Home Boiler System - Need help sizing boiler circ pump

TheMichael

Hello. Home boiler heating system. Main floor and basement radiant heated - approx 5000 sqft over 7 zones total. Viessmann Vitodens 100W boiler (100kbtu). The Grundfos zone circulator pump (in pic) failed and we replaced with a larger Taco 0026E. Boiler circ pump is an older Wilo Star S 21. It seems like the zone circ pump is not able to keep up with the zone pump and the system 'stalls out' - where the outlet of the low loss header is only luke warm when zone or zones calling for heat. DHW always works fine. If I choke back the valve on the zone return line a bit - the feed line heats up nicely and supplies good hot water to the zones.

Some days the system works great, others it has a hard time getting zones up to temp - as I believe there is a flow mismatch or something on either side of the LLH.

Supplier recommended the Grundfos UPS 26-99FC. I don't want to get too low a capacity pump, but also not too high.

Edit: Supplier suggested a Grundfos UPS 15-58FC as a good match for my system.

Hope this may make sense - original mechanical contractor no longer around.

Any help or suggestions on a new boiler circ pump would be appreciated.

Note: Grundfos pump in pic is now a Taco 0026E set to AutoAdapt.

Thx!

hot_rod

are you using a system supply sensor connected to the boiler? Id mount it a foot downstream of the distribution circulator it measures and adjusts the boiler supply temperature.

TheMichael

I haven't heard of this, so no. When the system seems to be 'stalled', increasing/decreasing the boiler temp via the dial has little effect on the temperature going to the zones. Manipulating pump speeds and/or valving gets things in sync again.

hot_rod

thermal equilibrium is when the heat emitter is taking the full output of the boiler

Supply and return temperature will be stable at that point, not moving up or down

The actual output of that boiler mihht be around 93,000 btu/ hr. So all you need to circulate is 9, 10 or so gpm. Did the previous circulator do okay? If so no need to upsize

If the new larger circ is moving the flow faster the temperature difference will be smaller between supply and return

What we don’t know is what is controlling the boiler operating temperature? ODR outdoor reset or a fixed setpoint!

GroundUp

If that Alpha was doing the job, the 0026E should be more than capable. Is it possible that the auto adapt function has slowed the flow to a point that it's causing trouble? This has been an issue with the Alpha but I have no personal experience with the 0026E performance. Perhaps there is an airlock in some of the loops after the circ swap which is causing the strange performance?

hot_rod

one of these 3 conditions is present in a low loss header Notice how the arrows change color responding to flow variations, fig 10

So if the new system pump is modulating, speed and flow, then the outlet water temperature will constantly change

That is why the boiler needs to be looking at the temperature after the new pump. If it drops to low the boiler cranks up its outlet temperature to compensate

Could be the new pump has the control logic confused

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TheMichael

I'm definitely seeing what's going on in Figure 10 more often than not. Going to try a Grundfos 15-58 - the flow profiles better match the Taco and on speed one look good for the boiler rates and capacity.

Thanks for all the responses!

GGross

This boiler does not have the option for a supply sensor at the LLH, it will only use the built in sensors. Possibly you added air to the system when changing the pump and the little air vent on the LLH is not getting it all out, might need to purge, or possibly the pump has some air in it. Also double check the pump is installed with the correct direction of flow

do you ever have pressure issues? the expansion tank and fill valve are located on the supply in not the best locations but if the system operated well for years that is probably fine, generally I would leave the fill valve closed so I know if the system drops pressure so I can find a leak and fix it. Leaks mean air, and makeup water which means more air, that 1/8th inch air vent might not get the microbubbles out well

TheMichael

The system has been running (relatively fine) since 2010. I check for air in the llh often and open the bleed screw on the top of it. Not much air seems to come out any more. Pressure is always around 15psig - in part due to the water feed setup (at the top). I'l report back when I swap out the pumps. Of note, with the return ball valve closed to approx 40% - the system seems to be happy with existing old Wilo circ pump still in place.

GGross

You don't want to choke down the boiler pump usually, these are flow sensitive boilers, they need correct flow to operate well. When you change out the pump I would try with valves all the way open to make sure it works like that, it should!

hot_rod

What setting are you using on the 0026?

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The boiler manual should tell you what gpm, maybe even what pump speed the Wilo should be running. @GGross would know. I doubt you need to choke flow on the boiler side?

TheMichael

Taco is on ActiveAdapt. Any of the other speed settings seems to reduce the outlet temp to the zones.

The Wilo is on speed 3 which also works the best with the Taco on ActiveAdapt.

The valve used to choke down the flow a bit is on the zone (Taco) side. The Wilo boiler circ is free with no choking off. The system currently works the best with Wilo (boiler circ) on max speed and Taco on ActiveAdapt.

