my boiler room.
After a few weeks of operating, there is one issue that I am struggling with. While drawing from storage only, boiler not running, my mixing valves work fine (they are honeywell thermostatic mixing valves). However, when the boiler is running and the boiler circ is on (Grundfos 15-58), the mixing valves can't seem to control and fluctuate around the control point +/- as much as 20F. For example, I run the upstairs at 138F, and the temp will fluctuate from 120 up to 160F. I assume the boiler circ is "pushing" water through the mixing valves, causing the problems. I don't think this would have been an issue if I had plumbed the boiler to charge storage only acting as the hydraulic separator. This was my original plan but wound up changing it to the one posted because I thought it would be the most flexible in the event I wanted to heat the house and charge storage simultaneously. Rearranging piping to do this now would be a pain, but I realize I need to change something.
After a few weeks of operating, there is one issue that I am struggling with. While drawing from storage only, boiler not running, my mixing valves work fine (they are honeywell thermostatic mixing valves). However, when the boiler is running and the boiler circ is on (Grundfos 15-58), the mixing valves can't seem to control and fluctuate around the control point +/- as much as 20F. For example, I run the upstairs at 138F, and the temp will fluctuate from 120 up to 160F. I assume the boiler circ is "pushing" water through the mixing valves, causing the problems. I don't think this would have been an issue if I had plumbed the boiler to charge storage only acting as the hydraulic separator. This was my original plan but wound up changing it to the one posted because I thought it would be the most flexible in the event I wanted to heat the house and charge storage simultaneously. Rearranging piping to do this now would be a pain, but I realize I need to change something.
Well, I am afraid I don't know yet but I am suspecting I would. I was just finishing up the propane boiler this week and found out the pump was bad. Waiting on the part to fix it. I hadn't been too rushed to get it finished as I have some backup electric baseboard that would keep the house warm if there was an issue with the wood boiler.
Hmmm, I was hoping that if it was the same, it would just help highlight that its pumps in series causing the issue. I dont have any experience with dealing with the mixing valves, so Im pretty limited in what I can offer for a solution at this point.
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