See repost with proper paragraph spacing below.

If the EWT of 25f is based on 10 year average weather data, what happens to EWT when you get a winter colder (or much colder) than average?

Here is a copy of my original post with the paragraph spacing that was intended. (sorry, still learning to use the system)

I have been researching a new closed-loop geo system for my home and this forum has been very educational. Now I hope others are able to help me with a couple of key decisions.

I have bids from three suppliers that I am considering, and would appreciate input from impartial experts. The basics:

- I am located in St. Paul, MN so heating will be a bigger concern that cooling
- My home is 16 years old, 5,200 sq. ft with lots of southern facing glass and is very tight and well insulated. Recent energy audit with infrared and leak test confirmed this
- I am replacing a 92% gas furnace (bad heat exchanger) and power vent gas water heater
- Two suppliers performed a manual J calculation from plans and determined that the heating load is estimated at 72,000 Btu/hr, cooling is 42,000 Btu/Hr. Based upon an analysis of my utility bills, these numbers seem about 15% high from my actual experience.
- I have a 40,000 Btu sealed combustion gas fireplace on a thermostat that will remain in the house which may be a consideration for aux. heat
- All suppliers have recommended a 5-ton unit for my application.

My first question is should I go with an integrated unit with electric aux. heat, or a split system with a new 95% gas furnace? It turns out that with rebates and all of the different electric rates involved, the purchase price and operating costs are about the same for either solution, so I can choose which is most appealing. Looking for feedback on the pros and cons of each from those in the know who do not have a financial interest in the decision.

Second question, there is a significant difference in the design parameter of minimum EWT that the suppliers are proposing – one is using 25F where others are using 30-32F. As expected, the supplier that is using 25 is proposing a shorter loop field and a lower price, but I am concerned that the system will not perform properly in the dead of winter. What is an appropriate design min. EWT for a cold climate like Minnesota?

Lastly, I am considering equipment from Carrier/Climate Master, WaterFurnace and Hydron Module. If there are thoughts on which is more appropriate for my climate, please speak up.

Any help would be appreciated. If I missed any key parameters that need to be considered, please let me know.

Thanks!

An integrated unit is typically more efficient, that said it would depend on your preference and fuel costs. If you are keeping the fireplace I would use it for aux heat. The loop is the last place to cut costs so use the 30-32F minimum EWT, again this determines efficiency. The installer is more important that the brand so choose wisely and compare warranty....read the fine print. One other thing is duct work for retrofit applications, heat pumps need to move more air. Proper sizing of supply and return air ducts will have a big effect on your comfort and efficiency.

Fortunately, all of the suppliers have confirmed that the duct work is fine for a 5-ton unit. And you are recommending what I suspected on the min. EWT, even though the supplier that recommended the 25F number has strongly defended his assumption based upon payback and the law of diminishing returns. Lastly, good recommendation on reading the details of the warranties - while they are all quoting a 10/10 warranty, the details may be somewhat different. Thanks for the assistance.

The purpose of the loop is to bring the EWT within the operating parameters of the heatpump. A correctly designed loop supplies water with a minimum of 30 degrees F during the average winter. This allows for extreme outliers in weather conditions. A loop designed for 25 degrees F EWT will not have that safety margin. All the brands you have offers for are premium brands, with non of them you can go wrong. Hydron is optimized for winter , has a bit more refrigerant filling, puts out about 5000 BTU/H more heat. Hydron also has models with a lifetime warranty for the compressor and the refrigerant cycle, which you might find intriguing. Higher upfront costs, thought. Other than that all of them are excellent brands. Installer experience is more important, I would treat the one who proposes to cut corners with the loop with healthy skepticism. You can afford to have the loop properly designed, you cannot afford that the system fails in the peak of winter because of short loops. Internal supplemental heat is the best and cleanest way, unless you plan to use a lot of it. With 72KBTU/H heatloss, a 5 ton system should be appropriately sized, and not use much supplement heat. Hard to imagine that the upfront pay of an additional furnace would ever pay off.

Everything will perform as expected if properly designed. In fact better...
You mentioned that load calculations appear higher than your experienced usage, that is because most design software has a little safety fudge in it.
A system can work fine at 25* EWT. Better at 30*. Neither is right. They are simply different. One of the things I often find with other bidding contractors is that they simply ignore that (loop) portion of the design. Most of us around here happen to use 600-800' slinkies, so I manually enter feet to show an accurate EWT sometimes it's more than 30, sometimes a little less. Others leave program on auto which suggests more feet of ground loop than they actually install.
I believe your 25* EWT bidder is not "cutting corners" he is simply offering a design that offers a quicker return in his estimate. At least you know he put some thought into the loop design. Why not ask him to quote with a higher EWT and see how pricing compares. At the same time verify that others actually intend to put as much pipe in the ground as indicated. 
Not a fan of split systems (with furnace).
The brands you mentioned are all fine.
Good Luck,
Joe

Thanks for the input.

