Tankless resonance...

[tinrat]

Ok. I just started pumping heat into the floor for its maiden power-up last week. Return is up to 60f. imput runs 170f. In the tankless I am getting a vibration resonance. Is this common? And could it cause failure to the system. I hope this will go away when I get more BTU's pumped into floor. Seems like It takes a while to start back up if I give it break and shut the gas off for a few moments.


40x64x16 building, fairly well insulated.
3.5"tx8"d insulation at perimeter edge & 3" bottom for 4' inward. 2" under rest of floor.
110k tankless water heater
8 loop SS manifold x 300' for 2400' of 1/2" PEX.

(broken link removed to http://images33.fotki.com/v1072/photos/4/31394/9266030/P1010067-vi.jpg)

 

[Beetle-Kill]

Nice set-up. Could your "resonance" be a water-hammer thing? Do you have a reducer in your system? I only ask because I'm correcting that situation in my place.

 

[Hunderliggur]

Do you have an air purge on your system? You may be trapping air causing problems.

 

[jebatty]

Return is up to 60f. input runs 170F

Unrelated question. What I have read for concrete is that input temperatures in the 115-135 degree range is appropriate for concrete. Might 170F be too high? And if so, what effect?

My shop building is similar to yours, but 32 x 48 x 14, well insulated, 6 loops of a little under 300' each. My system has a mixing valve and I have limited input temperature to 110F maximum, typical 100-105F. The sensor in the floor is set at 61F with a 1F differential. Interior air temp is 55-60F with outside temps in the -10F to +20F range. This is a very comfortable working temperature.

 

[tinrat]

My only air removal is a double ball valve at the highest point in the system (its directly after the heater). This was my "save $100 and forgo an air removal valve". I am trying to get this up and running on the cheap and hoped that some of the components could be later added or totally avoided. Perhaps this is my demise? Should I possibly have another purging outlet directly after the return manifold and before the pump? I have no flow metering in the system so I don't know how fast circulation takes place. I would think that air pockets could remain in areas under low flow speed or pressure. Is there a recommended flow speed or pressure for given diameters that might help remove trapped air?

 

[woodsmaster]

What happened to your flow meters ? I have manifolds just like yours only with flow meters. I"ve heard not to use over 140 degree supply water. Dont Know why other than you may get hot feet.

 

[woodsmaster]

It's odd you have the Knobs to adjust the flow on your loops but no meters.

 

[b33p3r]

I installed radiant in 3" of gypcrete over my basement concrete slab. I was told by radiant supplier to run temps at 135*. I had a contractor tell me to run it at 90*. I tried both temps and I will tell you the 90* temps are much better IMO. Temps are more consistant across floor. Not warm then not so warm etc etc. Circulating pump obviously runs longer but also less surge on the boiler. Try different temps and see what you like.
As far as your issue is concerned, it sounds to me like you have air trapped in lines. Did you purge the loops one by one with pressurized water before turning the system on? It's hard to tell by looking at your photo but I don't see any drain valves that you can hook a hose up to to purge them? You should install an air remover on your setup also. If you didn't purge the lines of air with pressurized water your circulating pump is gonna have a hell of a time doing the job. It won't pump air. You can try to close all but one loop and let the pump try to purge that line. Then single out the next loop and purge..etc etc until all loops are happy. With 170 in you should see return temps better than 60* after a short period of time. Just thoughts.

 

[tom in maine]

You need an air removal device.

I suspect you have some air in the loops. Or your pump is too small, probably both.
Is the space heating up?

You should be able to run your hand across the floor and get a hint as to which loops are flowing.

How thick is the slab?

Merry Christmas!

 

[jebatty]

I am puzzled by your system design. 1/2" pex for radiant normally is spec'd for flow of 0.4 gpm. Assuming that was your design, 8 loops = 3.2 gpm total. With input at 170 and output at 60, delta-T = 110, and that equates to 3.2 x 110 x 500 = 176,000 btuh, which to me seems extremely high. Perhaps your flow is less. What is your circulator brand and model? What is your tankless heater output btuh rating? What is the tankless heater pressure drop? Are all of your flow control valves wide open?

Your building is twice the cubic feet of mine. I have 6" of insulation in the walls and R40 in the ceiling (two 2' x 4' windows, one 3' and one 4' insulated service doors, and one 12' x 12' insulated overhead door), and my calculated heat loss at base 60 and winter extreme of -30F was right around 40,000 btuh. So far this winter our low has been about -10F, and my actual average heat loss at outside of -10 to +10 is right around 16,000 btuh. If I was using a tankless heater, output sizing of a tankless heater probably would have been right around 40,000 btuh, although I did not research that.

