Tankless Hot Water Heating

[KenO]

So, I have setup that includes a 250 gallon aquarium, a 130 gallon aquarium, 150 gallon sump and 2 - 40 breeder aquariums all connected together. In total about 600 gallons of water. I had been heating my setup with 2 - 500W Titanium heaters from Finnex. I live in AZ so my heating need isn't that great, but my heaters would run for about 30 minutes per cycle to raise the water temp 0.2 degrees. In the winter I would let the water temp drop to around 75-75.2 degrees. Based on my Apex tracking I was consuming 240kWh/per month.

I had been reading about using tanked and tankless hot water heaters to heat aquarium water. The good news for me is that I have a NG Rinnai tankless heater right on the outside of my fish room wall. The hard part, I'm not confident in my wet soldering skills. I've done it before, but I didn't want to take a chance with this setup. So I set out to find a plumber that would do the job for me. I had several plumbers come out and listen to me and my hair brain idea, at least from their perspective. The project went nowhere. I then had one plumber who would do it, but the cost was very high, even with me purchasing the pump and titanium heat exchanger. I delayed the project and when I decided to move forward on it, he wouldn't return my calls. So back to square one.

This was my simplistic idea, now my diagram doesn't show any check valves or drains, I figured once I found a plumber, they could fill in the missing pieces. The idea is based on how my pool heater is configured in that the pool is like the sump and its an open loop design.

 

[KenO]

So I kept calling plumbers and finally I found one within a few miles of where I live who listed himself as a tankless hot water expert. Gotta love that word expert. So I called him expecting to get yet another answering machine, but he answered his phone. So we chatted for a short while on the phone, setup a date and time for him to come by and I shot him a text message with my layout on what I wanted. The plumber Eric, came by looked at what I wanted and said sure he could do it. He would supply the parts except the heat exchanger. Within a day he sent me a quote and the price while not inexpensive, was much less than the first plumber I had found.

We had some hiccups along the way. Eric couldn't find the pump he wanted and that delayed him getting started. I ordered the titanium heat exchanger and had it waiting. Now the first pump Eric found would not work. Remember that expert word. So he called his Rinnai rep and found out that the Rinnai tankless heater I have has 10' of head loss. Wow, plus the heat exchanger I picked up added another 3' of head loss. The pump he first put in could only handle 4' of head. So the pump would spin up, but then shutdown. It was a Grundfos pump, but I don't remember the model number.

The Rinnai rep recommended a Grundfos UP 3 speed pump. So Eric ordered that pump and installed it when it came in. We new the setup was going to work once we had the correct pump. Eric installed a set of fill and drain valves on the loop. When we filled the loop and left the one valve open, hot water was flowing through the loop and the heat exchanger got hot fast. I ordered a 55,000 BTU heat exchanger. My tankless is rated at 180,000 BTU's, but I didn't want a heat exchanger that was so large that the system would heat up too quickly. I looked online and found out that 55,000 BTU's is equivalent to 16kWh.

Here's some pictures showing the outside plumbing configuration. Eric installed several check valves. One on the main cold water line to keep the hot water in the loop from going back into the cold line if someone turned on the cold water while the system was running. The other ones are to keep the recirc loop from filling with water when the loop is turned off. These pictures were taken prior to the pipes being finished. Cold is on the right, hot on the left.

 

[KenO]

Now for the inside part of the project, Eric mounted a plywood sheet to the wall in the fish room and mounted the pump and heat exchanger to the board. When he was done, I added the Finnex heat controller as a backup safety for my Apex and I added the aquarium loop from my sump.

These pictures show the first pump which was the undersized pump. For the heat exchanger to work correctly the domestic flow and the aquarium loop flow need to go in opposite directions. The domestic loop enters the right side of the heat exchanger and flows to the left. The aquarium loop enters on the left side and flows to the right.

 

[KenO]

Once the new pump was installed we tested the setup and I was amazed at how well the system worked. As per the Apex data, the heat cycle lasts between 6-7 minutes to heat the system 0.2 degrees. I had been letting the tanks drift to 75 in the winter now I am planning on keeping the tanks in the 77.8 to 78 degree range. Right now I am in the 77.2-77.4 range. The cycle when it turns off will go up 0.1 degree higher. So when it turns off at 77.4 it will top out at 77.5.

