Ground loop for ERV run under graywater line
I did a lot of specific and general searches and haven’t found much of anything.
The build location is at 8500ft in Colorado zone 5a, frost depth can be up to 3ft.
I finished all the other groundwork, including the leach field. However, the ditch from the house to the septic tanks is 30ft and about 7ft deep, then the ditch from the septic tanks to the beginning of the leach field is about 300ft and 5ft deep. ERV is intended to be a Zehnder Q600.
This would make up to a 660ft loop from the house to the ERV, my only concern is that the glycol-based solution running through the heat loop might take too much heat from the septic system and cause a freeze/clog of the septic pipe.
My local inspector said he is fine with it so long as it doesn’t go anywhere into the leach field and I do whatever is needed to ensure nothing freezes.
I am currently trying to estimate the amount of heat traveling down the septic pipes.
My questions: Are there potential problems or issues, not mentioned above? Has anyone done this before and if so what was your experience?
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One option is to put the ground loop beside the drain and separate the two with a strip of vertical rigid insulation. This won't stop all the heat from the grey water side from moving into the ERV loop but will definitely limit it.
Typical water use is between 50 to 100 gallons per day, that water goes down the drain at house temperature, say 70F. Pick the middle usage so an average that is about 0.05 GPM.
Assuming the soil is around 40F, the grey water delivers on average 0.05gpm*30F*500= ~800BTU. So not much but definitely something.
Thank you for the answer. Since the only time it will matter is to prevent core freeze in winter. I think it will be fine so long as I keep the loop as far from the drain pipe as possible and bed it carefully to ensure it stays there. The loop is going to be about twice as long as Zehnder requires and the loop pump will only be activated when the outside air temp drops below 10F.
If this loop doesn't have enough heat on the coldest night or two of the year, what do you think of retrofiting a small heat exchanger placed on the intake side of the ERV loop. If the loop fluid temp drops below say 40F degrees, it could call for heat from the same source as the DHW/slab. ONLY added if the loop doesn't work as I hope it will. I am off-grid and it is much easier/cheaper for me to do that than to expand my solar array etc. to support the electric heater they sell with the unit.
I have a ground loop system. Based on the conditions you describe, I think you will be fine without that modification. If it's only called for below 10F, and you're in zone 5, that's not going to be all that often. I have mine set to come on at 32F, so it runs probably ten times as much and it's never run out of juice. We have 800' of 3/4" pipe, but it's overlapping coil in a trench only about 80' long.
Trevor, do you have any observations that you can share? I'm interested to hear your observations if you're willing to share some information:
1. The temp delta over the heat exchanger that you're seeing at your given supply CFM rate at the coldest temps
2. The pump flow rate you're using.
3. What climate zone are you in.
4. How deep are the coils?
We are in the midst of some yard work so I'm toying with the idea of doing a ground loop for preheat. We're in zone 7a, so the HRV requires some level of post heat (up to 300-400) watts running 5-6 months of the year.
I can only give you a ballpark number on temperature delta, because I only paid close attention the first year or so, which is now 6 years ago. I think the temperature delta was about 6degC, at around -15C. Maybe a little less. The incoming temp starts at about 12degC at the beginning of the season, and gradually drops to maybe 9degC. The outgoing temp has never gotten close to freezing, I'm thinking maybe 6degC. The airflow is around 80cfm.
I have a Grundfos pump with three levels, and most of the time I have it at level 1, which gives a readout of 2gpm. When it's really cold, I'll bump it up to level 2, which I think is around 3-4gpm. We're in zone 5, 6600 heating degree days, 0F design temp. The coils are about 4 feet below natural grade, which is built up another 2-3 feet, with R42 of foam and the house on top of that.
I looked on the Zehnder website and I couldn't see anything about using it with a ground loop, do you have a link to their instructions?
My gut feeling is you're making this way too complicated. For the most part infiltration is driven by temperature difference between indoors and outdoors, on the coldest nights of the year I would suspect there's enough natural infiltration that additional ventilation isn't necessary.
It depends on what you consider necessary. You're not going to asphyxiate, or get hypercapnia. You will definitely see CO2 levels rise dramatically, if the house is well sealed. It will be measurable within an hour or two, and perceptible to some people in not much longer than that.
Having said that, even though natural infiltration won't be enough, I think it's extremely unlikely the ground loop will get exhausted under the described conditions.
You can start with the ComfoFond specs:
https://www.international.zehnder-systems.com/download/e302becf775e408a85bf254324113cc5
The data on page 9 will be useful. Note this paragraph on page 9:
...... The ground collector pipes are not allowed to be spaced
less than 0.6 m apart in all directions, and are also not allowed to
be less than 1.0 m in any direction from pipes carrying water. It is
not permitted for the collector field to be built over or sealed.
