Heat coil fed with hw heater questions

[Tony7812]

Hello All,
Here's the deal : Bradford white 65 gal hw heater. Hot side has tee, the branch goes to mixing valve and dropped down to 120 for fixtures. The run on the tee goes directly to dishwasher and heating coil of air handler (ac coil bottom) heat coil on top. I've been tasked with figuring out another guys work. This is all new, and 84 units done this way. The original plumbers didn't install any draw off, gauges, etc. Purging is done at coil inside air handler. With tstat on heat (but off) my max temp was 135F. With the tstat calling and pump on, temp is a little lower. The tstat on hw heater is on the max setting both elements. Reset is fine. I can't seem to get the heat coil temp up. Side note, runs r kind of long, uninsulated, and done in cpvc. At one point I threw testers on the element and it was getting power. Threw the disconnect and ohmmed it out, read around 10 ohms. Any ideas, or could I really be having that much heat loss from the run and it being uninsulated? Any clearifications would be nice. Please bear in mind I'm an apprentice trying to learn the right way and fixing somebody else's handy work so go easy on me. Thanks for any help!!!!

 

[Jadnashua]

You may be getting some convection in the loop and, especially if the a/c is still on, you're cooling the water going back to the WH because of that flow. The cooler water is denser, therefore, heavier, so it would promote that circulation. During the cooling season, having either a zone valve or a simple shutoff into the coil of the heat exchanger is probably a good idea to stop convection. Insulating the pipes is also a good idea.

 

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[Dana]

Why do you think the coils need water hotter than 135F? Air handler coils can be operated at just about any arbitrary entering water temperature, as long as it's able to deliver sufficient heat at the temperature. At EWTs lower than 120F the exit air at the registers can be a bit tepid, but even 100F air at the register isn't too uncomfortable for most people (though 110F+ is preferred if the air velocity is high enough to induce wind chill.)

In my own home I have large-ish hydro-air zone that I run at an EWT of 120-125F, and it's just fine. Temps in that range are pretty commonly used when the heat source is a condensing boiler, to ensure 95%+ combustion efficiency.

 

[Jadnashua]

A lower inlet temp would likely result in the system running longer in a cycle which also tends to keep the temperature in the house more even verses shorter spurts with hotter air.

 

[Dana]

A bit more information such as the make & model of the air handler, and the size of the apartments/condos they are serving might be enough to give a WAG on just how low the water temp can be and still heat the units.

While many air handlers will have a label specifying it's output at a couple of EWT (140F and 180F are often used) at some nominal flow rate. The full spec usually shows a greater number of points on the curve. Most town-houses and apartments in MA have design loads WAY lower than the smallest gas furnaces, which makes smaller hydro-air systems running at domestic hot water temperatures attractive, since they can be more readily "right sized" properly to the lower loads, delivering higher comfort & efficiency at a lower cost than an oversized furnace.

For instance, a small 1-ton air handler like a FirstCo CW4 specifies the output at three different flow rates and four different entering water temps. It'll deliver 11,800 BTU/hr at 1 gpm at an EWT of 120F, (and output air of 97F), which is plenty for most 1200-1500' apartments in a multi-unit building, but at 3 gpm that rises to 14,500 BTU/hr. Crank the EWT all the way up to 140F and it's good fo 16,600 BTU/hr @ 1 gpm, or 20,300 BTU/hr @ 3 gpm. So even at 130-135F EWT and 2-3gpm it can still heat a pretty substantial town house or apartment, even though it's the smallest of that series.

With a non-condensing boiler behind it delivering 180F EWT, at 3 gpm it's good for 31,800 BTU/hr which is enough heat most ~2000-2500' new single family houses built to current MA code minimum down to -15F or colder.

Bottom line- don't sweat the water temperature unless you KNOW it isn't enough to cover the design heat load of the unit it's heating. Heat load is proportional to the exterior surface of the unit, so townhouse end units will have somewhat higher loads than middle units, and upper floor corner apartments will have higher loads than lower level units that aren't on the corner. In an 84 unit complex it's common to run a Manual-J heat load calculation on just the lossiest units and specify an air handler that will deliver the design day heat with 120F or 140F water, then use the same air handler for all units. That saves design time (one Manual-J instead of 84), and lowering the piece cost & installation cost by making all systems identical. That's probably what they did here, and if there is access to the design documentation for the building(s) you can probably find the load calculations rather than taking a WAG, but a WAG is usually going to be good enough, since they tend to specify this stuff conservatively (rather than risk the expense of having to replace 84 systems.) From a fuel efficiency point of view oversizing a hydronic air handler running off a water heater is "in the noise", hardly moves the needle at all. Lower operating temperatures also have lower distribution lossesl, but in an apartment building the distribution losses accrue to your neighbors' heat loads along the way.

"...kinda long..." runs isn't a well defined term, but say it's a couple hundred feet, or a hundred feet out, a hundred feet for the return. A couple hundred feet of uninsulated CPVC is a bit of a radiator, and the length of the run slows down the flow a bit, so it's no surprise if it's dropping a few degrees along the way. But it's nothing to be concerned about as long as the unit being served can still be heated at the water temperature & flow that arrives at the air handler.