Primary loop to buffer tank - circ sizing for gpm

Dave Carpentier · September 18

(Upgrading from oil to gas boiler plus piping)

I used the Grundfos tool to plot my primary loop circ ( a 15-42 3-speed) vs the head loss in the loop.

The IBC boiler has a flow/head table, as does the Dirtmag. For the 1" pipe I used an online calculator for 8ft of equivalent pipe (buffer tank will be beside boiler and the main notable is the 3/4" port restrictions going in and out).

IBC boiler primary loop curves with 15-42 circ.jpg

It looks like on Med speed, the primary loop would be flowing ~ 8.7 gpm , on Low it would be around 5.5gpm. The boiler's stated min flow rate is 2 gpm.

The secondary loops would be flowing anywhere between ~ 0.5 and 7 gpm I suspect.

It seems to me that, for normal day to day secondary flow, the circ will flush thru the buffer tank even on low speed.

I would like to reuse this 15-42 (recycling it from my existing oil system) but maybe it's just too fast ?

Can I help the situation by increasing the buffer tank delta, or turning down the Ramp on the boiler ? Introduce more head on the primary loop ? (a valve partially closed ?)

New system diag for ref

Dave Carpentier · September 18

Now with coffee instead of evening refreshments I wonder, am I overthinking it ?

Is there even an "ideal" gpm for the primary loop when the secondary loop is variable flow ?

Kaos · September 18

@DCContrarian did a bunch of simulations of different buffer tank configurations.

General conclusion is the buffer settles to either SWT or RWT pretty quickly depending on which flow is higher. The buffer still works, so don't worry about it.

The better metric for correct flow rate is to have a decent delta T across boiler and supply loops. You won't be able to set this till deep freeze though as you need to run all your emitters at near full output

An ideal setup would have somewhere around 10-20F delta across the boiler and 15-25F delta across the emitters. The reason for the higher delta across the emitters is so to ensure primary flow rate is always higher than secondary. This means full boiler SWT is sent to the emitters instead of a mix of SWT+RWT when you over-pump the secondaries.

I would set to medium speed for now. I would look at replacing that with an ECM. Once you have a good ODR dialed in, the boiler pump should be running close to 24/7 in colder weather, there is a pretty quick ROI for ECM plus gives you a bit more adjustability.

Dave Carpentier · October 2

Im thinking of going new circ for the install but having a hard time finding a fit.

I seem to need approx 7gpm (in the 6 to 8 range would be nice) but the head is only 1.4 - 2.2 ft.

The Grundfos UPSe 15-58 that the rep suggested would send me to ~ 10.2 gpm unless I could come up with a bit more head. 10.2 gpm is getting pretty close to 4 ft/sec in the 1" pipe.

Is there just no demand for a slow circ like speed-1 or 2 on the 15-42 ?

I looked around on the Taco site but didnt find anything yet.

Curves for a UPSe 15-58

HydronicMike · October 2

You might need more coffee. The piping to me looks almost completely wrong, in almost every way.
The way the boiler is connected to the buffer.
The 2 manifolds off the buffer.
The placement of the piping and mixing valve going to the radiant zones.

hot_rod · October 2

basically you find the closest circ and add a balance valve downstream to dial it in exactly

A balance valve is the fine tuning device in a system

The 15-42 on speed 1 will be fine

Dave Carpentier · October 2

My 15-42 is a high miler, I dont recall ever changing it in the 25yrs.

I was looking at those Caleffi Flowcal fixed gpm devices.

Then, when I was browsing around on supplyhouse, I noticed that there is still a UPS 15-58 (non "e").

Back over to the Grundfos site for the curve mapping..

Using that speed-1 , I should be able to keep up with my design-day btu with a 19f rise in boiler.

The UPSe 15-58 and UPS 15-58 are different beasts.

Im not sure why the pump finder on grundfos didnt mention this circ as an option.

Edit: Dang, unavailable from my supplier (discontinued)

GGross · October 2

standard 15-58 are still in stock. they are going away soon-ish in the US but SH seems to have bought them all up, they have like 30,000 lol

https://www.supplyhouse.com/Grundfos-59896341-UPS15-58FC-3-Speed-Circulator-Pump-1-25-HP-115-volt-4701000-p?utm_source=google_ad&utm_medium=Shopping_Heating&utm_campaign=Shopping_X_Heating_X_SSC_New&utm_campaignid=22340288845&utm_adgroupid=177827080713&utm_targetid=pla-304229468133&utm_product_id=59896341&utm_matchtype=&utm_keyword=&utm_adtype=pla&utm_category=Heating&gad_source=1&gad_campaignid=22340288845&gbraid=0AAAAAD_WAysozMxm0NhVIrVIyztlvUq0U&gclid=CjwKCAjwrP3VBhBbEiwAnaqpQ4x9t1TCYN86-Xc2T1wS09Ht0PLEr5VCskt8iUdyqoyW3jXUSD-8BxoCRnQQAvD_BwE

grundfos selector is abysmal in my opinion. it has defaulted to the most expensive option since it existed. you have to select several options to get the 3 speed pumps to even show up sometimes. they keep changing it so who knows! but you can still buy 15-58 pumps.

hot_rod · October 2

If you shop around you can still find the Alpha 15-55 and 15-29

In the Grundfos model the last two digits are the head in decimeters

So the 55 and 29 are a bit smaller circs than a 58

I think the Grundfos offering will be all Alphas going forward

Kaos · October 3

The + version of this has a 0-10V speed control input so you could adjust it exactly to the flow you want:

https://www.xylem.com/siteassets/brand/bell-amp-gossett/resources/brochure/ecocirc-20-18-sell-sheet-_-a-169-rev-2.pdf

You could also run it in delta T mode.

Dave Carpentier · October 3

If they had the -29 with the regular Alpha display on it instead of the HWR, that would be sweet. In any case, I dont recall seeing the HWRs on my vendors site.

It only just occurred to me to check the curves for my existing Alpha2 ( a 15-55) that I was going to reuse for the in-floor. If I instead use it for the primary, on speed-1 curve it should flow 6.6 gpm ! (21f rise to meet my peak btu needs). ( 70000 = 500 x 6.6 x 21 )

I'll get a new Alpha 15-58 to use on the in-floor (constant pressure mode). Its a +$75 to add the "Alpha" display/control but you get logs and data via bluetooth/app that I might find interesting ?

hot_rod · October 3

The 15-58 has two versions, a basic model and one with more features. I think one is the UPS 15-58E and one the Alpha 15-58. Maybe?

The Alpha 26-99 is just hitting the shelves

Primary loop to buffer tank - circ sizing for gpm

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