Simplest Pressurized Storage revisited

[Nofossil]

Isn't this somewhat system dependent?

*Everything* is 'somewhat system dependent' ;-)
You're absolutely right that stratification doesn't matter much during discharge if you have only low-temperature loads. That's a relatively small percentage of systems, though.

When charging, especially when heat loads have been satisfied, it's usually very helpful to maintain stratification so that you can supply relatively cool water to the boiler. With my non-pressurized storage, I have a hard time transferring all the boiler output to storage once the return from storage gets above 160 or so.

I have a client with 1000 gallons of pressurized storage (two 500 gallon tanks in series) charged with a Froling pellet boiler. Initially, their setup provided no almost stratification at all. As the tank charged, the Froling would have to modulate as it couldn't transfer it's full output to storage with 170 degree inlet water temperature. Some of the loads in this installation are baseboards, so their performance is reduced as they are getting water as cool as 150.



After fixing various system problems, we were able to achieve pretty good stratification. Supply temps are much more consistent, and the Froling has a much healthier delta T to work with:

 

[Nofossil]

I'm having difficulty agreeing with the T approach and single ports. When both the wood boiler circ and one or more load circ are "on," the circs work in series, and there is a probability that hot water supply will bypass storage supply partially to completely and feed the loads directly.....

I hadn't thought of that, and it's a really good point. If you have several different loads with different return temps, it's quite possible that you might have a situation where the return temp from the loads is higher than the temp at the bottom of storage.
In that case, the two port approach allows the return water to at least have a chance to rise to its appropriate level in storage while the boiler draws the coldest water from the bottom.

I have no idea how much difference that would make in practice. I've been trying for over a year to get a professor at a local college (who has a Vesta) to have his students do a series of experiments on stratification, mixing, and so on.

 

[jebatty]

I fully agree that stratification is important to keep relatively low temp water return to the boiler. As to your Froling example, I experienced the same thing at Deep Portage with its Froling wood boiler and 1600 gal of horizontal storage. The "fix" for a wood boiler may not work for a pellet boiler. But for the wood boiler, in addition to some plumbing changes, the "fix" to eliminate modulating (reduced draft fan speed) and even idling was to base wood loading on middle of storage tank temperature. Using 140F middle of tank temperature as an example, the instruction to staff simply was to load the boiler up to somewhat about the bottom of the firebox door opening if middle of tank temperature was less than 140F, and not load if above 140F.

At 140F middle of tank temp, at least 800 gallons of storage (probably more) would be available to absorb the boiler output. Assuming 190F maximum tank temperature, btu storage capacity available would be 800 x 8.34 x 50 = 333,600 btus. That amounts to 55 lbs of wood at 6,050 btu/lb. The wood supply is mixed hardwoods, including a little pine and light weight aspen, and loading to this point would be about 55 lbs. In almost all cases the system also would be drawing btus during the burn, so there would be little if any likelihood of ever over-loading the Froling to cause idling. And loading to this level would result in decreasing btu output as the wood load burns down, allowing the full btu output to be absorbed by storage without idling. Return water could be 180F, but because of the wood load being substantially burned down, btu output would be very low.

The same procedure is used at Deep Portage with regard to its Wood Gun E500 with 4000 gal of storage, and I use the same procedure (although with a little more care on the weight of the wood load if I want to push storage to the max) with my Tarm and 1000 gal of storage. If my middle of tank temperature is 130F and I want to charge to 190F, I load 83 lbs of wood: 60 x 8.34 x 1000 = 500,400 / 6050 = 83. Actually, I rarely weigh the wood anymore because from experience I know quite closely the weight of wood I am loading.

 

[ewdudley]

I'm having difficulty agreeing with the T approach and single ports. When both the wood boiler circ and one or more load circ are "on," the circs work in series, and there is a probability that hot water supply will bypass storage supply partially to completely and feed the loads directly, and based on return water temperature from the loads, return water may be "hot" and bypass storage return partially to completely, resulting in 1) returning "hot" water to the boiler and cause it to cycle or idle (even though water in storage is cool) and/or 2) failure to charge storage.

We have already been assured above that the Simplest Pressurized Storage Revisited returns water to storage optimally at the minimum temperature possible.

Any (straw man) "hot" water returning from the system will have to be dealt with sooner or later, so there cannot be a problem with some portion of that flow going directly to the boiler, since the only issue is when the "hot" water is re-heated, not if.

