Tag Archives: water to air heat exchanger

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.

Hydronic Heating Coil Replacement

Today was Hydronic Heating Coil Replacement day at the Ivany homestead. What is a hydronic heating coil you ask? It’s basically just a water to air heat exchanger which is really just a radiator like you find in a car. Here is a picture of the old one that I replaced today:

Our old water to air heat exchanger coil that was leaking

Our old water to air heat exchanger coil that was leaking

The house actually has an electric forced air system. At some point before we bought the house, the previous owner did a retrofit to add this hydronic heating coil system. I’m guessing it happened around the same time that heating with electricity got expensive and/or natural gas was brought to Kemptville. We heat water with a Polaris high efficiency natural gas hot water tank. That unit has two sets of outlets, one for the domestic hot water and the other for a heating system loop. I believe it’s more intended for use with radiant in floor heating but our system is basically just a radiant system with one loop.

This replacement isn’t something I plan on doing on a regular basis as these coils should be able to last for a fairly long time. For some reason, the coil I had started to spring these little pin-hole leaks. That meant I had water collecting in the plenum and that’s really not good. If you notice in the picture above, the unit on the right hand side is my blower and all of the electrical connections are on the bottom side. Further right are all of the relays and the actual electric heating elements. Yeah, that’s the side where the water was pooling. I had to break out the drill to put a few well placed drain holes in the bottom of the plenum to ensure I didn’t run into any electrical problems while I found someone to fix my system.

Now, I had a hell of a time trying to find a hydronic coil (aka duct coil, aka heat exchanger, aka water to air exchanger, aka wood furnace coil) because I couldn’t find anyone local who seemed to have a clue as to what the thing was. I contacted a couple of the local heating companies, even had one come in and just recommend we replace the whole system with a new furnace. One other local company managed to track down a coil, a different size than what I needed, but the cost to get it and have it installed would have been over $1000.

Instead I hit up Google to see what I could find. Now, the time line of this is a little out of whack. I actually started searching Google just before having one of the local heating companies come in. That was almost 3 months ago. The second company got back to me just before Christmas. In the meantime, Google had helped me find a whole lot of places where I could buy a replacement coil, once I knew what to search for.

Ironically, I ended up getting the coil through my neighbour two houses away. We were chatting one day and he started asking some detailed questions about the sizing of the coil. Only then did I find out he actually works with outdoor wood furnaces and they use hydronic coils for all of their heat transfer. Within a matter of a few days I had my replacement coil and I started collecting all the stuff I needed to change around the copper connections.

Oh, yeah, time for some plumbing! This isn’t something I had ever done before so I consulted my books, chatted with the guys at work and borrowed some tools. Now I’m sitting here, tired as a dog (holy crap I found muscles I’ve never used before) and happy as a pig in shit because I actually managed to solder a whole crap-load of connections with no leaks! Well, no leaks yet. ;)

The new copper for the water to air heat exchanger coil

The new copper for the water to air heat exchanger coil

The new copper was required due to the fact the original coil was a 1/2″ inlet/outlet coil and the new one is a 1″ inlet/outlet coil. My feed from the Polaris hot water tank is 3/4″ so I had to replace the 1/2″ copper with 3/4″. I also had to re-route the connections as this coil has it’s inlet and outlet on opposite ends. The old coil had both connections at the top. In the picture, the little blue cylindrical device is my Taco 006 circulator pump. It pulls water from the hot water tank (well, helps pull as it’s all under pressure) to be passed through the hydronic coil while my blower is on.

After reading so many websites and so much information on how these hydronic systems work, I don’t think I’ll be replacing mine anytime soon. Basically, I can heat water using any number of sources. Wood, gas, electricity and even solar could be used to heat my house.

I’ve become very interested in radiant in-floor heating systems now as well. Interestingly enough, I could run the forced air unit as one heating zone on a radiant system. That could be handy for a slow transition to a radiant system in the house. Since we will probably replace much of the flooring in the next few years, radiant is a viable solution for this house.