Hot water loop question

ceksenior

I have a question about my setup that I been wondering about. I have a hot water loop off a steam boiler which for the past approx 7 years has been working quite well. I included 2 pictures which I hope can show as to how the system works.

Basically the first picture shows the hot water loop off the steam boiler part of the system. The water leaves the boiler on one side goes through the wye strainer than the pump and then the heat exchanger which is about 6 feet long in distance. It then goes back to the boiler through the honeywell control and reuturns on the opposite side of the boiler aprrox 9 feet in distance.

The second picture shows the second part of the system which is seperate( boiler water does not circulate through it, it is its own sealed system).

  When the system is making steam and one of the other zones from the hydronic part of the system kicks in the pump to start the hot water loop zones, the pump is quite noisy and sounds like it is cavitating. im also sure that the pump probably shouldnt have water that hot going through it.

Since the hot water loop is so short Im sure a valve in between to regulate the water before and after the heat exchanger wouldnt change the temperature that much.

So is there any way to change the way the system works so that the pump wont turn on when the system is steaming, and only come on when the water temperature reaches a safe temperature?

Gordan

That circulator looks pretty beefy

It could very well be cavitating. On the boiler side, you've got water with a lot of dissolved air in it, under pretty much atmospheric pressure. The suction of the pump would further drop the pressure and cause air to come out of solution, especially at higher water temperatures. You should not need such a large circulator... what's the required flow through the heat exchanger and what's the pressure drop at that flow? The circulator SHOULD be a bronze or stainless one; an iron one will rust out due to all that oxygen in the water. The situation could be getting aggravated further if the heat exchanger is getting clogged up with debris.



I don't think that the circulator will mind any temperature that water in its liquid phase is likely to have at atmospheric pressure.



You might also look into replacing that wye strainer with a dirt separator, to protect the clog-prone heat exchanger.

ceksenior

Tim the Tool Man Circulator

No doubt the circulator is to large. That came 2 days after system was set up and I could hear the original circulator 2 flights up in my bedroom. The original pump was like the one in picture 2. The installer came with what he had at the time and I appreciated his effort greatly. Believe it or not the large pump is much quieter on startup when the boiler is steamin! I am looking to buy a bronze impeller circulator to keep on the shelf if this one fails.

I dont know the required flow or pressure drop of the heat exchanger. Its a triangle tube model.

I do blow down the wye strainer occasionally. It has a ball cock valve on it. Some crud and rusty water come out.  

Is there a better method than the heat exchanger or is this system destined for failure becuase of design?

Robert_25

Bypass...

In addition to a properly sized circulator, you should have a bypass installed on the condensate loop to keep the temperature manageable. Here is some good reading.



http://www.heatinghelp.com/files/articles/1360/346.pdf



http://www.heatinghelp.com/article/330/Condensate-Hot-Water-Heating/76/Condensate-Hot-Water-Heating-FAQ



http://www.heatinghelp.com/files/posts/9875/CounterPointJun04-B.pdf

ceksenior

Bypass

 From what Ive read here a bypass would only help if the condensate was actually heating a zone.  After the water ran through the whole system including piping and radiators than the return would have lost considerable heat. My zones are run off a seperate loop off the heat exchanger.

 The condensate loop off the boiler is only about 15feet in length from inlet to return. When I use an IR thermometer to read temperatures between the pipe before the heat exchanger and after its barely a 3 degree difference. 

  

SWEI

circ in first photo

3-piece Armstrong, quite possibly an S-25 which has a flat curve with high flows @ low heads.

Gordan

If you had less flow through the condensate loop...

...the temperature difference would be wider, so tempering would be more of a prospect.



But I still don't believe that you're going to have an issue with those temperatures.

Mike Kusiak_2

Flash boiling

Sounds like whats happening here is flash boiling in the pump. When the boiler steams the water in the boiler is at over 212F. When the pump runs it creates a low pressure area at the impeller. Water boils at a lower temperature at lower pressure so it is creating bubbles of flash steam in the pump. That's probably the noise you are hearing. When not steaming, the boiler water temperature is lower and less flashing to steam and  noise occurs.



A boiler bypass in the condensate loop will lower the temperature seen by the pump and bring it down to a temperature where boiling will not occur.



The existing pump looks like a B&G 100 which actually generates less head than a taco 007. There is an all bronze version of the 100 which probably would be the most suitable long life replacement.

ceksenior

As usual

All informative and knowlegable information given when Ive needed it. Thanks guys.

 Just a couple more things please.



1 Do you have a filter you can recommend and where would be the optimum placement of it?



2 Where in the loop should I locate the bypass?



3 What pump do you recommend?

SWEI

pump sizing

Will require knowing the heat load of the HW zone(s) plus head loss for the boiler, HX, and associated piping.



And I was wrong about the pump brand - I can see the B&G sticker on the side (but a Series 100 and an S-25 are basically the same pump.)

Mike Kusiak_2

Solutions

You want to connect the bypass between the supply and return of the steam boiler. This way some of the cooler return water from the exchanger bypasses the boiler and mixes with the supply water entering the pump, lowering its temperature. Include a valve in the bypass so that you can adjust flow through it.



My choice of pump would be the bronze version of what you have, the B&G 100 AB.

They are a bit pricey, but share common parts with your present pump so you old motor etc could be saved as a spares. These bronze pumps can sometimes be found on Ebay at reasonable prices. Alternately something like a Taco 005 BF2 bronze circulator might work, but it probably wont last as long with the dirty steam boiler water.