Open or closed radiant heat system

zmann96

I'm getting ready to set up my 75 gal gas fire hot water heater and tie it into my manifold. The system is new. I have 5 loops of 1/2" approx 320' each. The system will have a heat exchanger. I have everything to set this up as an open loop with a combo back flow preventer and fill valve and a heat exchanger. Now I'm having second thoughts about the open loop as opposed to a closed. I had read that a closed loop would cause the circulator to cavitate and make noise without some pressure in the system. I would welcome your thoughts.

Ironman

Are You planning on connecting it to the same water heater that does your domestic?

Jamie Hall

Open loop -- presumably your domestic hot water will be drawn from the same piping? -- is an exceedingly poor idea. Not that a lot of people don't get away with it, but it is a dream breeding ground for almost any kind of bacteria that can make you -- or anyone visiting you -- sick. Don't do it.

Further, a domestic water heater is the wrong tool for a heating system. It's not meant for continuous duty, and it won't last long -- and its efficiency is poor.

Now it seems, though, that you mention a heat exchanger? Is the idea that the hot water heater will circulate through a heat exchanger and the other side of the exchanger will circulate through the radiant tubing? That's not an open loop. Or is it that the hot water heater will circulate through one side of the heat exchanger and the radiant heating, and domestic hot water will be on the other side of the heat exchanger? That's not likely to work as the heat exchanger may not give you enough domestic hot water.

It is quite true that a circulator -- or any pump -- will cavitate if there isn't adequate pressure on the intake side. However, any piping which is subject to a varying temperature and which has a backflow preventer must have provision for expansion (and a pressure relief valve) downstream of the backflow preventer. So what is usually done is that the radiant tubing is treated as a closed loop, with an expansion tank on it, followed by the circulator, with a fill device -- such as a backflow preventer and pressure reducing valve -- and a protective device -- the pressure relief valve -- all hooked in at more or less the same location as the expansion tank. That all runs through the boiler -- or if you insist on a water heater also serving for domestic hot water, a heat exchanger -- on out through the loops and back around to the expansion tank. Usually kept at 10 to 15 psi.

Perhaps if you cared to post a sketch I could be a bit more specific?

Rich_49

Use the heat exchanger and set up the heating side with it's own Pressure reducing valve , air elimination and circ . Use only a bronze or stainless steel circ on the domestic side

zmann96

Hi Rich, yes my domestic will be on one side of the heat exchanger and my radiant will be by it's self of other. I have the pressure reducing valve ( if I end up doing the filler and back flow) I have an expansion tank, spirovent, both circulators for each side of the exchanger are SS, and both mixing valves are bronze. I'm just concerned with the cavitation and noise from the circulator on the heat side. what's the pros and cons of each closed or open. I will have back flow preventers to keep the heating separate from the domestic.

zmann96

Ironman said:

Are You planning on connecting it to the same water heater that does your domestic?

Yes, my domestic hot water demand is very low and I have installed a 10' length of gray water heat recovery from my shower.

Zman

Are you asking whether you should leave it completely isolated or tie it to domestic with proper backflow protection? In that case, they would both be considered closed systems.

zmann96

Jamie Hall said:

Open loop -- presumably your domestic hot water will be drawn from the same piping? -- is an exceedingly poor idea. Not that a lot of people don't get away with it, but it is a dream breeding ground for almost any kind of bacteria that can make you -- or anyone visiting you -- sick. Don't do it.

Further, a domestic water heater is the wrong tool for a heating system. It's not meant for continuous duty, and it won't last long -- and its efficiency is poor.

Now it seems, though, that you mention a heat exchanger? Is the idea that the hot water heater will circulate through a heat exchanger and the other side of the exchanger will circulate through the radiant tubing? That's not an open loop. Or is it that the hot water heater will circulate through one side of the heat exchanger and the radiant heating, and domestic hot water will be on the other side of the heat exchanger? That's not likely to work as the heat exchanger may not give you enough domestic hot water.

It is quite true that a circulator -- or any pump -- will cavitate if there isn't adequate pressure on the intake side. However, any piping which is subject to a varying temperature and which has a backflow preventer must have provision for expansion (and a pressure relief valve) downstream of the backflow preventer. So what is usually done is that the radiant tubing is treated as a closed loop, with an expansion tank on it, followed by the circulator, with a fill device -- such as a backflow preventer and pressure reducing valve -- and a protective device -- the pressure relief valve -- all hooked in at more or less the same location as the expansion tank. That all runs through the boiler -- or if you insist on a water heater also serving for domestic hot water, a heat exchanger -- on out through the loops and back around to the expansion tank. Usually kept at 10 to 15 psi.

Perhaps if you cared to post a sketch I could be a bit more specific?

I've attached my plan. I'm not concerned about flow going back into the feed. that's the only way it could be mixed.

zmann96

Zman said:

Are you asking whether you should leave it completely isolated or tie it to domestic with proper backflow protection? In that case, they would both be considered closed systems.

Zman, I'm asking if I should use the filler and back flow preventer or just let it be completely isolated which I thought would be closed loop without any flow in.

Zman

With proper backflow protection it won't ghost. I would get rid of both the circ check and the one on the cold side of the mixing valve. They serve no function and may isolate the circ from the expansion tank.

zmann96

Thanks Zman

zmann96

zmann96 said:

Thanks Zman

Would you keep the cold water feed?

Zman

What type of system? Are you concerned about leaks? It can be a pain to keep the system topped off if you don't either tie it in or use an auto feeder.

zmann96

Zman said:

What type of system? Are you concerned about leaks? It can be a pain to keep the system topped off if you don't either tie it in or use an auto feeder.

No, not concerned with leaks, and I don't want a maintenance issue so I guess I'll tie it in with auto feed and backflow preventer. Thanks so much

hot_rod

so Thai the tank provides domestic hot water?  If so separate the hydronic side, Heat Link and Utica Boiler make heat exchanger modules for water heaters used like thus

Doesbthe heater have adequate BTU capacity?

zmann96

hot_rod said:

so Thai the tank provides domestic hot water?  If so separate the hydronic side, Heat Link and Utica Boiler make heat exchanger modules for water heaters used like thus

Doesbthe heater have adequate BTU capacity?

Yes, I have adequate BTUs per hour for my use and I've sized my heat exchange also.

Jamie Hall

For reference. Most circulator type pumps, such as you will be using, will not cavitate, assuming they have a few diameters of straight pipe at the intake, if the intake pressure can be maintained at at least 17 to 18 psi absolute, which would be around 3 psi gauge in your appliaction.