Given the power hit with that ERV, the Comfofond at 5 to 70 watts does not look so bad, particulary if you're getting up to 2.5 kW worth of preheating from it.
Page 27 has some technical performance data in the installer manual that should give you an idea of the heat exchange/watts.
https://www.zehnderamerica.com/wp-content/uploads/2021/05/ComfoFond-L_Q_Installer_Zehnder_EN_2020.03.11.pdf
Looks like 1.9kW to 2.5kW at 350 to 600 m3/h flow.
Then, on page 91, there are the circulation pump settings based on the size of loop pipe, and the length of the loop to achieve the kW ratings as above.
Do you have an idea of the ERV CFM requirement? You'll need this to figure out the pre-heating power requirements which will correleate with your loop length/volume and pump setting, regardless of what loop system you're using.
M Henson,
Some slightly off-topic information: these ground loops make no economic sense. As I wrote in 2015:
"Just because a ground loop works, doesn’t mean the system is cost-effective. Many energy experts have speculated that the pump needed to circulate the glycol solution uses almost as much energy as the system collects. The results of one monitoring study indicate that these experts may be right; data gathered in Vermont suggest that the simple payback period for this type of system may be as much as 4,400 years."
More information here: "Using a Glycol Ground Loop to Condition Ventilation Air."
@Trevor, thanks for the information :-)
@Martin, yes one of those studies looked at the power consumed overall with electric preheat vs ground loop and they were pretty much a wash. However, the study was in a warmer climate than ours (7A). The Comfofond pump is likely not an EC motor deal either.
I'm using HRV post heat with automation. The system is often calling for 300 watts, with outside temps at -25C to temper supply to 18C. Additionally, the system runs defrost cycles at higher power use, then has to run at a higher CFM during ventilation mode, to keep the IAQ sensors happy. Defrost times at -25C can be 20-30 minutes per hour.
If the Comfofond is consuming 70 watts at max, while delivering up to 2.5kw of heat (maybe this includes the Zehnder core heat transfer?), something is not right with the numbers. If you're using a circ pump using 80 watts, to a 7C delta @ 80CFM (like Trevor) you should be about 220 watts to the good. As far as the cost for that efficiency, that's another story.
My system draws only 14-28W. And the temperature delta I said was for the glycol, not the air. Air temperature delta is as high as 25degC at 80cfm. I did some calcs way back when, and it was a lot higher than 220W, I seem to recall it being over 1kW. My system is DYI, and it cost me around $700CAD all-in. I have many regrets about decisions made on our house, but that isn't one.
Ah, yes, major difference there. Assuming for example that the supply air temp was raised from -25c to 0C, at 80 CFM, you're at 1140 watts. If you're getting that at 28 watts on the pump, then it would be no contest with respect to efficiency vs resistance heat.
Trevor, do you run the system in summer to pre-cool as well?
Thank you Martin, I did read that article. My concern is that we are off-grid and while I am installing 18Kw of panels and 30kwh of storage. The ERV core potentially freezing will be at the extremes of cold and cloud cover. The averaged annual cost of electricity is important to the cost analysis of a given system. In this specific case, it is more important for me to balance the quantity of electricity used over a short time. The pump pulls 70 watts, the preheater pulls 2670 watts.
Practically speaking if the backup generator has to start the price of the pre-heater per year goes way up. The capital cost to me is negligible between a preheater versus a Comfofond and a few hundred feet of pex.
If what I am missing is that the pre-heater has to be on for a much shorter time than the fluid pump and the total amount of electricity used is the same over a 8-hour period then yes, I would prefer the pre-heater. Can someone verify one way or the other?
I read and reread that article and something is wrong with the calculations. My guess is that there is no reference to the kWh saved in the summer by the ground loop dropping the incoming air temp, and the kWh savings in the winter by the incoming temperature increase. There is no way a 2620kW (ComfoAir Q600 ST technical specifications page 2) resistance heater uses less or the same total kWh per hour as a 70-watt fluid pump. Further, if this were true no ground loop or heat pump would make sense. Every heat pump in existence air or liquid is based on this very efficiency. I am not arguing installation costs, but for new construction (which I am doing) the ditch had to be dug anyway. 500ft oxygen barrier pex $238, a couple fittings $12, the price of the ComfoFond. Which I cannot find a price for and can't find on the website of my Zehnder distributor. It appears these products have been or are being killed for reasons that escape me. Guess what, I am going down to Autozone tomorrow and buy a car heater core for $50 I am going to put it in the incoming air supply with a couple of fittings, some size up/down aluminum duct pieces, a clear nylon drain hose and a 70-watt pump (with an alarm if it stops spinning, a programable thermostat switch, and a gallon or two of premix food safe antifreeze. I will spend two to four hours making it work it will cost about the same or a little more than the preheater. I will track the incoming and outgoing temperature delta for the first year I am in the house and bet anyone the delivered air deviation will more than pay for itself. What ground loop heat pump won't? That entire article admits to unverified data to prove ground loop heat pumps don't work better than resistance heaters! Yes, I hope this upsets someone enough to try and prove me wrong.
