primary/ secondary with a modulating boiler

Mark Eatherton

as important as the separation of the tee's. If there is too much turbulence, the primary water may "skid" off in to a side branch tee and cause unwanted flow. I believe the rule of thumb is 5 times the pipe diameter in straight pipe before and after the secondary tee's.

You don't have to use Vito's LLH, but you DO have to use their prescriptive venting system, unless you don't care about the warranty...

But then again, you don't HAVE to use Vito boilers:-)

I once tried to "make" my own LLH using Viega ProPress fittings, and found that purchasing a pre-manufactured LLH from Caleffi to be cheaper, quicker and a HECK of a lot easier to mount and insulate. And you get the benefit of an excellent air removal system, dirt separator and PONPC connection.

It's a free country.

ME

tdk

primary secondary with modulating boiler

I'm struggling with the manuf's of modulating boilers, on the principal of having to install a boiler circulator. Can someone explain to me why full flow through the boiler is neccessary if the boiler is not at full capacity? In this case a 150k boiler able to ramp down to 30k would only need 3 gpm min. to be at max eff. Thanks

[Deleted User]

Point of reference...

Your mod con is an extremely intelligent piece of equipment. It can "see" load increase and decreases and anticipate those loads and modulate accordingly. In order to do so, it needs one constant, and that constant is flow. If you vary the flow through the boiler, the onboard controls will be confused and may not perform as designed.

If the flow rate through the boiler is constant, then the variable for load is delta T. As it sees a drop in return temperature, it knows there is a hill ahead and starts ramping up in anticipation. As it see an increase in return temperatue, it knows it is cresting the hill and can go back to idle.

It's all about variability, and one of variables has to be a fixed constant (flow) in order for the micro processor on the boiler to do its job.

The parasitic trade off is minimal compared to the overall energy reductions.

BTW, not all mod cons "require" an individual boiler pump, The TT is one that comes to mind. And when watching them run under these conditions, I hear them "hunting" a lot, which is to be expected until the load stabalizes.

ME

TK03

Mark

They say you need P/S if you want frost protection on page 4 I believe. Is this something not to be concerned about with the cold temps you experience where you are. That heat exchanger could get down to 44f pretty quick with a good insulated home.

klaus

Another main reason..............

For P/S piping and pumps w/ most mod-con boilers, is their is a fairly big pressure drop thru the boiler heat exchanger. You will not get the flow thru the boiler that is desired and still be heating your zones, w/ the same loop and pump. The simple solution to maintain what you want is a dedicated pump for the boiler flow, seperate from your system flow.


Merry Christmas!

Mitch_6

On thing I think you should be looking at is

capacity in gallons of the boiler, there is typically not much water in them. they thrive on water quickly movement through them so they do not flash off. That is why you need the ps so you are always moving water through the boiler regardless of required load at the time.



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Hvacman

Thank you Wallies...

Thanks for the very thorough information on this subject! Every time i think I "know" something The Wall teaches me more!

Perry_5

I rarely disagree with Mark; but,

In actuality - constant flow is not required to make a Mod/con boiler work.

The Vitodens 200 has a variable speed pump built in - which may be used as the primary pump in certain heating systems (no primary/secondary piping and 2nd pump at all); but may require a secondary loop for other heating systems.

However, It would not surprise me that other mfrs restricted their boilers to a constant flow situation as in some cases it makes the control system simpler.

Perry

Mark Eatherton

The difference is in the CONTROL logic Perry...

The Vitodens is in control of the pump, hence it KNOWS what is going on. It's in control of the pump, and it knows what the flow rate is, hence it is in CONTROL. It is still watching the system for overall delta T so it can respond accordingly, hence the reason they like to see their systems done though a low loss header. The problem with variable speed in a conventional system is that it has no idea what kind of head is really necessary t overcome the highest pressure drop circuit so the far ends of the system, or the zone with the highest pressure drop suffers.

