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An installed radiant floor system was working fine for one week, until a tenant inadvertently changed the mixing valve setting from the hottest position (6) to the coolest (position 1). Two days later, the tenant complained about the floor being cold. I have not been able to restore the system to its previously balanced state, and the floor hardly gives off any heat at all.
Oddly enough, the flow within the loops seems to have reversed itself, with the hot water entering the cold manifold, and cool water flowing from the hot side!
Would the addition of a check valve prevent this flow reversal? System pressure is 45psi. I am concerned that the check valve might be forced shut at it would be located right next to the mixing valve, which is also next to the manifolds. Could the close proximity of these devices cause some sort of “thermal or gravity” block or interference?
The owners have told me that the five 240’or so 1/2″ floor loops are not of equal length, nor are they sitting in the same plane under the slab. In one area, apparently, the loops sit 6″ lower than in the others. There is underslab insulation, but no perimeter insulation.
I am at a loss to explain this flow reversal. Thanks, Pierre
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The first thing I would check is what type of mixing valve it is. If it is a four way mix valve it is possible that the gate (or whatever the technical name for it is) inside of the valve could be in the wrong position, forcing the water in a strange path. A three way valve is not likely to do this. The only way I see for anyone here to help you is if you post a diagram of the piping, since the location of pumps/mix valve/check valve, etc. determines how it works. also you should check out "the Wall", which is a web site for heating guys. I don't remember the address offhand but a search here or on a search engine should turn it up.
*Here's the URL Nick referred to" http://www.heatinghelp.com/I've seen 3-way mixing valves get stuck. Maybe from crud in them, but I never tracked down the reason. But cycling them from hot to cold and back again seemed to fix the ones I've seen do that. Leave a pump running and feel the pipes going in and out. Fiddle it back and forth, checking after each cycle if the inputs and outputs are finally making sense.Sometimes flow can go slowly backwards in a loop if its pump is off and another pump is on. Typically it is not a lot of flow and won't pump much heat, backwards, out that loop. But just by feeling the pipes, you can't judge flow, just temperature. So it may seem that a lot of hot water is flowing the wrong way when just a little is.To give an example of the above: You might have two pumps running and three pumps not running. The loops that have pumps running will have the hot side hot and the cold side cold. The loops that aren't running will be the reverse. But, again, not much heat will be going out through those loops.If instead of multiple pumps, you have one pump and multiple solenoid valves, this probably wouldn't happen. The closed valves would typically prevent reverse flow. It would depend on exactly how the manifold and tempering valve are set up - in some configurations, you could get reverse flow with a single-pump system as well. But, once more, not much heat would go out those "off" loops because there is so much less driving force.Best guess: The tempering (mixing) valve. Try cycling it. The "reverse" flow may be a phenomonom that has gone on for a long time and which you just now noticed. But at the tempering valve, "hot" should definitely be hotter than "mix" which should be hotter than "cold" whenever a pump is on. -David
*Nick and Dave, thanks for the heads-up about the mixing valve.After cutting out the MV and crimping a straight-through loop connection, each zone was purged in sequence of 60F to 62F water by a hot feed,then brought on line one at a time. Twenty hours later, my readings are HOT SIDE: 100F @45psi; COLD SIDE: 77F and rising. The water heater is now firing - something else that I should have paid more attention to - the gas furnace and DHW heater reserved for domestic needs are also in the utility room. The system was stalled out by the MV, and the hot water was being pumped through one - not both - MV chambers right back to the DHWT, before it even made it to the loops. It warmed them enough, though, to fool me into the reversal quandary. I just wouldn't believe the MV was defective and was reluctant to cut it out. After all it had delivered normal flows for a week, i.e., "hot" was definitely hotter than "mix" which was hotter than "cold" whenever the pump was on, and the floor was warm. The blazing hot return flow downstream of the MV I attributed to hot water partially (rather than completely!) refused by the MV.Part of the mystery remains: how could enough heat still convect and conduct through the MV to heat the floor (@45psi) for the time that it did - for one week, until the MV was throttled down. Time for me to study thermodynamics a bit. It seems the MV may have failed after one week of service, probably a random event. Surely, turning down a 110F-140F MV’s setting valve should not stall floor loops.It would be neat to install the MV into a testing loop/heater to find out if it is operating properly, or why it has failed, or…if I installed it backward. Yet the embossed flow arrows and color codings were matched up properly. I have my start-up and running notes too.Dave, when you say “so it may seem that a lot of hot water is flowing the wrong way when just a little is”, you’ve hit the nail on the head. Thank god the pump was at least pumping into a hot loop, rather than back against itself - something that I’d checked for and will always check for. Saw that one.Time to spend some time at HeatingHelp.com. Interesting stuff. Perhaps another technical book purchase is in order.Thanks for the diagnosis.PierreX