"Further, if this were true no ground loop or heat pump would make sense."
You may be on to something about the ground loops.
You may be right. The problem is that every heated/cooled envelope, climate zone, and soil is different so it would be hard to establish a general principle. My guess is that climates with the most stable year-round temperatures benefit (if at all) the least.
If you read my last couple of posts, You will see a plan to try and get verifiable, repeatable, data for zone 5a, with very rocky, clay soil, with a loop that is not under the house.
If you can DIY than it makes sense. You still have to do a bit of design to make sure it works properly.
The big issue with any pre-heater is most of heat it produces is sent right back out since your core is somewhere around 80% to 90% efficient.
Say the pre-heater is warming 0F air to 20F at 100CFM. That is about 2000BTU or ~600W. If your ERV is 90% efficient 60W make it to the house. If your pump is 50W, that is a COP of a bit above 1. Better pumps and less efficient core would make the COP better.
To get any decent BTU at low delta T you will need a much bigger core than a car heater core. Maybe one of the larger hydronic toe kick heaters or the smaller Sapcepak Water PAK coils might work. The official Zehnder pre-heater is pretty large for this reason.
To throw another variable in to the pot, if you're pre-heating (and avoiding defrost cycles) the overall vent rate will be quite a bit lower (in my case, about 40% lower at -25C) to hit IAQ targets. At lower flow rates, core efficiency will increase. At 100CFM I'm at 67% sensible efficiency, vs 50 CFM which provides about 80%.
80%-90% are not realistic, real world numbers. The best HRVs are in the 90% efficiency range, but only under specific conditions. At higher flow rates, the numbers go down.
More importantly, HRVs don't work much below 0degC. So all the pre-heating you can do below that temperature counts at full value, and only above that should it be de-rated by the efficiency of the HRV. ERVs can operate at lower outside temperatures, but they are less efficient than HRVs to begin with (about 10% less, all other things being equal), and efficiency also goes down as outside air temperature down.
M Henson,
You claim, "That entire article admits to unverified data to prove ground loop heat pumps don't work better than resistance heaters." In fact, I wrote the article with an open mind. I sought out the best available data at that time, and in order to arrive at my conclusions, I interviewed or quoted respected experts like Alex Wilson, Marc Rosenbaum, Peter Amerongen, Andy Shapiro, and Stuart Fix.
I'm always on the lookout for data, and if more data exist now than when I wrote that article, please share it. In 2015, I wrote, "The best monitoring data I found were provided by Peter Schneider, a senior project manager at Vermont Energy Investment Corporation."
I had no agenda when I wrote the article. I followed the data.
I will be the first to admit I am no genius. I was waiting for a server script to complete at 4am when rereading that article. Thank you for being gentle Martin, you could have roasted me. Please accept that there was no personal offense intended toward you.
That said it feels like there is something wrong with the data. Out of respect for Martin and all those smarter than me, I will take the time to do the math properly for surface area, heat exchange, etc. spend more time than I may get paid back to size everything properly, and put in some thermistors and a humidity sensor, bag the condensate from the unit, and do a real-world test. My initial goal was to keep my core from freezing (that is worth $500 to me anyway), now the goal will be to determine if the added heat delivered to the house in winter and the added cooling in the summer, delivers enough btu/kWh's to offset the primary heating/cooling in the house, and if they do, is enough to provide a real-world cost benefit. I will install a proper box with an easily removed and cleaned fiber filter so the exchange radiator doesn't get fouled.
Zehnder went all the way through concept, design, and production manufacturing, it is unlikely they didn't do all the science on the product ahead of time. But I have seen companies make that mistake. This will only be a case study in zone 5a with air density at 8500ft in the high desert, but it will at least give another data point.
