Noisy potable-side circulation pump on radiant system

sjhudson

I've recently taken over maintenance on a rental unit my parents have. It has a Hydro-Smart radiant floor system with with a combined potable/radiant instant boiler. The system works, but the circulation pump on the potable side seems to make a lot of noise and vibration - see video linked below (I'm not sure how to do it inline). I don't have a reference for what it "should" sounds like, so I'm not even sure if this is normal or not.

Some notes:

• I have tried bleeding the pump using the screw in the center of the pump; it doesn't help.
  
• I have tightened all of the hold-down clamps on the panel piping.
  
• The pressure is a little high in the video, but the noise does not change if it is lowered, e.g., by running a hot water tap.
  
• I have no idea how long it has been like this. It is possible it has done it since installation about 5 years ago. The previous tenant never complained about it, but they also didn't seem to use the system much, if at all.
  
• The panels are mounted to the outside wall of the unit, and the sound seems to be from the pump vibrating the wall like a soundboard. The noise inside the unit is significant.
  
• The panels came assembled but were not professionally mounted, so it is possible there is some sort vibration dampening missing. If so, what can be done?
  

As mentioned, I'm not very familiar with radiant systems, so I'm not sure what noise level is "normal". It is my impression they are supposed to be quieter than this, though.

Thanks for any help and advice you guys have!

Link to video on google photos... hope this works!

Alan (California Radiant) Forbes

Is that a Grundfos 15-58 pump?  I would turn off the two isolation valves on each side, remove the 4 screw that secure the motor to the volute and see if there is anything that could be rubbing on the impeller. If nothing is there, replace the pump. 

hot_rod

Looks like a private labeled Grundfos with Stiebel Eltron name on it. Something stuck in the impeller or a broken vane throwing it off balance.

It is a stainless pump, but you could buy a cast iron Grundfos 15-58 and swap out the head like Alan described, it it is damaged. It will be a fraction the cost of a SS version.

GroundUp

Those Stiebel circs are notorious for tearing chunks out of the impeller. That appears to be the 26-110 cast iron version, and a replacement can be purchased at Menards. Or if you want to do it properly, replace it with a Grundfos 26-99SFC. Yes it'll be more expensive than the cast iron Stiebel, but it's the right way to run domestic and it'll outlast the Stiebel threefold.

HomerJSmith

I would take the nickle screw out of the back of the pump motor and see if any water comes out. Is it possible you have dry bearings?

You will probably not discover the issue until you take the motor off of the pump and pull the rotor out of the motor housing. That's a sound of something really wrong. Report back.

sjhudson

Thanks everyone for the responses. It's a Stiebel-Eltron CP3S26-110BFC, 3-speed 1/6 HP. The motor housing is stainless, and the pump section appears to bronze. I think Hot Rod is right about it being a private label Grundfos, it looks very similar to the UPS26-99SFC, although I can't find an exact 3-speed 1/6 HP Grundfos in bronze.

I've taken it apart, and the impeller seems to be fine. No pieces missing and the vanes look intact. It vibrates the same way when I power it up on the bench (briefly, I assume these don't like to run dry), and there is a bit of slop on the shaft, so I think the forward bearing is worn out.


It looks like there is a replacement rotor/impeller cartridge available for the Grundfos, which I'm fairly sure would work in mine. Any thoughts? It's 1/3 of the price of a new brass/stainless pump.

If I replace the entire pump, it's my understanding that shouldn't use a cast iron model, since it's on the potable side.
Is this correct?

Here are a couple shots of the pump and motor housing...

GroundUp

That's correct- CI in a domestic water system will work, but it's not ideal. Glad you found the culprit. The Grundfos motor "should" fit the Stiebel volute, but I've never actually done it.

hot_rod

price it as a replacement cartridge and also a complete cast iron Grundfos 26-99
Im pretty sure you can swap motor and impeller, leave the bronze volute you have in place 
Find some bargain prices on E-bay from time to time. Usually new, but old date codes models
A couple thin screw drivers will remove the rotor from that stainless rotor can, good chance that ceramic bushing up top is destroyed

Lack oif fluid for lubrication , or it cavitates in that application. If it is pulling through that 2 cv chrome mixing valve, that is not a good lifestyle for a high head circ 🥴

sjhudson

That's a good idea, so the cast iron 26-99 should still have the same stainless motor housing and cartridge? I'd like to replace the motor as well because it looks like it's been running a bit hot and might have a little water ingress, but I was willing to leave it to save $400. But there are 26-99s on ebay for less than the cartridge alone, so that would be a good way to go if the volute (just learned a new word!) is likely to be the only difference.