GGross

That wilo is kind of a goofy size for the boiler, according to Viessmann charts the wilo should be on speed 2 to get you as close to the spec you want, if it's on speed 3 it might be a bit too much flow for the boiler

TheMichael

The local supplier recommended the Grundfos 15-58 on speed 1 would be a good match. The Wilo may also be failing as well - I was told.

Dave H_2

the TacoAdapt on the 0026e will change speed if it sees a pressure differential occur. And it has to run for 90 minutes continuously and then it will start to adapt to the system needs.

TacoAdapt (as well as other auto modes built into circs) are designed for constant circulation system with say TRV's and such type systems. If it is already installed, I would put it in the Low setting. It will vary its speed as zones valves or actuators open and close.

hot_rod

That black low loss header keeps the two circulators separate from one another. Hydraulically speaking. So one pump should never interfere with the other flow wise.

Although there should be a gpm spec on that header. Typical 1" LLH are good for 10- 11 gpm on either side.

So the setting changes you are making don't make sense. The boiler on speed two, all valves wide open, as @GGross suggested, he is a Viessmann pro..

The Taco 0026, which seem like overkill, if a small Alpha was working well?

The system pump setting would be based on the system side requirements. Are there zone valves involved? Any idea on loop length.

Sounds like someone is getting a commission on selling you circulators 😉

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TheMichael

The smaller Alpha never really kept up when multiple zones were calling - did fine when only a few were though. Main floor has approx. 3300 feet of line. Double this with the basement zones. I believe zone 3 upstairs and the DHW use a Caleffi zone valve which opens when that zone calls for heat. I've had 2 Wilo's on the boiler circ fail over the past 14 years.

GGross

DHW? is the domestic piped in as a zone? if so, and you have in floor heat, where are the mixing valves? Are you able to take current pics of what the system looks like now? There may be a combination of things happening here. Has your boiler relief valve ever discharged to your knowledge?

TheMichael

Yes, DHW is Zone 1 which is on priority. When DHW calls for heat, all other zones get disabled and the boiler runs at a higher setting for DHW. This is controlled by Azel ilink zone controllers. The lines feeding the indirect water tank are the two larger 1.25" lines on the right heading towards the floor - and over to the left where the tank is. I don't have a picture of the right hand side where the zone valve feeding the upstairs is.

I've never heard or seen any relief valve action. Boiler has been serviced often and exchanger cleaned regularly. This is the older version which didn't have a dedicated DHW outlet. It does have DHW contacts (driven by the blue zone controller) which call for the high fire for dhw.

GGross

Ah a wb1a, I don't see many of those around here, figured it was the next series in line

hot_rod

so how many loops total in the building? 8 loops, or 16. Is it hydronic O2 barrier Pex?

Ballpark 1/2 gpm per loop, unless you have actual design flow rate. A complete design would spell out the exact pump spec, gpm and ‘ of head.

8 loops at .5 is 4 gpm, should be easy for the Alpha. 16 or more loops is pushing it, but without all the accurate data we all speculate.

WAG 16 loops at .5 would be 8 gpm. That low loss header should be able to deal with that flow number without any crossover. I’m not familiar with with the inside of that llh, some brands have a tight mesh that can plug and cause flow problems

What is the failure mode on the boiler pumps. Stuck, noisy, open winding? I’m curious how the service person knows a circ is about to go bad? Unless it is rattling.

If pumps are locking up, water quality is a common cause.

TheMichael

The picture with the loops was for the main floor. Adding the basement this would essentially be doubled. One Wilo rotor assembly seized up and now seems like 'inconsistent flow'. Sometimes you can hear what sounds like the pump slowing down. Adjusting the speeds down, then back up sounds like it gets running faster again. Water has been tested and is acceptable, but is now essentially water only (low glycol content) after several flushes. It's also of note that when the system is 'stalling or slow' - the flow will not always keep up to the boiler and it will cycle on and off as the water temp hits high limit or setpoint of the dial on front panel. When it's behaving normally, the flow will keep up and the boiler will fire and hit higher flame to heat the water. I rarely see the boiler hit high fire, which tells me the flow is not sufficient to remove the heat??

hot_rod

A fixed speed circulator that speeds up or down by itself often has sludge or magnetite in the rotor can. Air in the loop can cause pump surges also.

Taking apart a failed circ will often tell you the cause.

15- 20 years is not uncommon for a wet rotor running on curve in clean fluid.

If it had a high percentage glycol mix the circ needs to be up sized for that % mix

The heat transfer should be a bit better with plain water also.

Anytime I come across an old glycol system I want to flush and run a cleaner , fill with DI water and conditioner

The only way to know for sure what is happening us to have some gauges attached. Temperature in and out of the boiler. As it ramps down when the delta gets to wide, probably

Pressure gauges across the pump could determine gpm being moved

Flow meters are helpful but more complicated to add on

Without any actual or stable data it will be trial and error. An expensive and frustrating trial in some cases