I don't have any data on my NG bill since I'm in the middle of my billing cycle. I found when I added a whole house generator system which runs once a week for about 15 min at a time, I didn't even notice a change to my NG bill. We have also had a couple of times where the generator kicked on for a power outage for approximately 30 min each time. Even with the generator running for those 30 minutes cycles I still didn't notice a change in my NG bill. So we will see how this works out.

Here is the final layout with a few Apex screen shots.

 

[joseserrano]

very cool, what is the comparison on energy consumption for you running the tankless for 7 minutes x amount of times VS the submersible heaters?

 

[KenO]

I'm going to say the energy consumption is about the same. So I was logging about 8kWh per day with the electric heaters now this was also at a lower temp setting of around 75 degrees. Im running the tanks at 77.2-77.4 right now with the goal of getting to 78. I'm getting 4-6 heating cycles per day right now with each heating cycle at 6-7 min. which is also equivalent to around 8kWh if I convert BTU's to kWh. The difference will be that NG is less expensive vs the same amount of electricity for where I live.

I also want to test different aquarium flow rates through the heat exchanger. The pump I am using right now can only go down to about 750 gph. I have another pump I'm going to swap in this weekend that will go lower than 750 gph. My thinking is that if I reduce the flow in the aquarium loop it will pick up more heat faster, reducing the heating time to less than the 6-7 min time right now. There isn't a lot that I can reduce it by but if I can get to a 3-4 min. That would also reduce the energy consumption. Does that make sense?

 

[KenO]

When the system turns on, there is also a slight delay before hot water actually gets to the heat exchanger. They say instant hot water with a tankless, but that's not quite true. The longer your pipes are the longer it takes to get hot water. My run is fairly short. If I had a longer run, it would make sense to delay the aquarium loop until the heat exchanger is hot, but I'm not going to go through the trouble of doing that.

In my house it's just my wife and myself. The tankless hot water heaters are NG, so is my dryer, my range top and ovens and my outdoor BBQ. My monthly NG bill is around $18.00. That also includes the 20kW generator running 15min per week during its test cycle. Our electric bill if I was to use no electricity is over $30 in just fees. Start adding electricity usage, plus time of day uplifts and delivery fees and it can spiral out of control in a hurry.

 

[Blue Spot Octopus]

Is it possible to have a tanked water heater placed outside and use that instead of the tankless water heater. A 40 gallon water heater uses about 40K btu and hour and the tankless uses 199K BTU and hour. If your tankless cycle quickly it could get real expensive fast. My friend loved his gas fireplace until he got the bill. I know a guy that has a large grow station outside and he uses tanked water heaters to heat his setup. I fear for your bill.

 

[KenO]

Is it possible to have a tanked water heater placed outside and use that instead of the tankless water heater. A 40 gallon water heater uses about 40K btu and hour and the tankless uses 199K BTU and hour. If your tankless cycle quickly it could get real expensive fast. My friend loved his gas fireplace until he got the bill. I know a guy that has a large grow station outside and he uses tanked water heaters to heat his setup. I fear for your bill.

A tankless unit will vary its BTU usage based on a number of factors. The maximum BTU rating is if you are running the unit at its maximum flow capacity, with the lowest incoming cold water temp. Maximum flow for my unit is 9gpm. Mine for example also states it's min BTU usage is 11,000 BTU's.

Now when the loop is running initially the BTU usage will be higher, what that amount is I don't know. It's based on flow and the initial water temp. Once it starts to heat the water and the water flows through the loop heat is extracted from the heat exchanger, but warm water is flowing back to the tankless. So now the amount of energy that is needed to raise the water temp back up is less. Let's say the initial water temp is 65 degrees and the tankless is set to 130. So the tankless needs to use more BTU's to raise the water by 65 degrees. The water when it goes through the loop and returns to the tankless it is now 95 degrees. So the BTU usage is less to get to 130 degrees. Now the cycle only lasts 6-7 minutes and for me the number of heating cycles is 4-6 per night. It's like someone taking 2 long showers per day. Even that may not even be accurate, because in that situation the incoming water temp is always lower vs the warmer water returning in the loop. When we have family/friends staying with us for an extended period of time our NG bill will go from $18/month to $22/month. That increase also includes more dryer time and more range/oven usage.

I'm in the middle of my NG billing cycle so I will have to wait for a full cycle to get a true cost. When I get those bills I will post the amount vs the previous month.

I think having a separate tank kept warm all the time would waste more energy vs using the tankless.

