Does anyone have experience doing ACCA Manual S for Water Furnace equipment? As far as I can tell they do not provide enough data to properly perform a Manual S. 

Tags: Furnace, GSHP, Ground, Heat, Manual, Pump, S, Source, Water

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Hi Jeremy, manufacturer data is readily available for sizing ground source heat pumps. However, the procedure is different than for air source equipment since it involves the ground loop flow rate. Also, the procedure is different for water-to-air versus water-to water cooling. That makes it difficult for folks who follow a 'cookbook' method for Manual S.

I do Manual S calcs by hand so I can't speak how any given software works, but if you post what method or software you're using and whether the equipment is w2w or w2a, perhaps someone familiar with that method or software can step you through the process.

Hi David,

Thank you for responding. The person I am helping is using the Elite Software and it is Water to Air. I have uploaded a screenshot of where he is at in the software and of the data sheet he has available. Our outdoor summer design temp is 90 degrees. 


Not being an Elite user, I can't help you configure their software for a water-to-air system. Unless an Elite user chimes in, your best bet would be to contact Elite or their main training guy John Walsh. Both numbers are at the top of this page.


You will need to do a bit of gymnastics here to pull that off. 

As you know, the delivered cooling capacity is dependent on (1) Entering air temp, (2) Entering humidity or wet bulb, (3) airflow and with air to air equipment (4) outdoor temp. But with earth coupled units, the outdoor temp is replaced with the ground temp or entering loop water. And unlike outdoor air temp with air to air stuff, the ground loop temp is a moving target for two reasons. (a) the heat sink turnover is more like 30 days instead of a few hours and (b) the design of the ground loop. 

The ground loop design has to consider soil conductivity flow rates and the loop configuration. Water Furnace software provides answers to the questions. The software asks for heat gain from your manual J and the other pertinent questions about the ground loop, etc. 

I always run the load. Then I go to the WFI program to design the loop. The output of the loop program will give you high temperature enter loop water based on your inputs. Then you go to the detailed cooling tables to make the equipment selection. It is a bit of a juggling act bouncing back and forth between what ifs. But in the end a proper design is worth it. 

I also watch the heating side so I can avoid antifreeze. But that is just my preference. 

Danny wrote: "It is a bit of a juggling act bouncing back and forth between what ifs."

Exactly. You're trying to solve a multi-variable problem where you have only limited control over some of the variables, depending on whether you're the one designing the loop.

Loop design should be based on a soils test, eliminating what would otherwise be the biggest unknown, and the loop should be sized to handle the cooling or heat load, whichever is more constraining. That's going to depend on the mean soil temperature at loop depth, rather than which load load is larger. In other words, the heat load may be more constraining than the cooling load, even if it's numerically smaller. Or vice versa. Also, if the loop is heat-load constrained, it's an expensive mistake to size the loop to handle 100% of the heat load. There are two reasons for this:

a) Chasing that last few percent dramatically increases loop size (and thus cost), tilting the economics heavily in favor of supplement heat, even though it costs several times more per BTU delivered. A modest sized supplemental heat kit is relatively inexpensive to install.

b) An accurate MJ heat load will always overstate the actual load since MJ doesn't account for internal gains, solar gains and thermal mass. This worked out OK in 1980's vintage homes when MJ procedures were developed, but in modern homes, especially beyond-code homes, these non-accounted for gains loom large relative to the envelope loads that drive Manual J heat loads.

Loop design, soils and building loads ultimately determine the seasonal low and high entering water temps (EWT), which is what equipment selection must be based on. Keep in mind that loop temperatures will vary widely over the course of a heating or cooling season, with worst case temps typically occurring late in the season as the ground becomes thermally saturated. An accurate loop model generates these data points.

Once you know the expected min/max EWT's, the Manual S objective is to select the most appropriate equipment (capacity), blower speed and pump flow rate based on the sensible and/or latent loads, whichever is more constraining. GPM-per-ton rules-of-thumb are useful guidelines but a good designer knows when and how far to vary from the 'recommended' flow rate to meet overall design objectives. It's important to select the pump so the design flow rate falls within its efficiency 'sweet spot'. Likewise with heat pump/air handler selection and blower speed setting, except in that case you're looking to achieve the highest sensible efficiency, i.e., the highest sensible-total split that meets the latent load.

Well said David Butler. One more thing I would add is if you have some flexibility with the loop design, you can frequently downsize the equipment as its capacity increases with lower EWT. In other words, if you are stuck between say a 2 and 3 ton, two stage unit, you can improve the loop performance, lower EWT and possible slide under a 2 ton. But I always watch the low EWT to avoid antifreeze. The loop design software is key to exploring different configurations. Then you double check the pressure drop to minimize circulator size keeping Re numbers >2500.

ECM circulators should be standard and I always specify the header to be installed inside the basement or crawl. My product of choice is the GeoCal by Caleffi.

I also did a Webinar for Caleffi some years ago that might be some help. Anatomy of a Geothermal Heating/Cooling System



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