primary loop vs secondary loop gpm & velocity?

[pybyr]

can one of you who really knows what you are doing, professionally, remind me of whether the flow/ gpm/ diameter/ velocity/ btu/hr of the primary loop in a primary/ secondary system-- matters all that much

I got ahold of Siegenthaler's 2nd edition "Modern hydronic heating" via interlibrary loan and tried to digest and remember as much as I could within 60 days, and (I think) I basically remember him making the point that the circulator in the primary loop mostly needed to make the water and BTUs keep going round & round, without a whole lot of angst as to the gpm or velocity or BTU/hr, and that the inputs and outputs would sort themselves out as the primary flow went round and round.

but since my boiler (150 Mbtu), heat storage tank (1200-1300 gallons unpressurized) and duct coil (24x24, 4 row, 1600 cfm, 125k btu peak load on coldest day(big old farmhouse) ) all represent substantial loads of GPM flow and BTUs, I do not want to undersize the pipe diameter or circulator size on my primary loop.

that said, I also "get it" that there is little to be gained, and a lot to be lost, by gratuitous seat-of-pants assumptions that bigger is always better

thanks, as always, for the knowledge/ experience and willingness to share it

 

[Nofossil]

I'll go out on a limb here and hypothesize that the primary loop GPM should be at a minimum as great as the greatest secondary loop, which would presumably be the biggest heat source. If it's not, then that secondary loop will be recirculating a portion of the water, raising the boiler inlet temperature to something higher than necessary.

Just a thought experiment on my part - I'll wait for the experts to chime in.

 

[jebatty]

My out on the limb guess is that btu's through the primary should equal greatest btu draw in secondary. Primary temp may be greater than secondary demands, so adjust for gpm and temp.

 

[Nofossil]

My out on the limb guess is that btu's through the primary should equal greatest btu draw in secondary. Primary temp may be greater than secondary demands, so adjust for gpm and temp.

I think by definition the BTUs through the primary are always equal to the sum of all the BTUs through the secondaries. I guess that might not be true if you had two heat sources active at the same time, but it must be a pretty good approximation.

If you have a source that's delivering 100,000 BTU/hr, the only way it can be doing that is if all of the loads together are absorbing 100,000 BTU/hr.

My conclusion (for what it's worth) is that if your boiler is circulating at 15 gpm, then it seems like the primary loop would have to be at least 15 gpm to avoid the boiler drawing its own water backwards against the primary flow.

I can't think of any reason why you'd want the primary flow to be much higher than that, except that higher speeds mean less temperature drop as you go around the loop.

I've never done this kind of system, so I'm making this up as I go along.

 

[in hot water]

really no need to size the primary loop piping or circ any bigger that the amount of energy you intend on putting into it. If your boiler(s) input all add up to 100,000, and you decide on a 20F delta T as your design, then you size pipe, and pump to move 10 gpm within the loop itself. the primary loop circ really doesn't, shouldn't, effect all of the secondary circ sizing..

If all of your loads add up to more than 100,000 BTU/ hr and you only intend on having the input of 100,00, really no need to size the loop beyond what the heat source(s) can provide. Where would the additional HP come from?

Now, if you plan on adding additional heat sources, solar, geo etc, sure, size the primary loop to accomodate any future upgrades.

What CAN change pipe size, circ sizing, and flow rate in the primary loop is the delta T you select to operate at. If you chose 30, 40, 32, whatever, then you need to calculate based on that data.

hr

 

[Sting]

in hot water

That's the best advice posted yet - its all about the delta t and the available energy that you control to that bench mark

You should copy and past this in the stick about the subject above - or maybe Goose Rider could

 

[pybyr]

Thanks HotRod & Sting- now you helped me "get it" that despite all of the other variables that become interactive moving targets the first (and always) benchmark is to follow the BTUs- and then start picking & optimizing all the other parameters outwards from that benchmark.

Learning about hydronics has been and continues to be fascinating, but at times it gets to seeming like being lost in a fog of interacting variables and formulas- and you just made it clear what's the "compass" in that fog.

 

[in hot water]

Some basis design info will help keep you out of trouble, it sounds like you have a good handle on principles.

Hydronics is a fairly forgiving medium to work with. You can be "off" a few %'s here and there and it still accomplishes the task.

The fun part for those that do it on a daily basic is chasing down all those % to sneak up on the ultimate design and installation. With all the new technology and desire to enlist alternative energy options, it is again an exciting industry. Like the game of golf, you're never really an expert or done with the journey.

hr

 

[heaterman]

One thing that you need to remember in the instance that your secondaries come off the primary in succession is that each secondary will see a correspondingly lower temp to work with. Another way to put it would be to visualize a row of say 4 pri/sec tee's all in a row on the Primary loop. As each secondary takes water from the primary and dumps it back in, the primary loop temp will drop, lowering the "heat" available for the next loop. You can design around this by increasing the primary GPM in some cases. dumping all the secondaries into a common main tee on the primary or by increasing radiation on the "downstream" loops. Point it, you don't want to leave any of the zones sucking the "hind teat" as the farmers around here say.