 

[Opus]

Did you calculate how long it will take to recoup the cost of the new setup? I've always wanted a tankless system but I'm electric only and I've read the electric tankless systems just aren't that good.

 

[Blue Spot Octopus]

Ok it should be interesting to know how it all pans out. Can you link your power strips that you have on the wall?

 

[Joesig]

Did you calculate how long it will take to recoup the cost of the new setup? I've always wanted a tankless system but I'm electric only and I've read the electric tankless systems just aren't that good.

yeah, you don't want to go electric it costs too much and for low output.

Nice set up. I would be less concerned with the gas usage and more with the ware on the unit. Most manufactures reduce your warranty when you add a circulating pump to the tankless system. but your system looks nice.
If your heater goes out might look into replacing it with a noritz with a stainless steel heat exchanger.

 

[KenO]

Did you calculate how long it will take to recoup the cost of the new setup? I've always wanted a tankless system but I'm electric only and I've read the electric tankless systems just aren't that good.

I didn't run a cost analysis. If you did the plumbing yourself. The titanium heat exchanger and the pump were about $300 each. I would guess another $300 in valves, fittings and pipe. I already had the Rinnai tankless. I also had the pump for the aquarium loop so say another $100.
I did it for a couple of reasons. I wanted to get rid of my electric heaters. I wanted to run the aquariums at a higher temp in the winter. I hate my electric company, it's a long story lol. I have solar, but I don't have a battery system to store my excess for use when I need it at night.

 

[KenO]

Ok it should be interesting to know how it all pans out. Can you link your power strips that you have on the wall?

Will do. Here are those power strips
Amazon product

 

[KenO]

yeah, you don't want to go electric it costs too much and for low output.

Nice set up. I would be less concerned with the gas usage and more with the ware on the unit. Most manufactures reduce your warranty when you add a circulating pump to the tankless system. but your system looks nice.
If your heater goes out might look into replacing it with a noritz with a stainless steel heat exchanger.

Yes the manual for the Rinnai says adding a loop will reduce the warranty. I find it interesting since the manual shows various configurations for infloor heating, hydronic hot water HVAC systems, etc. Honestly for the number of cycles per day, I don't think it will affect the unit. I have 3 Rinnai tankless units on the house. The kitchen unit gets the most cycles, with the amount of use our kitchen sink gets.

 

[Xanthurum]

Are you concerned about running your tank water thru copper pipes?

 

[andrewkw]

Thanks for posting such details. While I'm a ways off from wanting to do this, it's good to learn more about it so when the time comes I'll have a better idea of what I'm trying to do. Especially if I have to explain it to plumbers.

 

[KenO]

Are you concerned about running your tank water thru copper pipes?

The tank water only runs through the titanium heat exchanger. If you look at the heat exchanger you see 2 white pvc fittings on the bottom. The aquarium water enters on the left exits on the right. The heat exhanger has a series of twisted tubes that the domestic water goes through. The aquarium water flows through the shell portion of the heat exchanger flowing around the twisted tubes.

The house water is flowing through the copper pipes.

 

[KenO]

Thanks for posting such details. While I'm a ways off from wanting to do this, it's good to learn more about it so when the time comes I'll have a better idea of what I'm trying to do. Especially if I have to explain it to plumbers.

When the time comes, send me a message I would be happy to help. Feel free to use the diagram and any of the pictures to show your plumber. The key finding for us was the head loss through the tankless and heat exchanger. You need a recirc pump large enough for the head losses. Also the heat exchanger listed its loss in psi. There is an online calculator that converts psi loss to head loss.

 

[Opus]

I didn't run a cost analysis. If you did the plumbing yourself. The titanium heat exchanger and the pump were about $300 each. I would guess another $300 in valves, fittings and pipe. I already had the Rinnai tankless. I also had the pump for the aquarium loop so say another $100.
I did it for a couple of reasons. I wanted to get rid of my electric heaters. I wanted to run the aquariums at a higher temp in the winter. I hate my electric company, it's a long story lol. I have solar, but I don't have a battery system to store my excess for use when I need it at night.

You mentioned electricity cost so thought that might have been a big factor. From your reply I guess you can't change electric companies. I can't because I'm on a Co-op but most can in Texas. I have coworkers that change companies every couple of months and go with whoever is offering the cheapest rates. I've thought about solar but it is such a huge $$$ investment and with all the hail storms around here not sure it is such a great idea. Have a good friend that her panels got shattered from a hail storm last year.