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Hydronic Heating Coil Sizing

As some of you may recall, I recent completed a hydronic heating coil replacement at my home. The actual replacement of my heating coil was very straight forward and easily something that the average DIYer could do themselves. Replacing a coil is simply some standard plumbing – cutting copper piping, soldering, etc. I say very easy as I managed to do it! ;)

Finding a replacement coil was the hardest part of my task as I didn’t know what I was looking for. I also didn’t know much about the heat output of a hydronic heating coil. So, first place I went to was Google where I searched for “water to air heat exchanger”. After pouring through a few links I found Heat Exchangers Online which has a fantastic table detailing hydronic heating coil dimensions as well as potential BTU output. Note, this is potential output and varies on a whole lot of factors. One of those factors is the temperature of the water you are going to be sending into the unit.

In my case, my Polaris hot water tank is set to 60°C (or 140°F). Using the table and based on my plenum size, I was looking at a 18×18 coil. That would potentially produce around 70k BTU. Now, there’s a bunch of other factors involved here. The flow rate and pressure of your water going through the coil also come into play. Note that I didn’t have to do the sizing math to figure out what coil I needed – I had a coil already that I was looking to replace. My guess was that the existing coil was capable of heating my house so a similar sized coil should also be able to heat my house.

I just tracked down the specifications for my Taco 006 Circulator Pump that is used to draw water out of my hot water tank. I’m realizing now that I’m not going to get the potential 70k BTU out of the coil as that rating is based on 12 GPM. The 006 is only capable of a maximum of 10 GPM. I should probably have a Taco 005 Circulator or Taco 008 Circulator to be able to move more water through the coil and thus increase the potential BTU output. Of course, a higher volume pump would draw more water from the hot water tank which in turn could cause it to heat the water more often thus consuming more gas.

One other thing I looked at was the output of my old electric forced air system. It has 4 electric coils, two are 5kW and two are 4.6kW for a total of 19.2kW of heating capacity. Since electric heat is 100% efficient, we can do some quick math once we know that 1kW is equal to 3432 BTU (thanks again Google). Therefore, my original electric system was capable of around 66k BTU. Good, I was in the right ball park anyways with the heating coil size I was looking at replacing.

The way my system is connected is very simple. The Polaris hot water tank has a secondary set of water inlet/outlet that you can connect for this application. Basically, my circulator pump turns on when there is a demand for heat. This draws water out of the hot water tank and pushes it through the coil. After exiting the coil, the water returns to the hot water tank where it mixes with the other water in the tank. Eventually, the Polaris will detect that the water temperature has dropped and it will perform a burn to heat the water back up. Basically, I’m not burning gas the whole time I’m heating so this should result in a reduction in the amount of gas I use.

Since this was a forced air retrofit, when there is a demand for heat, my existing 1/2hp blower also turns on. The blower will stay on as long as there is a heating demand (plus 5 or so minutes after the demand has been satisfied). I’ve read that ideally you should have your blower turn on after the circulator has run for a bit to ensure that you are going to be blowing hot air. Also, you want to have your hottest water entering at the “exit” side of the heat exchanger. For example, if your air is traveling left to right through the finned section of the coil, you should have the water entering the coils on the right hand side and exiting on the left hand side. This way, the air is heated first by the cooler water and heated last by the hottest water.

Now for the downsides of this type of heating system.

The system is slow to recover if the temperature in the house is set lower than “normal”. I bought a programmable thermostat soon after we moved in but I haven’t been able to make much use of being able to setback the temperature at night or when we aren’t home. It simply takes too long to recover the temperature to our preferred 22°C.

One other related issue is on really cold days (-25°C and below), the system can barely keep the temperature at the set point. If you leave the door open too long, the house can drop a degree or so and it will be an hour or more before it recovers. I don’t think this one only affects my heating system but it would be nice if it could recover a little faster.

The system can often lead to a lack of hot water for domestic use, especially first thing in the morning when we are taking showers. Now, this one is partially due to the heating system drawing off of the hot water tank at the same time. It’s also likely due to a malfunctioning proportioning valve (aka anti-scald thermostatic mixing valve) that I have to install on the domestic side of my hot water tank. Since I run at 60°C, that can cause serious scalding so I need to have an anti-scald valve to keep the water temperature lower. This mixes outgoing hot water with cold water coming from my supply. Obviously, in the winter here, the water is coming in at less than 10°C so the mixing valve could be part of my problem.

Finally, the biggest problem with this system is no one knows anything about it. ;) It can be a huge pain in the butt when you need to get it serviced. Last winter I had some issues with my Polaris tank and thankfully I rent the tank. The difficult part is explaining to the service tech on the phone that yes, I have no hot water but I also have no heat. One point of failure is not good. Thankfully I still have my electric coils so as long as I have power, I can heat the house.

Would I recommend this style of heating? Sort of. As a retrofit it works and it’s cheaper than installing a whole new furnace. I’m not sure if I’m really saving any money as my gas bills are roughly the same as other people I know. I’d much rather have in-floor radiant heating though and I’m seriously considering a transition of this house to radiant heating. We’ll have to replace much of our flooring in the next few years if we stay here and a retrofit to in floor radiant would be fantastic.

I’d love to hear about other people’s experience with a hydronic retrofit as well as any pointers on ways I can improve the heat output or address my issues.

2 thoughts on “Hydronic Heating Coil Sizing”

  1. I just purchased a townhome in portland, oregon. It has a similar airhandler with a heating and cooling coil in the attic. Now there is a crack on the copper tube of the heating coil and it is very close to another copper tube.

    So the brazing to fix the crack seems almost impossible from the service guy I called. So the option is either replacing the heating coil or replace this one with a new furnace. I am not sure how much tools and skills I need to replace the heating coil my self. I have never done any plumbing before.



  2. Fabulous post! I have a Polaris combi system in a 2BR apartment in NH, and have been trying to find a replacement heat exchanger for it. I was having a hard time even figuring out what they call it in the HVAC trade. They are hard to find!

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