The other mod cons on the market don't have speed control of the pump, but do have control over the burn rate, hence it needs constant flow in order to have total control over the flame and consequently, output.

BTW, I've asked a couple of the modcon manufacturers to give variable speed control the boiler pump to no avail. I think they're afraid of what might happen :-)

The Vitodens boiler has a little piece of a certain German electrical engineers brain in each box, and he is one smart cookie (currently residing in Canada). He "gets it". Just like you.

I forgot about that little technical (Vito V.S.) detail :-(

ME

PS, I have seen conventional modcons improperly installed, and misapplied, and they short cycle like a banshee. It is due to a lack of proper, continuous flow across the heat exchanger.

ME

Mark Eatherton

JGB...

Yes, it is required to avoid freezing a HXer, but I have pleaded with the modcon manufacturers to give us, the field contractor, the ability to disable freeze protection selectively in the field (glycoled snowmelt boiler) to no avail...

Make certain that if you do a snowmelt boiler, that you control the boiler pump independently from the distribution pump, or the HO will get a REAL shock in their utility bill....

ME

Dave Behrens

Increase Pipe Size of Hydraulic Separator

In a recent install, I saw some "forcing" of hot water into a zone which was not active (five 3/4" zone pumps w/IFC) when one zone was active. I reduced flow through the boiler (slightly closed ball valve on isolation flange on discharge side of primary pump into the boiler) and the situation was solved. BTW, the distance between the tees which formed the P/S loops in this application was 13" and both loops were made with 1-1/4" Pro Press Type L copper.

I'm considering increasing the pipe size between the tees which form the hydraulic separation between the primary and secondary loops. I think this will allow more "slippage" between loops. Also, decreasing the distance between tees (and strictly following the 12" rule) may help. When I do another P/S loop in 1-1/4" copper, maybe 2" copper with a 10" center to center between tees may work better.

What do you guys think???

P.S. To all you Viessmann fans out there: it really pisses me off that Viessmann pushes their $250+ separator when I can make one cheaper and smaller for MUCH less. Ditto for their Vitodens vent system.

Dave Behrens

Tight Install

The install I posted about was VERY tight (think nightmare).

Perry_5

I strongly advise use of the Vito vent system or the 29-C4 vent

Dave:

I have a prediction for you if you build a vent system using PVC pipe with a Vitodens....

The boiler HX will fail - and potentially in just a few years... and will not be covered by Warranty. Quite honestly, I think that is one of the reason other SS mod/con boiler HXs are failing -- from the chlorides leaching from the air intake and outlet vent piping.

Feel free to use the AL 29-C4 alternative they recommend. I doubt it is cheaper.

Of course, it's your business and it's your customers you will have to face if my prediction is true.

Turns out that Viessmann is probably way ahead of the pack on this one too... I have some literature from them in 2006 where they claim that they have never had one of their HXs fail in service. That may even be plausable to me as I have been thinking through the issues that can affect 316 SS in general (regardless if its "L", "Ti" or just plain 316).

Perry

Dave Behrens

I Follow the Manual

I wouldn't attempt a Vitodens install without their ($500+) vent system, or their overpriced hydraulic separator. I LOVE the quality of their product; I've been to their classes in RI (NICE!!!); I don't like their prices. Neither do my clients. They are a really hard sell.

Mark Eatherton

MMMMm, yes... 5 pounds of sugar...

In a 2 pound bag :-)

If it were me, and I understand that you inherited this red headed freckle faced step child with a non existent budget, I would have gotten rid of all the pumps, put in one distribution circulator, like a Wilo Ecos or Stratos, depending upon your needs, and cleaned up all the piping to get rid of the 4 wheel drift occurring in the primary piping system, AND reduced their wattage consumption.

But, that's just me.

I would have to say that most problems associated with ghost flow in todays hydronic heating systems (pipe primary/secondary) have more to do with turbulence issues causing water to bounce around and skid off into side branch tees.