It seems that there are at least a couple of thoughts on the appropriate min. EWT that should be used in the design of the loop field. The last two responses pretty much sum up the various opinions of the installers that are bidding. I should add that the installer sizing the loop field for 25F is also proposing a split system which would provide plenty of heat for any unusually cold periods in the event the geo unit cannot provide adequate heat or that the energy company shuts down the unit as part of their dual-fuel program.

I have confirmed the amount of pipe that each bidder would be installing with their proposal. And I did ask the installer proposing the shorter loop field for an option to use 30F as the design input. The result was, as expected, an increase in cost for the longer loop field and bigger circulation pumps offsetting much of their price advantage. It seems to boil down to what design philosophy is most appealing to me and my willingness to use a gas furnace during the coldest days.

When you mention dual fuel I feel the need to speak up. I am in MN also. Our electric co-op shuts off all electric to the geo unit during the coldest days of winter for an average of 4 hours at a time. With my first geo system(which I replaced this summer) I relied on a gas fireplace for backup heat. The FP did not keep up, and some areas of the house got pretty uncomfortable. The new system has electric plenum coil to assist the geo if needed AND a gas furnace for use during times of load management by the electric company. I wouldn't do it any other way and urge you not to make the same mistake i did 11 years ago.

I am not a fan of split systems generally, but would certainly yield to dafr's experience in your locale.
RE minimum EWT, I looked at a 20 year old open loop CMaster last year that owner wanted to convert to closed loop. That model's minimum EWT was 45* (which called for about 5 miles of ground loop). Not a good plan.
My point is it wasn't so long ago that 30* minimum EWT would have been poo pooed as "cutting corners" to negligent.
From the sound of your last post 25* guy may offer cost advantage over others even with extra loop. Either way, I don't want you to discard him over EWT prejudices when he sounds like he is being thoughtful in his design practices.
j

WATERFUNACE RESIDENTIAL AND LIGHT COMMERCIAL HYDRONICS APPLICATION MANUAL "3) Determine the minimum entering source water temperature at design conditions to be used in selecting the capacity of water-to-water equipment needed. For open loop applications, the minimum source temperature would be the average ground water temperature, usually 45 to 70° F (7 to 21° C), depending upon geographical location. For closed loop systems, the minimum source temperature would be minimum water temperature used in sizing the ground loop piping, usually 30 to 50° F (-1 to 10° C), depending upon geographical location."

Pretty much all the manufactures specify the desired entering water temperature to be at a minimum of 30 degrees. While the operating limit is specified at 20 degrees, designing a loop for 25 degrees leaves lesser of a margin to account for the less loop performance in the first year due to the compaction of the soil around the pipes, or longer periods of excessive cold temperatures, or may be even both. I have seen loops fail and shut the systems down under those circumstances, luckily not my own. Maybe Joe is an experienced installer who can eliminate many other pitfalls, but why take a higher risk of failing? Can you put Prius tires on a Corvette? Sure, as long as you never go fast, they might do OK. Are the cheaper to install? Yep. But then why bothering then getting a Corvette? And what is gonna happen if you ever take a corner fast? Speaking of corners again, designing a loop for 25 degrees EWT in Minnesota weather is cutting a corner you do not have to cut. You must be able to afford to pay for a correctly designed loop, since you must not afford a system failure after your significant investment into geo. The heatpumps are relatively bulletproof, the are designed to work reliably within their parameters. So far, every system we had to fix for other installers had design flaws with the loopfield, resulting in low EWTs.

Dafr, thank you for sharing your experience, as I too, want to avoid having a situation where I have inadequate heat to keep our home comfortable. In order for us to qualify for the dual-fuel program, our local energy provider requires an alternate source that can meet 100% of the homes heating need, so the fireplace would not qualify. My thought on using the FP as an auxiliary heat source would be in place of running the electric plenum heater if we go with a packaged unit or to keep us from prematurely going to the gas furnace if we have a split system. And if we go with a packaged unit, we would be forced into an all-electric program that would not be subject to our electricity being cycled off by the power company. Again, thanks for taking the time to post and sharing your experience.

Your co-op shutting off the electricity at peak time certainly changes the dynamics in regards to supplement heat. In case you consider radiant, in europe the problem is partially overcome with extra large buffer tank as a thermal reservoir, and they have radiant slabs most of the time with a large thermal mass.

Unless you have that kind of scenario there, a dual fuel setup with a gas furnace is probably the best way to go. It can be a relative cheap, 90% efficiency furnace, since it will hopefully not run much.

the "regional concerns" ie utility turning off heat pump during peak periods, are exactly why i try (and often fail) to avoid blanket statements about split systems and loop sizing et al.
a 25* loop design, may be more than adequate if heat pump is shut off for 4 hours during peak usage. i can picture a 25* design in my minds eye that never goes below 30* since the furnace actually runs during coldeswt weather.
my point is to hear-out bidding installers before discarding any due to comments we make that may not be relevant in your area.
doc and i have had a spirited debate about loop sizing before. in his area compaction is an issue that he handles with feet of loop vs man hours of careful back filling. in my area we enjoy damp sand and gravel on most sites which makes compaction as easy as pushing the soil back in the trench. neither of us are wrong in our own geographic context. but my methods would be wrong in his town.
j

If the loop never drops below 30 degrees F, then it is the correctly designed loop, no matter what area you are in. I earlier said "I would treat the one who proposes to cut corners with the loop with healthy skepticism.", I did not suggest to generally discard his approach. But he needs a good rationale, most of the time there is none.