I don't think the pumped loop pulls through that mixing valve, that's where the domestic hot water comes out and it mixes water from directly from the boiler with water after the heat exchanger (and pump) - the only flow there should be when hot water is drawn. But the flow through the boiler was 3.9 gpm with the pump on high and 3.1gpm on low, which is much less of a difference than I would have expected and way below what the pump is capable of (lowest spec is 7 gpm @25 ft head), so something in the system is quite restrictive - I figured it was the heat exchanger or boiler, but I'm obviously learning as I go on this. The pump had been running on high since installation, and I can see how it may have been cavitating under those conditions.

I'd been wondering how much domestic-water-side flow is actually needed in a system like this... I'd rather keep it on low, or maybe even use a smaller pump. It's less than 1000 sq ft of heated floor on 6 loops, and a pretty mild climate (coastal California... "cold" here is in the 40s). My understanding is that the radiant floors were done professionally during construction but not used immediately, and the equipment was installed by my dad several years later, so it was never professionally engineered as a system. I'm sure it's a deep subject, but is there a way to get a rough how much flow should be needed?

I took the rotor out, and yeah, there's a lot of wiggle in the bushing on the shaft, and I can feel a worn spot on it. I wasn't expecting it to be all ceramic like that... learn something new every day.

Steamhead

Does the potable water mix with the water in the radiant loop?

If so, I'd add a heat exchanger of some sort. This can be a perfect recipe for Legionnaires' disease.

sjhudson

Steamhead, no, there's no mixing. It's an off-the-shelf system with a heat exchanger and additional primary and secondary loop pumps using treated water on the radiant side. For reference, here's a photo of the whole thing.

I'd be curious what you guys think of these combined domestic/radiant boiler systems... seems to make it a lot more complicated, and it takes forever to get hot water to the taps when the heating isn't running because of all the extra pipework. And even then it doesn't get very hot. Hence the added electric water heater on the left...

hot_rod

The only way to speed up DHW would be to maintain a hot plate HX. The transfer through the HX would only be a few seconds. That is how the combi boilers work. You chose cold start, slow delivers, or put it into maintain mode. Which of course cost more fuel to operate when you maintain the HX.

mattmia2

Use an indirect....

GroundUp

Using a tankless water heater for space heating is a crappy setup at all times, IMO. It's obviously a lot better the way you have it than an open system, but still far from ideal. Waiting for hot water is just the nature of the beast with cold start tankless WH and combi boilers alike

sjhudson

Thanks for the replies everyone. I took Hot Rod's suggestion and ordered a new cast iron Grundfos 26-99FC on eBay and replaced the volute with my brass one (fortunate I could do that, because one of cast iron ears broke during shipping.)

I installed it and bled it using the center screw before starting and several times afterward, and also bled everywhere else I could find. It is MUCH quieter. However, there is a not-too-loud "rattling" sound from the pump. If I restrict the outflow valve so the flow through the boiler is less than 2 gpm, the noise stops. With the valve open, it moves about 3.3 gpm. This is with the pump on low speed.

Is this noise likely to be cavitation rather than air somewhere in the system? Could it be what killed the old pump? Is there likely to be a restriction somewhere, possibly the HX? The system has only been in use about 4 or 5 years.

Thanks again everyone!

GroundUp

Is there a check valve in the volute? That's a common source of rattling.

hot_rod

Cavitation can occur when the pressure at the suction side of the circ drops below the vapor pressure of the fluid, or water. This can happen if there is a restriction in the circuit.  Plugged strainer somewhere? Ball valve chocked down? Undersized valve? Plugged HX? Plugged boiler HX? 

If the noise goes away on low speed, look for  a plug somewhere. A gauge at the suction side would be a way to see that

The check in the circ is a plastic, soft seat, rare but possible that it is the noise issue. Easy to check and clean.

sjhudson

Thanks for the feedback. Here's the latest...

