As with any heat exchanger, in a GSHP there are resistances to heat transfer (R=1/U) due to the metal wall, the "clean" U on each side, and the fouling factor on each side. I wouldn't expect any significant fouling factor on the refrigerant side, being a very clean fluid with a limited mass just being recirculated. But clearly the fouling resistance on the water side could become significant over time, especially for an open loop system, and would be more so with lower water quality.

At least Climatemaster publishes the performance tables of its units. Does anyone know if those numbers are for "clean" service, or perhaps average numbers for "mid-cleaning cycle" performance based on some typical fouling rate?

The reason I ask is because my heat loss calcs for the house about to start show a total load at 93-98% of table value for a 2-ton Tranquility 27, depending on actual EWT and assumptions about air leakage/HRV losses. The calcs are based on 68 inside, -5 F outside temperatures; the town's website says design minimum is -3, with relatively few hours per winter below that (I've seen -14 here). It would seem that the 2-ton unit would be appropriate, with reliance on 3rd stage heat (or firing up the small wood stove) for the few hours each winter when the temperature is below design. Roughing out the extra cost of resistance heating, a whole two weeks of 24/7 supplemental 1 KW heat at 15 cents/KWH comes to 50 bucks. Payout for the next size up (3-ton) would be something like 80 years.

Furthermore, a 3-ton unit would make it even that more oversized for summer A/C operation, when worst-case load is about a ton, half of which is dehumidification of ventilation air (this is why I started the Climadry thread). With the 2-ton unit in dehumidification mode, I stand a better chance of avoiding the problem of overcooling without dehumidifying adequately.

But having the advertised unit capacity just a few percent above calculated load brings me back to the issue of water-side fouling. What sort of allowance should I make for fouling if I don't want frequent cleaning?

Climate master has a published standard of base line water quality standards to answer your question. The point being it is a recomendation not a gaurantee. % of diminished capacity of the unit over time based on water quality is also a "swag". There is just no way to predict anything with no data.

I would guess that published performance data assumes squeaky clean waterside heat exchangers. My recommendation would be to gather baseline data for a system upon commissioning (unit current, airside delta enthalpy, and waterside delta-T) and the let measured performance / degradation drive maintenance intervals.

In other words, WP is right - it is almost impossible to predict required maintenance vs. water quality for any given installation. If you go open loop, plan on at least annual performance checks and heat exchanger flushing. I've had open loop since 1996 with excellent results. Your Mileage Will Vary.

Hi Curt,

Actually the climatemester water quality standards are fairly broad in regard to minerals, tds, and ph.  They do not specify how your mileage may vary or the service interval to address a given water uality.

Just because a heat exchanger is rated to handle certain levels of various minerals and other characteristics doesn't lead me to the conclusion that the unit connected to that heat exchanger will perform at AHRI specs for the life of the unit without heat exchanger flushing.

I got the impression that the recommended maximums were intended as guidelines to keep the heat exchanger from failing completely during the warranty period. I don't remember reading anything to the effect that exceeding certain limits would trigger periodic maintenance.

I think we are on the same page.

My recommendation would be to gather baseline data for a system upon commissioning (unit current, airside delta enthalpy, and waterside delta-T) and the let measured performance / degradation drive maintenance intervals.

I agree - ideally the COP would be measured at installation time and compared to specs and then the installer would return at 1 year and measure COP again and clean the heat exchanger. At this point, you have a good idea of how often cleaning is needed. Of course measuring COP accurately can be a challenge.

The measurements aren't so hard to a reasonable degree of accuracy- clamp and multimeter yield power. Water side GPM and delta T yield btuh on water side, air side delta enthalpy and CFM yield btuh on air side.

If system is operating within ESP limits and uses an ECM blower, we know CFM to within 5% or so. Airside delta T across strip heaters of known KW is another option for determining CFM

All that said, accuracy isn't as important as trendlines. If a system drew 7 amps in cooling low stage last year and draws 9 this year under similar operating conditions, heat exchanger fouling is a good place to start looking.

Since the manufacturers provide nice tables, I'd normalize what ever COP readings one gets as a percent of rated COP. So it at install if you get 95% of rated COP and it drops to 85% later on (likely to be somewhat different conditions), then something is wrong. COP changing from 3.5 to 3 doesn't mean much by itself.

I agree it could be done, but are any installers measuring COP for each install?

This one does.