Garn connection to existing boiler question

Sawyer · Aug 14, 2010

Can I get adequate heat transfer to the existing shop boiler loop with the closely spaced tees located as shown in the drawing? I was concerned with the existing boiler pump location on the return side of the boiler and the tees on the supply side before the radiant heat tubes.

If this schematic would work it would save me tearing down and rebuilding the plumbing on the existing boiler as this is a tankless boiler with a compact installation. There is not room for two tees on the return line the way it is installed.

Thanks.

Holley · Aug 16, 2010

Yes George, the configuration you have shown will work. With the right design flow rates and heat exchanger selection it is also possible to cut the HX directly into that supply line and eliminate the system side HX pump. With the right HX selection the pressure drop across the HX can be low enough for the system to perform well with only the existing boiler pump

Instead of the two close tees as drawn I would suggest the use of a Webstone Primary/Secondary purge tee. The Webstone p/s tee is essentially a set of close tees with an integral ball valve between the tees. If you stay with your existing drawing configuration the ball can be shut when initially purging the air from the HX loop, and opened for normal operation. If you decided to eliminate the HX pump you would operate with the integral ball valve closed. You would save the cost of the HX pump, flanges, and control relay to operate that pump. That's at least $200 which could more than offset the cost of increasing the HX size by say 10 plates. At that point you might also want to consider eliminating he mixing valve and crossover and instead using a Tekmar 356 to vary the speed of the GARN side pump to provide outside reset control of your system temperature.

TCaldwell · Aug 17, 2010

Sawyer, On my prior garn piping the injection loop was on the return rack just before it entered the boiler, again due to existing piping that i did not want to move. It worked great but needlessly ran the supply water through the boiler before it got to the supply rack/zones. After seeing your diagram with the injection loop at the beginning of the supply rack, if you turned the oil boiler aquastat all the way down, it is pretty close to a series piping job. I guess if the expansion tank is before the injection tees and your zone circs are on the supply side you will maintain a point of no pressure at the expansion tank.
tom

in hot water · Aug 17, 2010

What about a 3 way zone valve and single pump? A setpoint control with sensor on the HX would tell the ZV to open to HX or boiler.

The ZV would go on the return side if that is a high head pump just to alleviate any cavitation issues with restriction at the circ inlet.

hr

Sawyer · Aug 17, 2010

Chris@FHS said:
Yes George, the configuration you have shown will work. With the right design flow rates and heat exchanger selection it is also possible to cut the HX directly into that supply line and eliminate the system side HX pump. With the right HX selection the pressure drop across the HX can be low enough for the system to perform well with only the existing boiler pump

Instead of the two close tees as drawn I would suggest the use of a Webstone Primary/Secondary purge tee. The Webstone p/s tee is essentially a set of close tees with an integral ball valve between the tees. If you stay with your existing drawing configuration the ball can be shut when initially purging the air from the HX loop, and opened for normal operation. If you decided to eliminate the HX pump you would operate with the integral ball valve closed. You would save the cost of the HX pump, flanges, and control relay to operate that pump. That's at least $200 which could more than offset the cost of increasing the HX size by say 10 plates. At that point you might also want to consider eliminating he mixing valve and crossover and instead using a Tekmar 356 to vary the speed of the GARN side pump to provide outside reset control of your system temperature.

Chris,
For clarification the HX was given to me, it is a AIC LA14-50X, ¾” outlets. My plan is to run ¾” copper to connect with ¾” tees to the ¾” supply line. I need 31,000 BTU max so I thought this HX would work. The HX pump is a Grundfos 15-58. The boiler pump is currently a Grundfos 15-42F.

I do not know if this configuration would work with a Webstone p/s tee and the existing pump only.

Sawyer · Aug 17, 2010

TCaldwell said:
I guess if the expansion tank is before the injection tees and your zone circs are on the supply side you will maintain a point of no pressure at the expansion tank.
tom

Tom, I wish were as knowledgeable as many on this board, are you saying that the "point of no pressure" is a good thing or bad?

Holley · Aug 17, 2010

Well that was a nice gift George

That LA14-50X is already over-sized for your application. I generally design radiant systems around a 15 degree temperature drop across the floor circuits. A 31,000 btu/hr load would require approx 4 GPM of flow. At 4 gpm your HX will introduce less than a foot of head loss so yes you can get away with only the boiler pump by putting the HX in series in the supply piping.

But what is the existing boiler George? Did you mean tankless as in it has a tankless domestic hot water coil. If so, neither the configuration you drew and the alternative I suggested will maintain the boiler temperature to supply domestic hot water. Or is this possibly a condensing wall hung gas boiler? Given that you are currently supplying radiant floors and there is no mix down shown on the existing piping maybe that's the case.

Up until about 15 years ago it was standard practice in this country to put circ pumps on the boiler return line and the expansion tank on the supply side (as yours is shown). Standard practice....but not very good practice, because it sometimes led to air problems with the potential for some locations in the circuit to actually drop below atmospheric pressure so that a small leak or an air bleed fitting could actually introduce air into the system.

Today it is standard practice to always pump away from the expansion tank which is the point of "no pressure change" that Tom referred to.

