Vitodens and pool heat exchanger

rumn8r

Should a pool heat exchanger be oversized when using a Vitodens mod con to increase delta T and run at a lower temperature for greater condensing?

Ken_40

There are so many ways...

to induce low return water temps., I would simply size the HX to match the boilers net output. Given that pool water should not go above 110F at outlet, and will be between 65 and 80 (unless a hot-tub) at the inlet, the application should be awesome especially if you make the piping bypass/diversion setup from the filter pump so that you control how much water goes through the HX and how much goes into the bypass.

Let us know what you decide - and how it pans out.

rumn8r

The boiler will be used for hydro air space heat as well. In that case I was advised to oversize the hydro coil to increase the dT to drop the return temp for a mod con boiler. Need to consider the rate of temperature rise as well as the size of the pool to get minimum size of the HX. However, is there any downside of oversizing?

rumn8r

Ken, I wasn't thinking of using a bypass/diversion setup from the filter pump to the pool HX. Why do you think it would be a good idea?

Ken_40

We found

the variables, e.g., the resistance to flow, "head," changes so wildly from a clean (just backwashed) filter bed and a almost full (almost in need of a backwash) can run from 12# to 18# of just filter bed head, will wildly alter the dT through the HX (and therefore the boiler flow rate) that a bypass allows adequate flow in all scenarios. A non-bypassed desgn has no ability to deal with that dynamic.

The filter pump also has a huge GPM rate, far exceeding the capacity of any typical boiler intended for home heating and/or a pool app.

Because the variables can take on a life of their own, a simple way to adjust flow gives one almost infinite control over dynamics that may occur, as well as the extremely inexpensive valves and piping (all in ABS or PVC) as well as a cool feature of installing a digital thermometr using a downstream tee and well to place the sensor, and the small wire those $25. thermometers provide, inside a nearby sheltered display location.

I did a solar array, two-pipe R/R and the head loss was zip squat. The few real HX's we installed however, had hefty Cv values that mandate bypass considerations.

I suppose ot all comes down to the actual Cv rating of your HX at the huge flow rates typical in that array?

Leo G_99

Ken a silly question

you say poolwater should not be above 110 at outlet. How do you control that? Here the pool guys just ask for a HX to tie into, then away they go. No one has ever mentioned what temp should be going into the HX from the boiler, or going out of the HX to the pool! We usually set the boiler temp to 160* to the HX.

Do you use a variable control with sensors on the supply to the pool or some such set-up?

Leo G

Ken_40

Well Leo...

The 110F outlet max is not some "golden" rule, just my notion of a somewhat arbitrary temperature that would prevent some little kid's butt or leg from getting an inch or less away from the outlet fitting. We all know anything over 110F has some potential for discomfort or an actual "burn." That's why a discharge temp gage is nice.

Controlling the actual outlet temps is the real purpose of the bypass valve. The balance between flow in GPM's and temperature are somewhat proportional. It works sort of like this (assume a 70F pool water start temp.): "Which would you prefer, 5-gallons a minute of 73F water outlet or 10-GPM at 72"? The math of BTU's/lb of water converted into galons vs. filter backpressure swings from clean to near backwash points - is a pain.

By simple dynamic testing for a day or three, one can see how resistence to flow and bypass settings can maximize the raising of cool pool water temps. It is easy because it really has two prme settings - and variations possible, ONLY between those two maximums.

Those two maximums being: either all the pool water goes through the HX, or all goes through the bypass and "around" the HX. The former theoretically taking all the BTU's the HX can add; the latter providing no HX flow. Somewhere between those two extremes the resistence to flow vs. the heat the HX can impart will find an acceptable adjustment.

By a simple test, I found from 10 to 4 PM in summer, my home-made solar garage roof mounted HX made from ABS made 40,000 BTU/hr. During the "dog-days" of August, my pool went above "refreshing" and I would turn off the HX and allow straight by-pass flow only! 84F was the point at which we felt the water was warm enough! In the 15+ years we had the home-made HX on the garage roof, I think we closed off the HX maybe 3 times and for less than a week each time.

And no. We did not use "variable controls" per se, unless you call a hand operated, full-opened or full-closed PVC ball valve, "variable."

Obviously, a more powerfull heat source and HX would potentially require more, not less, adjustment and control. Putting 40,000 BTU's into the 17,500 gallons of water the pool contained for the longer May and June days extended our pool season by almost two months on either end! Which was our true intention. Being the cheapskate I am, the initial cost of ~$300 for the all the ABS tees and lengths used, coupled with the free sunlight my south facing garage roof afforded, coupled with the 15+ year time frame we used the HX at no additional cost, or any maintenance cost whatsoever, I thought it the best of all options.

Hope that helps.