I ran a few numbers using Trevor's estimates of a 25C air temp delta across his loop coil, and 80 CFM, with ERV core sensible efficiency at 75%. I also assumed that the larger deltas would occur at the lowest air temps. The excel sheet lets you play with temperatures in F (I just adjust supply in the spreadsheet to get 75% efficiency).
At -10F, the ground loop system (preheat) is providing supply at 63F, while the post heat system requires 329 watts resistance heat to bring supply temps to 65F.
Somewhere around 25 F ambient (where Trevor's 45F delta is not likely as the ground temp is getting closer to the air temp), the difference between adding some post heat and using 30-40 watts for a pump start to come in line. If tests were done where ambient temp are in that area, I can see how the results would be a wash. However, if you're spending a few months at -10 to 10F, there is a much larger difference.
Now, add in cooling and perhaps the numbers look a lot better for a given climate zone.
"the ground temp is getting closer to the air temp"
The ground temp, if you go deep enough, doesn't significantly change with outside air temperature. If your loop is under an insulated slab, then it basically doesn't change at all. If I was installing a system buried in exposed ground, I'd probably put some insulation on top of it.
Trevor, yes understood. What I’m saying is that the supply air temp delta across the ground loop coil would decrease as ambient and ground temps converge.
I misunderstood, I thought you were saying the ground temperature was changing. Yes, certainly as outdoor temperature goes up, there's far less energy "available" to recover. Any temperature rise to the incoming airflow that is above the minimum safe operating temperature of the HRV/ERV is reduced by a factor that is the inverse of the efficiency of the ERV/HRV.
I called my local distributor in Denver Main Stream Corp. The price for the Comfofond is over $4K US. That is probably the main reason these systems don't sell. They don't have them on their website because they only sell 3-4 units per year.
At that price, it is worth my time to design and build one inline as described above. Even if it takes me 15 hours it will be worth it. So I am doing all the flow calculations, pump sizing, etc., and will put a cheap Raspberry Pi to control it with a wifi interface, so I can write some scripts to live monitor the temperature delta, humidity, monitor pump health etc. For a future owner, I will go ahead and purchase the electric pre-heater and leave it disconnected. However, switching out the fluid pump will be as easy as closing two valves and swapping the pump, plus sliding out the filter and cleaning it simultaneously as the ERV filter replacement.
I will also monitor the loop temp outgoing and incoming with a script to increase or decrease the pump flow rate and find the sweet spots for flow rate based on the outdoor temp. It seems wise to oversize the filter protecting the radiator core(s) so there is negligible airflow resistance to the ERV. My only mild concern is that the amount of moisture drawn from the incoming air in the summer may impact the household humidity levels since we are in a dry climate, that is easily fixed if it even becomes an issue. The total loop will be just over 800ft and be a straight run out and back with 1.25" or 1.5" pex. This will definitely keep the ERV core from freezing and should pay back the cost in a reasonably short time with what it will add to the whole house's heating and cooling..
I think your concern over humidity is baseless. It's hard to get blood from a stone. For the same reason those scam "water from the air" devices don't work in climates where you'd actually need them, I'd be surprised if your system captures enough moisture to worry about.
Just for the fun, let's run some numbers on an example scenario. You say it's a dry climate. That's hard to quantify, but let's just say the outdoor temperature is 30C/86F and the relative humidity is 20%. That gives a dew point temp of 4.6degC/40F. In order for your system to collect ANY moisture at all, the coils have to be below that temperature; they aren't going to be.
Thank you Trevor,
The humidity here is usually about 30%. The summer high temperature is usually in the high 80'sF, in the summer rarely going over 100F on the hottest days. The winter low temps are usually 5'sF to 10'sF but drop down to -15F on the coldest nights. As you said it is probably not a concern.
I have two tractor-trailer loads of recycled Polyiso coming (basically free to me) so I will also scratch out some numbers for insulating above the loop. Since this will be a case study I probably won't. In normal installations, I doubt anyone regularly does that unless they are running the loops under a building and even then the loops should be well below the insulation. If DC and Martin are correct that ground loops may not even be economically worthwhile then I will have spent some time and money on my continuing education.. Something I am never afraid to do.
There will be quite a few folks very interested in your data once you have the system running.
If you are seeing a dry 100F in summer, you may want to sort out a summer bypass for the ERV to maximize cooling as well.
If you look at the spreadsheet I loaded up then compare energy costs vs the price of a dedicated ground loop system, contractor installed, then the economics don’t look so good. However, if the loops are being installed along another project, and you are DIY’ing the system, different scenario.
With an on grid system, it may only require 2-3 PV panels to completely offset resistance make up post heat in winter. Zero moving parts, less expensive and far simpler.