Primary / Second- too much flow?

KBP&H

Yes, there are 3 second circs, all on low speed, which is still way above ( at least twice the flow rate) of the spec sheet on the system,, When I asked rep to order the 10-15 circ he said why, etc. Called back week later and said to stay w/ superbrute, set on low,,, still think too much water going back to the boiler-- this is the only zone thats a problem. Delta T seems to be huge, Did 3/4 primary loop because it is a short run and trying to control the water, I would think that is taking as much heat away as is available since the boiler is getting knocked out ( indicating too much BTU thru it?),
Did a quite a few other sys w/ different rep ( other place) and had no problems-- but he also ordered pumpsto match the system, not what they had on the shelf,,, I don't think the boiler is undersized since it is more than the req DESIGN BTU, Just tooooooo much water thru slab..

KBP&H

Too Much Water Flow?

Recently installed system- 55,000 BTU net output gas boiler, three zones- first thin slab,onix radiant 112*, .8 gpm, 7500 BTU DESIGN;; second- concrete floor- 1000 1/2" ( 4-250' loops) pex- 108*, 2.3 GPM, 23316 BTU DESIGN, third- hi temp baseboard- 26'@ 550 BTU ft= 14300....... Total DESIGN btu = 45000 app BTU. SUpply hse spec'd Grundfos superbrute S1558FRC for three zones on prim / sec loop, prim pump is Taco 007 w/ app 60', 3/4" loop, when the slabb is running it is knocking the boiler temp to almost nothing and taking a long time to recover I think the superbrute circ is way oversized- resulting in way too much water going thru the slab even though I have the balls almost closed, Grundfos lit says this should be a 10-15 circ... The rep says stay w/ the superbrute??????? any Ideas....
MY LUCK- The homeowner is also an atty?

Brad White_24

Questions

What jumps out at me is the difference in potential pressure drops. The 26 feet of baseboard compared to the 250 foot PEX loops is one thing. The thin slab somewhere in between obviously.

I would think that with proper balancing it should be solved, but to do that, do you have the means such as near-linear balancing valves? (Tour and Andersson type for example). The fin tube would take the most throttling obviously, while the circulator head would be sized for the 250 foot PEX and manifolds.

What I would do is plot the pressure drops of the worst case run and the total flow rate onto the Grundfos pump curve and see where you are. If this is a UPS (3-speed) you have a little wiggle room if not on a good spot on the curve.

Other thoughts:

That the slab running at first drops the boiler temperature, that could be a function of the mass not warming up yet. The initial Delta-T can be quite high until things settle to near design.

The primary loop at 3/4" is just a tad undersized in my opinion. I would use 1 inch but that is just me. My concern is that there may not be proper de-coupling of P/S? Secondary flow might be affecting the primary? Just a reach here to eliminate it as a variable.

Are you saying that each zone has a Super-Brute even at fractional GPM's? Or does the one Super-Brute serve all three? That is not clear to me. If each zone has a circulator (3:1 secondary/primary) then, yes, I would say you are taking "pumping away" to an extreme. Have to ask...

Seems your secondary flow rate calculates out as it should (a little under-radiated compared to the boiler output perhaps, but that is another matter).

All that comes to mind for now...

Best,

Brad

p.s. HO is an attorney? Put your house in your wife's name.

Brad White_24

I would think that a single Super Brute

would be plenty for all secondary loads. But then you would be using control valves or other means to gain control. Yes, over-pumped. Naturally that extra flow has to go somewhere... are the cirulators piped in parallel to each other or sequentially from a series of closely spaced tees?

If your secondary loops have too much flow, when they return to the main (3/4"), occasionally the flow will reverse to the exiting high-flow tee, forming a T-mix at that point. My concern about the main size is more about turbulence, asking for two-way traffic on a narrow street.

You say "huge" delta-T but do you have a number? If you have too much water flow, the delta-T would be narrower, a lesser number. Can you get measurements?

If series of tees I can see the cumulative pressure easily in my mind's eye. If in parallel, cumulative flow. Do you have a diagram to share? That would help.

When you say boiler is getting knocked out, you mean on high limit? Or just falling behind on low temperature? You must be using several temperatures, how are they controlled?

Have you let the slab get a good heat soak? That might stabilize things.

More to chew on for you and for me!

Brad

Unknown

we have done single loop zones with low speed super brutes many times.

You say this is primary/secondary... what is the mixing method?

siggy

\"knocking the boiler down\"

I'm not sure if I can picture the installation from your description, but one thing you don't mention is a mixing device that can measure boiler inlet temperature and prevent the slab load from extracting heat faster than the boiler can produce this heat (e.g. a "thermal clutch" between the load and the boiler). You must provide this capability on high mass loads, or you will always get the marked drop in boiler temperature that you describe. An injection mixing system or a 3-way or 4-way MOTORIZED valve with controller can do this. I really don't think you have a problem with the circulators.

Siggy

T-RACY

to much delta t to me ,what is temp out and what comes back?

we use thermometers onradient supply and returns so we know our delya t.if we drop 40 instead of 20 degrees, You may in jan. in Maine You will double the load until the slab warms up.more a product of increased heat transferdo to temp dif. between slab and water temp than increased flo rate.

KBP&H

Loops

Yes- it is a standard primary loop- 30' across basement ceiling to a manifold that has 6 tees ( 2 for each zone) BB comes off first ( 180*), then conc slab-233000 BTU-Conc slab ( 108*) then small bath 112*, standard sparco mix valve 80-120*, so a circ for each zone, - My opinion, if there is alot more GPM than design- doesn't the BTU load change- so instead of 233000 BTH , slab may be 30-40000 BTU because of 3 or 4 GPM instead of 2.3 ( doesn't this make sense).
Thanks for the help> I've been to lots of Dan's seminars , and have his books, plus lots of other stuff, so I'm not new to this,,

siggy

You can not protect the boiler from low temperature operation with a single thermostatic mixing valve regulating supply temperature. You MUST have a mixing device that responds to boiler inlet temperature by partially closing the hot water flow through the mixing device. Here's one way you could pipe in a motorized 3-way valve along with the existing P/S piping.

Siggy

Unknown

I'm with siggy, but I would pull the two radiant zones off and use variable speed injection to feed them. would be a bit cheaper and effective. Then you get boiler protection and real-time temperature modulation to the radiant. maybe even boiler reset depending on the controller you use.