It's not something that you can "see". It's something you have to "FEEL" in order to understand the dynamics at work. I use my ooohhhh AHHHHhh detections system more than not...

ME

TimS

space between P/S tees

when the manual say 12" or 4 pipe diameters that is the maximum I dont believe there is a minimum I biuld mine very close together and a minimum of 6 pipe diameters of straight pipe on downstream and upstream of tees ,try this with a larger secondary loop in the vertical position supply on upper end should fit well even in tight spaces.

hot_rod

too much pipe size

will reduce the velocity and could lead to potential air elimination issues?

There are some important numbers and formulas to follow when you build your own hydroseparator, mainly the barrel diameter to branch diameter relationship.

I've found that yiu really need check protection on the supply and return takeoffs on PS piping. The check in the circ is not enough. this

This is particularly true if you pipe pumps off the top of a loop. You may get some two way flow in the un-checked return side pipe.

hr

hr

Perry_5

There will be a minimum

There will be a minimum spacing if you use 2 Tees.

All primary/secondary piping does is use a heat exchanger to separate the boiler flow from the circuit flow.

2 Tees - or a Low Loss header device - is a "Direct Contact" heat exchanger.

There must be adequate time for the fluids to mix to a uniform temperature. Given that you are in a laminar flow regime (and not turbulent flow) it will take some distance.

Without modeling it - I suspect that it would take 5 pipe diameters to just get a steady result - and more than that to get uniform mixing.

Perry

[Deleted User]

Perry, Brad et engineering associates et al...

What are your thoughts on these "non vs conventional" methods of thinking?

I've often wondered....

Sometimes, we get so used to doing things the conventional way, we forget to think outside the pipe :-)

ME

Perry_5

Do not use conterflow

Counterflow in a direct contact HX works very poorly from a home heating and control perspective. I see no advantages and several possible disadvantages (and I'm not sure there are any advantages for industrial uses either). For a set-up like the Vitodens LLH where a key temperature sensor is located in the LLH - using counterflow could severely mess up how the boiler operates.

All the "normal" or "parallel" flow setups will produce identical results - regardless which side (or rotation angle) is used for input/output - as long as there is sufficient mixing going on (the concept of crossflow is not an issue - and the water has no way of knowing which side it comes in and goes out on).

Perry

Mark Eatherton

HMMMMmmmmm....

Am going to have to roll that one around in my brain for a while, and think about why it is that heat exchanger manufacturers PREFER counter flow as opposed to parallel flow in flat pate heat exchangers...

ME

Perry_7

Once you separate the fluids...

Mark:

Counterflow is preferred once you separate the fluids - be it a tube or plate HX.

The reason is that you get the maximum temperature change in a counterflow HX - if the fuids are separated by a barrier.

If you have a direct contact HX and are mixing the fluids - then you also have to worry about flow patterns developing that substaintially negate the mixing. You also can - depending on how pumps are sized and pump curves - get some funny stuff happening with the pumps (they can fight each other). Thus you want to keep direct contact HXs in a parrallel flow arrangment to automatically avoid several problems.

Of course, if you wanted to throw sufficient engineering at it - and precisely design the sytem and pumps - you could make a counterflow direct contact HX work as well. Home heating systems aren't on that level.

Bottom line - direct contact counterflow HXs are a great theory; but in practice... may not work so well.

Perry

Buzz G.

counter flow

There is a process called countercurrent distribution where two separated fluids flow in opposite directions yet are connected across a semipermeable membrane between them. The kidney uses this to get the concentrating mechanism to work for various things that pass out via the kidney. Brewers cool their mash by running it one direction in tubing that is inside a tube in which water is flowing in the opposite direction. By having the coldest water contact, first, the coldest mash in the inner tubing, the transfer of heat is faster. The hottest mash contacts the hottest water first as well. Just info from two of my areas as a non-professional heat engineer.