In reality it's often a matter of how much re-piping you are willing to undertake when you are adding a wood boiler. Ideally when you are operating multiple boilers the boiler that is NOT in operation should not have flow through it but that generally involves more re-piping. I like Hot Rod's solution with the three way zone valve. With a small system like this that's a pretty inexpensive valve.

Chris,
For clarification the HX was given to me, it is a AIC LA14-50X, ¾” outlets. My plan is to run ¾” copper to connect with ¾” tees to the ¾” supply line. I need 31,000 BTU max so I thought this HX would work. The HX pump is a Grundfos 15-58. The boiler pump is currently a Grundfos 15-42F.

I do not know if this configuration would work with a Webstone p/s tee and the existing pump only.[/quote]

Sawyer · Aug 19, 2010

Chris@FHS said:
Well that was a nice gift George That LA14-50X is already over-sized for your application. I generally design radiant systems around a 15 degree temperature drop across the floor circuits. A 31,000 btu/hr load would require approx 4 GPM of flow. At 4 gpm your HX will introduce less than a foot of head loss so yes you can get away with only the boiler pump by putting the HX in series in the supply piping.

But what is the existing boiler George? Did you mean tankless as in it has a tankless domestic hot water coil. If so, neither the configuration you drew and the alternative I suggested will maintain the boiler temperature to supply domestic hot water. Or is this possibly a condensing wall hung gas boiler? Given that you are currently supplying radiant floors and there is no mix down shown on the existing piping maybe that's the case.

Up until about 15 years ago it was standard practice in this country to put circ pumps on the boiler return line and the expansion tank on the supply side (as yours is shown). Standard practice....but not very good practice, because it sometimes led to air problems with the potential for some locations in the circuit to actually drop below atmospheric pressure so that a small leak or an air bleed fitting could actually introduce air into the system.

Today it is standard practice to always pump away from the expansion tank which is the point of "no pressure change" that Tom referred to.

In reality it's often a matter of how much re-piping you are willing to undertake when you are adding a wood boiler. Ideally when you are operating multiple boilers the boiler that is NOT in operation should not have flow through it but that generally involves more re-piping. I like Hot Rod's solution with the three way zone valve. With a small system like this that's a pretty inexpensive valve.

Thanks Chris, it is a tankless domestic hot water coil. The temperature is controlled by an aquastat. Luckily the system has been running flawlessly for ten years. I also like Hotrod's idea, I may redo this setup when I add the Garn supply.

Holley · Aug 20, 2010

Sawyer said:
Chris@FHS said:

Well that was a nice gift George That LA14-50X is already over-sized for your application. I generally design radiant systems around a 15 degree temperature drop across the floor circuits. A 31,000 btu/hr load would require approx 4 GPM of flow. At 4 gpm your HX will introduce less than a foot of head loss so yes you can get away with only the boiler pump by putting the HX in series in the supply piping.

But what is the existing boiler George? Did you mean tankless as in it has a tankless domestic hot water coil. If so, neither the configuration you drew and the alternative I suggested will maintain the boiler temperature to supply domestic hot water. Or is this possibly a condensing wall hung gas boiler? Given that you are currently supplying radiant floors and there is no mix down shown on the existing piping maybe that's the case.

Up until about 15 years ago it was standard practice in this country to put circ pumps on the boiler return line and the expansion tank on the supply side (as yours is shown). Standard practice....but not very good practice, because it sometimes led to air problems with the potential for some locations in the circuit to actually drop below atmospheric pressure so that a small leak or an air bleed fitting could actually introduce air into the system.

Today it is standard practice to always pump away from the expansion tank which is the point of "no pressure change" that Tom referred to.

In reality it's often a matter of how much re-piping you are willing to undertake when you are adding a wood boiler. Ideally when you are operating multiple boilers the boiler that is NOT in operation should not have flow through it but that generally involves more re-piping. I like Hot Rod's solution with the three way zone valve. With a small system like this that's a pretty inexpensive valve.

Click to expand...

Thanks Chris, it is a tankless domestic hot water coil. The temperature is controlled by an aquastat. Luckily the system has been running flawlessly for ten years. I also like Hotrod's idea, I may redo this setup when I add the Garn supply.

Ah now I understand

...you are using a tankless hot water heater as a boiler. (as opposed to having a tankless hot water coil in a boiler). That being the case, your original configuration, the one that I suggested, or HR's will all work. I think you might be better off leaving the pump on the return. On a tankless hot water heater and other wall hung gas boilers the pumps are often put on the return to maintain the highest system pressure at the boiler hx to prevent the water from flashing to steam there.

If you install a three- way zone valve as suggested by Hotrod ,look for a valve with a CV of around 7 or larger to keep the pressure drop low. I've attached a drawing with each method. The Tekmar will vary the pump speed to vary the radiant water supply temp as outdoor temps change, and it will give you a remote reading of GARN temp, radiant water temp and outdoor temp. It's a very reliable control and it's about $350. The zone valve method will probably cost about $250 for the thermostatic mixing valve and the three-way. In the end it might come down to whatever is the easiest re-piping of your existing setup. That's what's fun about hydronics...there are always several good solutions.

Sawyer · Aug 21, 2010

Thanks for the explanation and the drawings Chris.

You even made me look up CV, so I learned more than you expected ;-)

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