All that being said, I have no idea whether any of that info applies in your scenario.

 

[Willman]

Would the mono flow tees have any use in this piping scenario ?

Will

 

[pybyr]

Would the mono flow tees have any use in this piping scenario ?

Will

those devices (monoflo tees) are ingenious for what they are, and within the context of the era that they date from. but they introduce head loss, and don't seem to make much sense for pumped (rather than parasitic/passive) secondaries.

The Taco Twin Tee, though, seems to make a lot of sense, design-wise, for building an efficient but very compact primary/ secondary system.

Plus I have carefully checked and confirmed that the Twin Tees don't have the same issues that "regular" closely-spaced (but separate) tees do about needing substantial minimum distances of straight pipe before and after tees.

I have my supplier working on some pricing, and as long as the Twin Tees are not prohibitively expensive in relation to the options and flexibility that they provide, am leaning strongly towards those

 

[heaterman]

I use these in all sizes and they work very well. The ball valve in the middle really speeds purging on the secondary loop.

(broken link removed to http://www.webstonevalvespurgetee.blogspot.com/)

 

[Willman]

Not knowing much I used ball valves with drain screws on them on the return line for bleeding purposes. Worked out ok. Will go with more professional stuff on my new install.
Will

 

[pybyr]

I use these in all sizes and they work very well. The ball valve in the middle really speeds purging on the secondary loop.

(broken link removed to http://www.webstonevalvespurgetee.blogspot.com/)

thanks- but for my application, I am still leaning towards the twin tees, on account of the fact that, unlike any of the "series" type tees, which require some length of straight pipe before inlet and after outlet, the twin tees can be put right next to each other, and other fittings.

 

[MNBobcat]

I use these in all sizes and they work very well. The ball valve in the middle really speeds purging on the secondary loop.

(broken link removed to http://www.webstonevalvespurgetee.blogspot.com/)

Mine arrived tonight. I'm trying to get my head around how they work. When I think of purging I think of some method of pumping water through a circuit in one direction and then reversing the direction and pumping water the opposite direction. The idea being to dislodge any trapped air. Is that correct?

So.... With the valve open lets say water goes through the left leg and through a pump and returns via the right leg. Closing the valve, wouldn't the water still want to go through the left leg, through the pump and return via the right leg? So what's the point?

 

[heaterman]

I use these in all sizes and they work very well. The ball valve in the middle really speeds purging on the secondary loop.

(broken link removed to http://www.webstonevalvespurgetee.blogspot.com/)

Mine arrived tonight. I'm trying to get my head around how they work. When I think of purging I think of some method of pumping water through a circuit in one direction and then reversing the direction and pumping water the opposite direction. The idea being to dislodge any trapped air. Is that correct?

So.... With the valve open lets say water goes through the left leg and through a pump and returns via the right leg. Closing the valve, wouldn't the water still want to go through the left leg, through the pump and return via the right leg? So what's the point?

The point is that you can put both the primary pump and the secondary pump in series to move the water/air around through the secondary. Or, if you have a purge tee setup in the primary loop somwhere (that would be a boiler drain/ball valve/boiler drain configuration) you can connect an external purge pump that will REALLY blow some water through the system.

 

[MNBobcat]

Oh, so the theory is that by putting both of the pumps in series you can move enough water to dislodge any trapped air?

One of the reasons I ask is because of another discussion where it was suggested to configure a water to air HX in a furnace in a counterflow configuration where the hot water enters the hx at the top and the cooler water exits at the bottom. The obvious drawback of that configuration is that it would not purge trapped air, because the water flows through the exchanger from the top down. By using these purge tees, I had thought it might solve the problem of purging the air but originally I thought it somehow reversed the flow.

 

[Blevesque]

Taco has a great series of videos that explains primary-secondary piping with pump sizing. The videos are labeled multi-temp-multi-load but goes in depth and explains it all. I hope I linked the right video.

(broken link removed)

 

[Mid Michigan]

My wheel are turning. So let me add my ignorance to this.
Sizing a system, primary, or secondary off of a delta T - Would there not be more energy in 180* water as compared to 130* water when using 20* delta T for control. This seems to me to be a issue. If we are running storage and design our system for 180* out of the boiler with 20* delta T and size accordingly, and then switch to storage and are down to 130* out of storage with same 20* delta T would this not throw all the calculation off?
Is it best to design to our practical minimum supply temps we will be using out of storage?