Pennsylvania Ideal Cycle Length

Discussion in 'Maintenance and Troubleshooting' started by cweinhofer, Feb 14, 2023.

  1. cweinhofer

    cweinhofer New Member

    The short version of my question is
    What is an ideal cycle length (i.e. not short cycling, most energy efficient, least wear and tear on the system) for a water-to-water geothermal heat pump?

    Here are the details: Last year, (with the help of a local HVAC contractor) I installed a w2w geo HP and the mechanicals to do hydronic heating in my newly built home (built myself). I've been generally pleased with the system, but want to make sure it's optimized for energy efficiency and system longevity.

    I recently bought a clamp-style energy meter (Shelly EM) to add to my smart home setup. One of the things I was interested in was more details about the energy usage of devices that can't be monitored with smart outlets, like my water heater and heat pump.

    After looking at the history data, it appeared that the heat pump was cycling on at an average of every 13 min for around 3 min at a time (i.e. on 3, off 10). The cycle length seems to be predominantly determined by the differential of the buffer tank tank, which was set at 4 deg F.

    I figured (and the HVAC contractor confirmed) that this was too short of a cycle, so I started experimenting with the differential. The highest I've tried was a 36 deg F differential, which resulted in a 2:20 cycle length with around 0:40 of runtime (i.e. 0:40 on, 1:40 off). But a 36 deg F swing seems like it's going to put a lot of strain on the tubing. I figure I need something in between those two extremes.

    So, I'm hoping to get some knowledgeable opinions on what would be a reasonable cycle length for a system like this?

    System Specs:
    Hydron WT048 4-ton, 2-stage
    Geo-Flo 40-gal buffer tank
    6 zones with zone valves and variable speed pump
    house is located in SE Pennsylvania, 2K sq ft with a load of approx 45K BTU
  2. gsmith22

    gsmith22 Active Member Forum Leader

    no one has bitten so I'll give it a try...
    Tough questions and there probably isn't a right answer. I don't have a water to water heat pump so kind of spitballing here but you are probably dealing with really two seperate questions - a good cycle length for the heat pump and keeping the temp differential minimized. I suspect on the heat pump side, you could really look at any heat pump/air conditioner to answer this question. Most references I have seen want a min 15 mintues on and 45 off per hour (25/75). This is probably for single stage equipment. I suspect the answer would be more like 50/50 for two stage and maybe something like 72/25 for variable speed. Maybe cut your temp swing in half - say 15 degrees and see what the compressor duty cycle is. I suspect there is no one right answer but shades of grey and whatever you are comfortable with. I agree though that your original setup isn't right and your setup at 36 is too long. Try 15 and then adjust up and down from there. you certainly have the boundaries discovered!
  3. cweinhofer

    cweinhofer New Member

    Thanks for the reply.

    After consulting with the local manufacturer's rep, it turns out there was one more factor in all this that needed to be considered. When widening the differential, you have to make sure your differential and tank target max put together don't exceed the HP's recommended maximum. For example with my HP, 112 deg F is the max. So if I had a tank max of 105 and a differential of 20, on the absolute coldest days the HP would be trying to produce 115 deg water, which the rep said could damage it.

    So after discussing options with my installer, I think I'm going to have to settle with somewhere in the middle of the road. I set a tank max target temp of 105 F and set a a 12 F differential. That means the HP should never exceed 111 F, which is one extra degree of safety below the manufacturer's recommendation.

    This setting gives me a runtime of 8 min with a total cycle length of 42 min. It's less runtime than I would prefer, but at least the cycling of 1-2 times per hour seems more reasonable.

    And in the end I'm realizing that the buffer tank size is also playing a role in all this. Mine is only 40 gal, but I realized if they had installed an 80 or 120 gal buffer tank, the cycle length could be longer without having to excessively raise the differential.

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