I checked the pressure on the intake side of the pump, it pegged the 50psi gauge I had. So unlikely to be cavitation, I think.

Next, I took the pump out looked at the check valve... it seemed to be moving okay, but seemed a bit loose. I pulled it out, destructively unfortunately. Reinstalled pump, bled it, and no more noise! So that was the trouble. Since it's not cavitating, I really wonder now what killed bushings in the old pump.

Next problem... I didn't think I needed the check valve, but it turns out it needs to be there to keep cold water from looping backward through the pump to the hot side of the mixer when the system is in DHW mode with the pump off. I have a new check valve in the cast iron volute that came with the new pump, but I can't figure out how to get it out without destroying it. Know any tricks? I haven't had any luck trying to shim it around the edge.

Otherwise, it looks like it's $50 for a new one... a pricey little piece of plastic. I'd plumb in a separate check valve for $15, but there really isn't room for it on this panel without moving things around.

Thanks for any advice!

GroundUp

Since you already have the motor unbolted from the volute, you should be able to see the back side of the check from inside the volute. I always just take a flat screwdriver and poke it throigh the volute against the rim of the check and give it a few love taps. It'll just push out in one piece that way instead of 3.

hot_rod

That is really the worse place to install a check on a circulator, it is in the most turbulent condition. But... I suppose it is the lesser of two evils, backflow, or bad location

Maybe get a typical potable water (no lead) spring check and install it 6" or so downstream, above the HX maybe?.

Looking at this pic, the chrome mixing valve is a ASSE 1070 listed valve. As such it is shipped with check valves, same type as in the pump in both H&C, brass tailpieces unless they were removed?

But the tee to the right of the air purger, the top connection, where does that go? It would need to be checked also.

Looks like a Y strainer to the right of the pump, did you open that up? It could also restrict flow to the pump and cause problems.

sjhudson

Thanks GroundUp, that did the trick. I didn't realize it was just an O-ring and friction holding it in there, seemed like there were some tabs I would have to release to get it out without breaking it. It popped right out with a little encouragement.

I put it in the working pump, and I can hear the rattling noise again, although not quite as noticeable as it was with the old checkvalve. The noise itself isn't a problem, it's not loud enough to hear in the building. I just wanted to be absolutely sure it wasn't cavitation, since I really don't want to have to replace the pump again in a couple years. I'll leave it running on the low setting, since high only yields about 20% more flow and it uses a lot more power.

I do wish I knew why the bushings on the old pump were so worn. I suppose it's possible that it was run with no water (or at least no water pressure) at some point, but not intentionally. The stator coils had obviously gotten pretty hot at some point too - is that normal on a five year old pump? (see photos at top of thread)

Hot Rod, thanks for the suggestions, I'll keep that in mind in case the built-in checkvalve causes more trouble. The issue is probably moot now, but to answer your questions, the top of the tee is incoming hot water from the boiler. It doubles back behind the tee and comes from upper of the two lines with the big temp/pressure gauges. The pumped circulation is counter-clockwise from that tee --> air purger --> HX --> pump --> tee to cold water inlet --> strainer --> T/P gauge --> boiler --> T/P gauge --> and back to the tee in question. See my July 17th post with the photo of the whole system.

The mixing valve does have checkvalves, but they don't help this situation. The pump shuts off when DHW is drawn through the mixing valve (there is flow sensor after the MV connected to the controller on a priority zone input), and without the checkvalve, cold water was traveling backward through the pump, clockwise through the HX, and entering the mixing valve from the top (hot) port (in the pass direction of the checkvalve), so it couldn't mix correctly.

As an aside, since that photo was taken, I've actually replaced that MV with a Caleffi 521500A because the old one wouldn't go higher than about 108F. I'm pretty sure that was a separate issue with that MV though. It's been a problem since the system was installed and made the DHW feature of the panel practically useless - my dad had installed an electric water heater after the panel since he thought the boiler just couldn't handle it, but it works well now with the new valve adjusted to around 120F, and I should be able to get rid of the electric heater.

Thanks again to everyone for your help and suggestions troubleshooting this issue! It's been good experience getting to understand how the whole thing works - it was just a maze of copper pipes when I started.