I have a water to water geothermal heat pump supplying a radiant slab. My question is how long should the pump cycle on and off to reach maximum efficiency? At this point when the home is calling for heat the pump runs for about 24 minutes turns off for 8 minutes and then restarts. The unit is a waterfurnace 3ton water to water. The water is put into a turbomax indirect exchange tank, the radiant loop runs through this and is supplying the house with 90-95F radiant water. I can manipulate the run time through a few different methods. changing the start and stop point DeltaT or changing the flow rate through the floor. so my question is what is the correct on/off cycle for a geo-heat pump when calling for heat? Should it run continiously?
  I have control over the flow rate through the floor, but no good way of measuring this. The installer did put a pressure gauge on one end of the radiant loops, but not the other, so I can't calculate using pressure drops. I am getting a 20F delta on the return water for the floor.

Tom Pittsley
[email protected]
www.eebt.org

the best way to manipulate eff. is changing the water temp the cooler the water in the buffer tank the more eff you system gets
( hold on now because Im about to contradict myself ) long run times are good and at design temps the system should run nonstop . but if you try to achieve this artificially ( raising the water temp ) you are decreasing the systems efficiency . The best way to increase eff. is to add an out door reset to modulate the water temp to meet the btu requirement of the home based on outdoor temp
all that being said your current cycling sounds fine ( maybe off time a little short could add a time delay or increase it to give you a longer off and longer on with no negative impact , but the only positive there would be wear and tear )

I considered a outdoor reset, but opted out for a couple of reasons. The house is a slab on grade, super-insulated house with some unique windows that store solar energy when it shines, complicating the thermostat settings enough without adding outdoor reset into the mix. I would need a PLC to set that one up. We currently have the heat completlely turned off and the house is a comfortable 67F. Dismal outside all day and 47F right now. I would probably consider an outdoor rest if it just turned the system entirely off when not needed, rather try to maintain a tank when heat is not needed for days.
I think the storage tank is to small for the system. It is a turbo max 26 gallon tank for a 3ton heat pump. I do have another tank that I could use back this up, but to what benefit? When the heat pump exchange to the tank reaches 110F it turns of and the radiant loop continues to circulate. I have a couple of thermal data loggers placed in the system checking to make sure the deltaT was correct for the radiant loop. They also show when the system kicks on and at what temp. I can put them on any part of the sytem and record whats happening. I just reviewed the last batch of data and he is how it was running when last on and calling for heat. The radiant system was putting the water into the floor at 93-97F and the return was coming back at a steady 74F. The heat pump was putting water into the exchange tank and running for 90 minutes and then turning off for 15 minutes. I thought long run cycles where better, but not so taxing on the system as to run continuiously for hours or days on end. The floor is a large thermal mass and takes a very long time to move the temp, even one degree. I have a data logger in the floor as well, it has been between 65-67F. The room temperature fluxuats from 65F-72F depending on the amount sun that hits the house. The data used for the previous post, was before reviewing the last months info, I tweeked the flow rate through the floor by just a little and the on/off cycle changed dramatically. This just goes to show how careful you need to be when setting up the heat pump for maximum efficiency. I still think a larger storage tank would help with the off cycle times, may another 10 or 20 gallons. All in all the system is running pretty good and last february's total energy bill for the house was $157(Massachusetts), all electric and ready for solar conversion.

Tom Pittsley
[email protected]
www.eebt.org

most if not all out door resets offer warm weather shut down
for your cycle times a larger tank is not worth it