Tankless coil not working well

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DavidP
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Location: Hudson Valley

Re: Tankless coil not working well

Post by DavidP »

Thank you, Skip -- very interesting. I don't find that my burner runs too often right now, although it may run more frequently if we raise the temp at which the water is reheated. If I do end up getting a separate hot water tank, I will certainly talk to whoever installs it about making it about doing something similar to what you have.

Russell Tavern
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Re: Tankless coil not working well

Post by Russell Tavern »

Just to second what Skip said, ours is very similar and hot water has never been an issue. We have 4 zones of hot water loops for baseboard heat and one for the hot water tank. One nice thing is that the boiler doesn't take the summer off, it does run periodically to heat the water tank. So no surprises when the cold weather hits.

I would think a water loop in the boiler itself would need to be substantial to contain enough hot water, what is a normal water heater- 30-40 gallons for a family?

TheFixer
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Re: Tankless coil not working well

Post by TheFixer »

I would look at the location of the thermostat, and the wiring setup. Being that winter is the issue for you, I would suspect that the thermostat may be getting fooled by the heat from the boiler and other zones? Do you know if the thermostat switches when the water gets cold? It would be interesting to see how the thermostat for the DHW and the boiler thermostat are both controlling the boiler. I think you have a condition where the thermostat isn't accurate to the DHW or the thermostat is calling for heat, and the boiler isn't getting the signal.

The type of thermostats and boiler might help.

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DavidP
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Re: Tankless coil not working well

Post by DavidP »

So. It turns out that I have something called a triple aquastat. See this page for some good info if you want. It turns out that the max and low temperatures were set lower than normal in this area for some reason (170/150 rather than 180/160). There is also a mixing valve that lets cold water mix with the hot water coming from the coil. This lets the hot water go further (for instance, if the ideal temperature for use is 120F, having the coil at 170 or so and then being diluted with cold as it leaves the boiler means that you don't run out as fast as you would if the water were 120). This also prevents one from being scaled by hot water. In my system the mixing valve was set to allow the maximum amount of cold water in. The tech adjusted this a little as well as raising the temperature.

It also turns out that there is another setting, called the differential, that causes the water in the boiler to heat up more than usual before the circulator brings in water from the radiators. So if the domestic hot water was close to the low point and the burner just came on to heat it, and then the thermostat called for heat, the differential would prevent the boiler from being overwhelmed by the influx of cold water before the hot water recovered. Pretty clever.

The guy who came last year was totally clueless. This guy knew exactly what was going on and I am now confident that I can do a lot better with my tankless coil, even if they have to come back again to make some additional adjustments. It's clear that there are things to work with.

I am amused (in an envious way) by those of you who have zones for heating and hot water. My "zones" consist of leaving all but one of the upstairs radiators turned off and turning on two, rarely all three, before I go to bed on very cold nights. It actually works pretty well even if it is not always convenient. To get real zones would cost me several thousand dollars of plumbing and new control hardware.

SkipW
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Re: Tankless coil not working well

Post by SkipW »

Glad you got a well educated tech to troubleshoot. Much better to resolve issues with an existing system than try and cobble one together or replace with new.

Smart aquastats are amazing little animals...that is how my domestic hot water takes precedence over the heating zones when they are both called for at the same time. Essentially like yours so as not to overwhelm the system and give us our preferred priority.

Thanks for the update
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