One obvious rationale would be a heat pump that is turned off 4 hours each day during peak load may easily exceed design temp on the loops by 20%.
Another might be a loop field that delivers 30 EWT at 6 feet depth but only 28* EWT at 5. If unusual conditions wouldn't permit digging past the 5 foot mark you could either pay for 30% more loop or take a 5% hit on heating op cost (in the case of the op cost calculation I refrenced about $20/yr for a 6 ton system). Cheap electricity in my area makes the choice simple for my customers. This is not "cutting corners" it is a sensible trade off.

As long as intelligence is employed in design and consumer is educated in design differences, one need not consider 30* EWT the "holy grail", though it is certainly preferred.

The only "wrong design" is that which does not fall within manufacturer's operating parameters or perform in the way consumer requires and was promised. All else is preference.
j

Well, I agree with Joe, there can be always special circumstances. I have once designed a commercial building with 60 tons total capacity with a 45 ton loopfield, because large areas of the building only had temporary use for a couple hours a week, so a higher capacity was needed for a short period of time. That way it saved the customer 15 tons of vertical loopfield install, and if the use of the building changes down the road, the headers and circulation pumps were designed for 60 tons, and we can easily add loop capacity later.
However, not everybody designing residential loopfields has the best interests interests of the customer in mind, and the apparent sophistication of Joe to understand the impact of the design.
This thread started with a general question what the appropriate minimum EWT design temperature is especially in a cold climate like Minnesota. The answer there is still 30 degrees F. I lived in Rochester, MN for 9 years, we had a cold period in 2002 (or 2003?) where it was between 0 degrees and -30 degrees 6 weeks in the row. A loop designed for 25 F min EWT with normal weather data, would likely have failed under those circumstances without appropriate backup heat. Now, it sounds like in order to deal with the shut off period, it is required to have a dual system with a capacity to take care of that load, so that issue seems to be solved.
Yes, Michigan is different, surrounded by "warm" lake water, creating a buffer for spiking cold temperatures. Buffalo NY now is different. Despite it's snow reputation, the coldest it got last winter was plus 5 degrees, and people here were complaining. People living in Minnesota would consider that a heatwave. So yes....I am one of the few guys who moved to Buffalo, NY for the weather!

The point is that I am sometimes outbid by companies who either wrongly size a system, or doing shortcuts later on. Then the system fails, and then people ask us to help with the fixing, and start to appreciate why our price was higher. Unfortunately, that way geo gets a bit of a bad reputation. That is why some of the parameters are standardized. Because that way the system will always perform. That is why the Climatemaster design software Joe was referring to is auto set to 30 degrees. You can overwrite it, but then you are take away the safety margins built into the parameters. You can do that if you know exactly what you are doing. Everybody else, please leave it on automatic!

docjenser and joe.ami, thanks for the spirited discussion and input on the minimum design EWT for my situation. I have asked the installer that was using 25F to change their design parameter to 30F as I believe this is appropriate for our circumstance and provides an appropriate margin of safety. If I end up spending more on the loop field that may be absolutely necessary, then so be it. I do realize that we are debating a point of diminishing returns here, but all things considered 30F seems like a better choice that 25. I consider this question answered.

As far as the second question on an integrated vs. a split system, I am leaning toward an integrated unit. This would provide the most efficiency, eliminate the additional equipment with a furnace, and avoid any control problems switching from the geo unit to the furnace and back. While I would give up the most favorable electric rates from our utility provider, I would no longer have to deal with our electric being cycled off during high-demand periods. I would still get favorable rates for having all electric heat, and those favorable rates would then apply to all of our electric usage and not just to the geo unit since there is no longer a split-meter involved. And in the event of a geo system failure, we would still have electric plenum heat and our gas fireplace as a backup. If there is something I am missing, please speak up.

Lastly, it is very clear from reading this forum that selecting the installer is the most important factor in this decision. I believe that the WaterFurnace dealer is the most knowledgable and experienced, and also the most expensive. I prefer the specs (particularly those for heating) and warranty on the Hydron Module equipment and the dealer has plenty of experience. And the Carrier/Climate Master dealer has many installations under their belt as well, however I have a niggling concern over some of their technical positions in my discussions with them, but they are the least expensive. This will be a tough decision.

Thanks again for all of the help.

Sounds like you are in the fortune position that you are not likely to go wrong with the installers. Out of curiosity (and jealousy), what are the electrical rates for your heat only rate, and the "shut off rate"?