Short Loop, Minimal Head, Over Pumped

CurtT

Looking for comments & suggestions before scrapping the system. Very poor installation, including poor choice of "Boiler". Short loop from Tankless Water Heater through Fan Coil at 140F, 3/4 " Pex/Copper, Coil is 5/8" or less. Pumped with Grundfos UPS 26-99 on High Speed (33gpm @ 0 Head, I believe). Tankless Water Heater is throwing over heat code. The Delta T must be absolutely miniscule. Will changing pump to much smaller unit, or reducing the speed on current pump, and reducing Tankless Water Heater from 140F (its Maximum) make a difference? I am thinking 120F and about 3 to 5 gpm given max heat load of house. It is about a $14K question where I live....!

EBEBRATT-Ed

Wrong "boiler" for the application. Is it tripping on high temp?

The ahu has to be able to reject the heat the tankless can produce or it will eventually reach high temp.

The tankless may have a high head loss.. Need to know more about the equipment to even see if this can work.

hot_rod

A plugged strainer on the tankless perhaps?

Tom_133

The pump may have been chosen to overcome the tankless, they are very restrictive. I would suggest a lack of flow rather than a pump issue. You sure you got all the air out?

CurtT

Not sure how this works to keep conversation going. There are about 50 units in a complex with the same equipment, same age, & most/many with same issue. The Bosch C1050ES tankless was used in a piping/pumping configuration for both DHW & CH through AHU. The graph provided by Bosch top out at 5gpm at near 60 feet of head, starting at about .5 feet of head recommending no less than 1.7gpm. An analysis of the piping scheme used reveals no fluid to fluid exchanger, heating fluid is integrated with DHW. Can't see how tankless unit caused headloss can be so great it requires a UPS26-99 on High Speed. These systems shut down due to tripping of ECO, requiring manual reset. Like previous post, is there a solution out there short of a complete replacement at $14K to residence owner?

jesmed1

How difficult is it to open one system up and check the strainer as @hot_rod said? If it's plugged, that's a lot less $ fix than a complete replacement.

jesmed1

The pump curve and the Bosch curve seem to cross a little above 3 gpm, and that's assuming no additional head from the heating loop. If you add that in, maybe you're already under 3 gpm, and then it wouldn't take much blockage to reduce below your minimum recommended 1.7 gpm flow rate.

Given the implications for 50 units, maybe worth your time and effort to open one system up and install a flow meter and check the strainer. The pump and Bosch curves suggest you may be marginal on the flow rate.

CurtT

Tankless flushed & cleaned. Coil not flushed. Can install flow meter. Good idea, given time & cost. Frankly, I suspect the way it is piped (not even close to Bosch diagram) has created a Closed Loop System, contrary to Bosch installation manual specifically stating that the unit NOT be used in a Close Loop System. I don't KNOW the consequences of it being a closed loop INCLUDING the Bosch, versus the closed portion being isolated by a fluid to fluid exchanger and the rest being open as a part of the DHW system. It would still be cheaper to pipe as per Bosch manual than gutting it all. Just installed a Navien Combi, after gutting it all, including coil. All good....but $$$$. Not every home owner can do that. I really appreciate the input. It is not available locally.

jesmed1

Funny how piping systems per the manufacturer's instructions seems to solve a lot of problems.

CurtT

If I might add to this challenge.... Given the Bosch is used for DHW as well and there is NO lack of flow (volume or pressure) on DHW, I don't believe it is any unusual restriction in Bosch. So my thoughts about nearly ZERO Delta T due to GPM is getting no traction? This is why the feedback to my challenge is very helpful.

jesmed1

CurtT said:

Frankly, I suspect the way it is piped (not even close to Bosch diagram) has created a Closed Loop System, contrary to Bosch installation manual specifically stating that the unit NOT be used in a Close Loop System. I don't KNOW the consequences of it being a closed loop INCLUDING the Bosch, versus the closed portion being isolated by a fluid to fluid exchanger and the rest being open as a part of the DHW system...

It might help if you could post a drawing of how it's actually plumbed. You say you "suspect" it's been plumbed as a closed loop. Note that the manual says that if it's plumbed in a loop for recirculating DHW, the heat exchanger warranty is only 3 yrs, LOL.

CurtT

I'll do what I can to "draw" up & post what was done versus what Bosch was expecting including the significant warranty reduction even IF done their way..... Give me some time to blow the dust off my drafting skills.....

jesmed1

CurtT said:

I'll do what I can to "draw" up & post what was done versus what Bosch was expecting including the significant warranty reduction even IF done their way..... Give me some time to blow the dust off my drafting skills.....

LOL, pen on a napkin works fine.

jesmed1

Also note that section 5.2.2 of the manual I'm looking at says "Direct recirculation flow through the tankless waterheater must be above 1.7 gpm for best performance. Flow can be verified in the diagnostic menu, mode P4-3d."

Then in section 7.7 Control Board Diagnostics, it steps you through how to get into the P4 diagnostic mode to menu item 3d, which will give you a direct readout of gpm (with no decimal). So a reading of 25 means 2.5 gpm.

Figure 41 shows how it's supposed to be plumbed for space heating. Note item #14, water-to-water heat exchanger. Not an air handler like you have. @EBEBRATT-Ed is probably right that the air handler can't extract heat fast enough from the coil. A water-to-water heat exchanger would extract it faster.

So even if you have sufficient gpm, if the air handler can't cool the coil fast enough, the return water is going to overheat the Bosch.

hot_rod

What size is the air handler? Even a basic kickspace heater can move 1.5 gpm, 15,000 btu/hr

CurtT

Thank you all. The gpm is outrageously high compared to the heat load, head, etc. The delta T is extremely low. And the whole system is CLOSED, contrary to the Bosch Manual. Owner has some choices, including complete gut and replacement, including a larger coil. I still say to start with reducing pump speed to its lowest and reduce boiler temp to 120F to give it a chance! Nothing to lose for the time being, until a decision can be made to re-pipe properly, or change pump to lessen gpm and gauge by measuring delta T, looking for the 20F differential....or close to it. Thank you all. Much appreciated. "Don't Blame the Boiler"

jesmed1

CurtT said:

I still say to start with reducing pump speed to its lowest and reduce boiler temp to 120F to give it a chance!

Can't hurt to try. After you turn the pump down, check the P4 diagnostic mode for the new gpm. Would be interesting to know what it is.