It would seem at your location in central Illinois vs mine in northern Minnesota that your heat loss would be quite a bit less than twice mine. Did you size your tankless heater according to a heat loss calculation, or how else did you size it? Over-sized heaters for radiant floor are not very efficient because once your floor is up to temp, the heater will cycle on and off excessively, and each cycling is inefficient. Did you size your tankless heater for some additional use? And if your tankless heater only is serving the floor, I suggest turning down the output temperature considerably, and especially once your floor is up to temp.

I don't intend to be critical in a negative way. It took me 3 re-dos of my system to finally get it to what I think now is right, with each re-do having plenty of cost. Live and learn, I guess. Maybe some of my questions will help you strive to an improved system as you finally get it all completed.

I am inclined to agree with the other comments on trapped air in the system. An air scoop and vent at the high point would be very desirable. You also might want to do a pressure purge of your system, one loop at a time, until all the air is out.

 

[RobC]

Hard to tell from picture but are the supply & return lines large enough to carry flow / Btu's. 1" is 71,000 btu @ 7.1 GPM. Could the water be moving so fast around the pex fittings that that could be causing some of the noise. Not familiar with this style set up but is there a mixing valve in loop or is one not needed with a tankless boiler ?

 

[in hot water]

air and or not enough flow.

With a small HX boiler like that you really want a microbubble type air eliminator. Those small bubbles, that develop when water is heated, tend to gather in the HX tubes, insulate the flame from the water and cause a peculation, resonance, or moaning type noise.

That boiler, depending on the manufacturers spec, may need a high head circulator to scrub away the heat energy that a small tube, large burner boiler can produce.

I'd limit the operating temperature to 140F or less.

I've never seen concrete "shocked" by excessive temperature but it will get uncomfortably warm to stand on if you don't have a limiting mechanism.

hr

 

[tinrat]

http://images55.fotki.com/v605/photos/4/31394/9266030/P1010046-vi.jpg
Been burping and purging. I believe that a larger pump may be the answer. Would another pump in line with existing pump work? If I put 2 of these pumps in line, would that increase flow, or would I have to get a bigger pump? Can I put a pump pushing hot water into the manifold? I have no knowledge or intelect on this. The whole thing started several years ago when I bought this new Thermar tankless LP heater in its original box (w/no manual) at an auction for less than $20. Am working on a bio to post later.... anyway... $30k later I have a shop with pex in the floor and the Thermar hanging on the wall. Had I exorcised a little more sense, I would have reserched things a little closer an much sooner. Hearth boiler room is my life suppport. I thank everyone for their jenerous help. Back to the matter at hand... I also noticed robC mentioning 1" tubing. I only used 3/4" to the manifolds. could I be undersized? After purging/burping indevidual loops best I could I have managed to add another gallon of coolant and increased the system pressure to 17-18#. I have put just under 25 gal into the sytstem. That was the quantity calculated to fill 2400' 1/2" pex., So I should be getting close to having the air removed from system. I will try to turn the heat down in the water heater. May be a few days before I get anywhere to look into parts and or pumps. Merry Christmas Everyone!

 

[tom in maine]

Hi Tin,
Merry Christmas!

I think I know what is going on.
Should've asked this first, except there are not many Thermars kicking around, most people have newer units.
Many of the early generation tankless gas heaters (Thermar is one) had significant flow restriction in the water circuit.
They used a diaphragm to sense water flow. In order to do this, the water path restricts down to about 1/8" to 1/4".

Many moons ago, I attempted to do the same thing you are doing with a different manufacturer's unit. They claimed that it would work.
I learned so much about the units, that the importer had my company remanufacture all their warranty returns.

A friend had a Thermar, that we tried to do this same set up with. We/he eventually wound up with a more conventional boiler.

All this being said, almost all of the newer Japanese wall hung tankless water heaters now use electronic flow detection to do what
all the water flow restriction accomplished. And these can function like boilers without any big problems. They should tell you this on their websites.

I hate to say this anytime, especially on Christmas, but I think you need to change out the boiler if there is a diaphragm in it.
You should be able to determine this from the manual. If not in the manual, it will be visible as a round brass cover about 2.5" in diameter
near the water piping at the bottom of the unit.

Rinnai is one that I know will work.

Merry Christmas!