Sidearm for DHW Supplies Boiler After Extended Periods

[rwh442]

I have a Thermo Control boiler that supplies a water to air HX in my furnace plenum in the house and also runs through a DHW sidearm heat exchanger. The piping is in series - the water runs through the sidearm first and then the furnace HX and then back to the boiler. I feed the boiler at 6 AM, go to work, and return around 5 PM. I am collecting temperature data on all the key points in the system and around 2 PM every day the DHW tank sidearm reverses the heat exchange process and starts to feed the boiler the heat it has stored up (the boiler fire usually starts to die around 10 - 11 AM). The data looks strange at first until you figure out what is going on - the DHW usually gets to 160-170 degrees and starts the backfeed process around 130 degrees.

My question - how to stop the backfeed from the DHW tank sidearm to the boiler. Would a zone valve and timer work to maximize the heat retention in the DHW tank? Say shut off the sidearm flow at noon every day? I would say that about 75% of the time we are fine during the evening but every once in a while you get the dishwasher running, washing machine and then its time to give the kids a bath and then there is no hot water and screaming kids. . .

Any ideas would be appreciated!

 

[Fred61]

Hopefully you have a tempering valve on the output of the tank. 170* is too hot especially if you have kids in the house. A tempering valve will extend the usable hot water. The dishwasher is going to take only hot water without mixing. 130* is adequate for dishwashing. Again the same for the clothes washer. I believe when it calls for hot, it does not sense the temp. It simply dumps a preset amount on that particular side. Have you noticed if the kids are screaming if is too cold or too hot? I've run my DHW down to 120* with no adverse effects.

 

[tuolumne]

I noticed the same problem on our setup. It was like having an 80 gallon water buffer. I have a valve on my sidearm, so I manually close this if I know the boiler will be going cold so that it does not backfeed the system when the zones call. I am not sure about the zone valve option. These are always closed; you need something that is always open. I am sure that others will be along with some schemes far more elaborate than mine!

 

[Hydronics]

You could put the zone valve in the thermosiphon loop, control it with a normally closed aquastat clamped to circulating loop on the other side of the hx. Set it to open @ your minimum desired tank temp. The only concern may be head loss through the zone valve, it may be excessive for adequate flow by thermosiphon (no circulator).

 

[Eric Johnson]

You could put a small circulator like a Taco 007 with integral flow control on the domestic water line and run it directly off your water heater thermostat. When the temp reaches the setpoint, it shuts off the pump and the flow check keeps the water in the tank from circulating back. I'd set it high so that you always have a tank of very hot water, and then use a tempering valve, as mentioned, to bring the water temp at the tap down, while extending your hot water supply. You'd probably need to make the domestic water flow into the bottom of the tank with this setup.

Obviously, by doing this you lose the advantage of gravity circulation and you use a bit more electricity running the pump, but it pretty much guarantees an endless supply of hot water, which will be more than made up for in enhanced domestic harmony.

I don't do any of that, by the way. I just try to keep my boiler water temp above 140 and we never run out of hot water. But everybody's situation is different.

You should use a stainless steel or bronze pump for this application, BTW, but I've used cast iron pumps in similar applications in the past and they ran for years without succumbing to corrosion or staining the water with rust.

 

[tuolumne]

I would not worry about head loss with Hydronic's method. I have a ball valve on my sidearm that stays nearly closed all the time. This was to prevent DHW demand from shortcutting through the sidearm during useage. My tank heats quickly and effectively. The thermosyphon flow will be quite slow anyway.

 

[Fred61]

Would a simple swing check valve work? Perhaps some of the more knowlegable contributors to this forum can tell you if thermosyphon action will overcome the resistance of the check valve.

 

[Eric Johnson]

I don't think it will. That's the problem.

 

[rwh442]

Thanks for all the replys guys.

I should have mentioned before that the DHW heater is propane heated. Hence I have no thermostat to start/stop a pump as Eric suggested. Right now I'm still thinking over Hydronic's idea.

I do have a mixing valve on the output of the DHW tank to avoid scalding. I also have a manual valve in the thermosiphon loop and need to play with shutting it off on weekends and getting a feel for how long the DHW lasts. Just wanted to get ideas on an automated methodology.

Speaking of a propane DHW heater - do you think I should cover the exhaust flue to conserve heat? I would have the pilot light off. So long as I remember to take the cover off before firing it up again . . .

 

[Eric Johnson]

Well, there must be a heat-sensitive switch somewhere to turn the gas on and off to maintain temp.....

 

[rwh442]

Eric,

There is some type of mechanical valve that opens the propane line using a mechanical thermostat but I'll bet its probably "encased" pretty good in the controls. I'll check tonight.

 

[Eric Johnson]

Any surface-mount aquastat would work.

My dad has untreated hard water and it clogged up his first sidearm. He put in a pump and hasn't had a problem since, which is to say around 15 years. I have a water softener and I still get crap accumulating on the bottom of my water heater tank. If I don't drain off 5 gallons every couple of weeks, my gravity-feed sidearm will quit working, and then I hear the dreaded "Honey, there's no hot water."

 

[Fred61]

Would a simple swing check valve work? Perhaps some of the more knowlegable contributors to this forum can tell you if thermosyphon action will overcome the resistance of the check valve.

I'm sort of a maverick so I'd try the swing check. It won't cost much to experiment! If the thermosyphon doesn't overcome the resistance, why am I spending so much money on flow-check valves?