In any event the Wood Circ flow will be on the order of 15-20 gpm and the Load Circ Flow will be on the order of 2-8 gpm, so the boiler will have all the more cool water it needs, and it will of course be preferable to mix the "hot" water in continuously rather than wait to process it all at once.

 

[maple1]

I'm having difficulty agreeing with the T approach and single ports. When both the wood boiler circ and one or more load circ are "on," the circs work in series, and there is a probability that hot water supply will bypass storage supply partially to completely and feed the loads directly, and based on return water temperature from the loads, return water may be "hot" and bypass storage return partially to completely, resulting in 1) returning "hot" water to the boiler and cause it to cycle or idle (even though water in storage is cool) and/or 2) failure to charge storage.

As I mentioned above, I use the T and one pipe approach. And just as I also experienced unwanted flow through the boiler at times earlier on (corrected by switching to an Alpha pump & reducing load flows), I also experience the 'storage bypassing' at times that you are speaking of here. It happens when I am starting with depleted storage, and a cold house. But, in my case - I consider it a desireable thing. That means that most of my boiler output is going to my house when I need it the most, and with higher supply temps. Then, as the house warms and zones (4 of them) get satisfied, more of the heat gets put into storage. My boiler won't idle, and the hottest I have observed my supply temps get up to when this is happening, is 180° (from typical 160° under normal conditions). There is usually still some water going through storage when this is happening, but not much. But, I have also observed the odd time the zones pulling more than the boiler is putting out which creates the odd situation of the storage slightly charging from the bottom up earlier on - the flow though storage is reversed. It doesn't seem happen for very long, and not very often, and I think it happens only when my fifth zone is also open (DHW).

So - I am very happy with the way my T'd one-pipe setup works. I did no number crunching or flow calcs before I did it, maybe I just ended up this way by being lucky. But it greatly simplified my piping, and I'm in a tight space with most of it. Little things like orientation of Ts can play a big part - and having some tunability can be important also. Switching to an Alpha for loads was big for me. I think I have all my zones throttled down a little bit now via ball valves since putting the Alpha in - slowing those flows a bit likely compensated for my lack of proper number crunching. I could also likely see using the two-pipe setup as reducing the likelyhood of flow related problems appearing later on. Or maybe not being as susceptible to some of those subtle things, or being more widely adaptable to different zoning arrangements.

 

[Nofossil]

....the Wood Circ flow will be on the order of 15-20 gpm....

Your wood boiler is a *lot* bigger than mine. Mine has an average output of 60kbtu per hour. Working with a delta T of 30-40 degrees, I think that works out to 3-4 gpm.

 

[ewdudley]

Your wood boiler is a *lot* bigger than mine. Mine has an average output of 60kbtu per hour. Working with a delta T of 30-40 degrees, I think that works out to 3-4 gpm.

The VT2218 in the Simplest Pressurized Storage Revisited system easily has 15-20 gpm to work with when dealing with "hot" return water. Depending the temperature of the water entering the boiler and amount of heat added to the water as it flows through the boiler, the flow rate as controlled by the VT2218 could be as little as zero.

 

[Nofossil]

Yes - I was just looking at average / typical flow rates.

 

[jebatty]

In any event the Wood Circ flow will be on the order of 15-20 gpm and the Load Circ Flow will be on the order of 2-8 gpm, so the boiler will have all the more cool water it needs, and it will of course be preferable to mix the "hot" water in continuously rather than wait to process it all at once.

I know we are talking about a system with Viridian and Alpha circulators, not 15-58's or loading units. The system I described has a boiler loading unit rated at just under 12gpm maximum and 4 - 15-58's on the load side, serving 4 zones. I don't know exactly what the load flow is, but it is certainly greater than the boiler flow.

 

[ewdudley]

The system I described has a boiler loading unit rated at just under 12gpm maximum and 4 - 15-58's on the load side, serving 4 zones.

Not that it makes any thermodynamic difference to boiler performance, but are not talking about your over-pumped mix-to-the-max straw-man system, we're talking about the Simplest Pressurized Storage Revisited system, which by definition has minimum load circuit flow with maximum deltaT.

 

[Nofossil]

So, back to my initial questions about this design. Based on these discussions, I think I'd make the following adjustments:

1. Discuss and clarify the bypass circ vs. Danfos type mixing valve for boiler protection.
2. Disclaim the Viridian as a wood boiler circ (needs a temp sensor hack that most wouldn't likely be comfortable with)
3. Present the alternative of one-port storage connections with suitable warnings about plumbing detail

Am I missing anything? I'm thinking that I'll write it up and ask a Mod to update the sticky.