Tekmar 406 with both radiant cooling and DWH preheat

JeffInCO

Last December, I had a Waterfurnace 504w11 W2W heat pump installed along with a Tekmar 406 heat pump control to provide radiant floor heating and domestic hot water heat as well, via an indirect tank.

The radiant floor heating system was existing, with Tekmar t-stats in each of the 12 zones (three 557's and nine 541's).

During the heating season, everything worked great... The 504w11 uses "vapor injection technology", which allows it to support entering water temperatures of up to 140 F, so it has no problem fully supplying our domestic hot water requirements. (Floor heating water temps are lower, around 110-120F on the coldest days here in Colorado.)

This summer, I replaced the two Tekmar 541 thermostats in the garage and workshop zones with 553 thermostats that can do radiant cooling. I enabled radiant cooling on the 406, with corresponding setup on the 553. The system was able to keep the garage in the high 70's most of the summer while continuing to heat the indirect tank for DHW, with water temps in the range of 55-65 F.

From a comfort and functional perspective, everything is working just as it should. From an energy efficiency perspective, there is a bit of a problem when both cooling the garage floors and heating the DHW tank. The problem seems to me to be a firmware bug in the 406, though I hope it is instead a configuration issue.

The problem sequence is as follows:

• One or both radiant cool zones are calling, but the radiant tank is already cool so only the tank pump runs supplying the radiant zones (no cooling call to the heat pump).
  
• The 406 detects that the indirect tank has fallen below the threshold temperature, so there is a new DHW call
  
• The tank pump continues to run for 1/2 an hour, with "purging cool" on the 406 display, but nothing else happens differently.
  
• After the 1/2 hour, the 406 turns on the DHW pump, which circulates water between the indirect tank and the heat pump.
  
• The 406 starts the heat pump unit in heating mode (so far, so good)
  
• After some time, the indirect tank reaches the target temp, at which point the 406 does all of the following simultaneously:
  
• ---- Shuts off heatpump
  
• ---- Leaves the DHW pump running (presumably to purge heat from the heat pump)
  
• ---- Turns on the HP pump (the radiant floor tank pump, which circulates water from the radiant floor tank through the heat pump).
  
• ---- Displays "precool tank"
  
• ---- This is the problem... The DHW pump is running at the same time as the HP pump! The DHW tank has just reached its target temp of 135 F, while the radiant tank is at about 60 F. With both pumps running, we're mixing the hot water in the indirect tank with the cold water in the radiant tank. The mixing continues for two minutes. The indirect tank cools down somewhat, while the radiant tank heats up somewhat.
  
• Both pumps turn off
  
• Sometimes the radiant floor tank has warmed up enough, due to the mixing, such that the 406 starts the heat pump in cool mode.
  

The piping of my system is, for all intents and purposes, identical to application A406-1 in Tekmar's installation and operation manual for the 406. The only differences are that I don't have an electric backup in the radiant tank, and I have more zones.

I think I can mostly fix the problem with a relay that locks out the DHW pump when the HP pump is running, but I'd like to avoid adding a relay if there is something I can configure differently to fix the issue.

Does anyone have similar experiences with the 406, radiant cooling, and a DHW preheat tank? If yes, did you have the same issue and how did you fix it?

Thanks!

Jeff