Heat-Pump Loop Temp

[skarpes]

There has been some debate amoung my collegues and I over the best temperature to run a Commercial Building's Heat-Pump Loop. Just thought I'd throw this one out there and see what others think.
We have several buildings with water-source Heatpumps in a loop with a Boiler/Circ-Pump(s)/Cooling-Tower to mantain loop temp. I have found some techs like to keep the water loop close to 70F (Boiler comes on at 70, with say a 5 degree deadband before Tower is staged on in steps at say 75, 78, 80) but I tend to disagree with this.
Everyone seems to set a water-reg valve on a water-cooled unit for about 100F Condensing right?? So, then why wouldn't we try running the loop closer to say 85F? (This would allow for any spots in the loop that don't have good water-return) The heatpumps would thereby run more efficiently in cooling mode and, then there would actually be some heat in the loop to provide for heating. Certainly 70-75F is much too cool to run the loop for cooling.
Another reason I have stressed this approach is that more than few buildings we service have been designed with over-sized boilers piped directly into the loop (no seperate loop with heat-exchanger) and at 75F The impingement/scale on the tube-fins becomes a major problem.
Anyway, I'm curious what others think regarding all this.

[willf650]

I set my loops up about what you are saying. I control the boilers to 70 deg and the towers between 80 and 85 deg.

[caosesvida]

thats about what we have also, except the older units, we have to run them hot in the winter (exteme temps) to keep up.

[airworx]

80 degree water is standard.

[dapper]

Around here the temps are usually higher for the tower. I looked at a job today where the damper opened at 90f, pump at 92f, low fan at 94f and high fan at 98f. These are not unusual here.

The boiler is usually controlled between 68f and 65f

[firstmech]

70 to 90 for boiler/cooling tower application, in some cases it depends upon the type of heat pump and the water flow through the heat pump I believe flow rates should be 3gpm per ton as standard and 2.25 the very minimum.The main idea of this type of system is to not to require any heating or cooling of the loop for the 20 deg between the 70 and 90 thus saving energy.

[skarpes]

OK, thanks for the input. No one wants to argue over the standard 100F condensing temp (for cooling), but still some debate. Well, in the case of buildings with boilers piped directly into the loop, i have tended to argue that 70F is too cold as boiler only fires for a short time and it never gets to run hot, thus the impingment/crud on the fins and a drastically shortened boiler-life.
As I've seen it, the heatpumps perform better with loop-water 78-90 and cooling-tower runs less. Thanks again for your input...

[thermo]

We always recommend 70 degree loop temp in winter (avoids nuicance trips on low temp) and stage 1st stage heat of rejection at 88 degrees with a 2 degree deadband....

[ac/dc]

I agree 70 for winter 80-90 summer in summer for energy
savings. You said you think the heat pump runs more
effeciantly in cooling mode if the loop temp is higher... I would think not...the head pressures would be lower (lower amp draw) and liquid line temps lower for increased
refrigeration effect with lower loop temps.
I was thinking of why your boiler is scaling up like that
even though it has a short run clcle? Is the water treatment for the loop correct?

[dapper]

To minimize boiler condensation it is best to pipe it as a secondary loop with its own pump and a bypass to allow heated water to mix with the boiler inlet

[skarpes]

Yeah, the scaling I mentioned is on the OUTSIDE of the boiler heat-exchanger...DAPPER has it right, "To minimize boiler condensation it is best to pipe it as a secondary loop with its own pump and a bypass to allow heated water to mix with the boiler inlet" (This is what I've recommended to any customers with this problem.) But like i said, for water-cooled A/C (or heatpumps)in cooling, 100F condensing-temp is what most manufact recommend. So, 70F is too cold for heatpumps in cooling!

[bradysmill]

The lower the head pressure the less energy used by the compressor......What kind of boiler do you have, I hope it is one that is designed for this installation. With the boiler tied in without a seconday loop of an injection type into the primary, you will have to have one that can take the condensation formed (acidic water) by the extreme temp. diff. between the 70 degree and boiler water temp. Usually these types of systems are designed with High eff. boilers that are like the "hydropulse". Regular boilers or normal setups can not tolerate any lower than say 140 degree boiler water going throught the boiler for the corrosion problem you are describing.

The real reason for the 70 degree figure is just that you need the span of degrees for the proper staging of all heat and cool stages usually 2 stages of heat and four stages of the cooling tower. 85 degrees for the final stage of the tower dosen't leave you the luxuary of any higher water temp in your loop in the winter.
100 degrees of condensing temp. has been a standard for some time now...but I have been seeing Trane RTAA commercial units now with condensing temps in the 60 degree range with the screw compressors and fan staging system they use.(floating the head" is what we called it in the Refrigeration business before), for energy efficiency. Or you floated the head by means of a "liqid amplifier pump" instead.

[Edited by bradysmill on 03-16-2005 at 10:01 PM]

[CSalsman]

What kinds of COP's do you get with these types of systems?

Does the boiler have the ability to modulate it's capacity?

What about energy use relative to water temps? Or is it a capacity issue when you say that you have to have the water temps that high for the heat pumps to perform?

What kind of refrigerant metering is used with this setup?

Forgive me, Im a newbie to geo/water-water systems, but Im a quick learner.