Trying to understand how an indirect tank leaks into a steam boiler

Synbio

I have an oil-fired steam boiler with an indirect domestic hot water tank. The water level (as seen on the sight glass) on the boiler continues to rise unless I close off the circulator loop/coil that goes from the indirect hot water tank to to inside the boiler.

What is the path at which cold feed water that goes into the hot water tank could leak into the steam boiler? Even if there was a leak in the coil in the steam boiler, wouldn't that be a closed system from the water inside the domestic hot water tank?

Jamie Hall

It's all a matter of pressure. The domestic water inside your indirect tank is at a higher pressure than the water in your steam boiler. A LOT higher pressure. If the loop from your boiler to the indirect is a closed loop -- which is highly unlikely (there would be an expansion tank on it, if no other clue) there'd have to be two leaks, but normally just one would do -- in the coil in the indirect. High pressure domestic water leaks into the coil and goes back to the boiler. No problem. If, on the other hand, the indirect is just a storage tank (no heat exchanger), and the water is circulated though a tankless coil in the boiler and back to the indirect, then the leak is in the tankless coil in the boiler. Same principle.

If you have a really fancy system with a closed loop from a tankless coil in the boiler to a heat exchanger in the hot water tank and back, then you need two leaks -- one from the domestic hot water into the closed loop (which I hope is at a much lower pressure! -- and one from the tankless coil in the boiler into the boiler..

Synbio

Thank you for the explanation! I have a SuperStor SS-40 which I believe is an indirect that has a heat exchanger in it, so the first explanation you described sounds like the issues that's going on.

Is there a consensus on what's a better setup: indirect with exchanger coil in the hot water tank or the storage tank with a tankless coil going through the steam boiler?

mattmia2

The second is probably a little better in a steam boiler setup because the boiler water in a steam boiler is not clean and will eventually scale and clog the coil in the indirect, but the outside and inside of the tankless will get scaled from the boiler water on the outside and from the minerals in the domestic water in the inside. The tankless coil is probably easier to keep clean.

ethicalpaul

The better setup is to separate your DHW from your heating system by buying a standalone gas (or even better especially for your oil situation, hybrid) water heater.

Why heat up a whole boiler all summer long with its standby losses? I bet your oil company loves the idea, though.

mattmia2

Is that really much of an issue if you size the tank so the boiler can be cold start and set the boiler up as cold start?

ethicalpaul

To me it is. It would drive me absolutely batty to know that after I used some hot water my steam boiler (400-600 pounds of cast iron plus some gallons of water) would have to heat up from room temperature in order to heat the water back up in my tank.

Then after it does that, it heats up the basement by giving off all its latent heat until it goes back to room temperature. Rinse and repeat.

I'd gladly take even take a resistive electrical water heater over that scenario (and I have).

mattmia2

I think you'd want to size the indirect so that the boiler pretty much only ran during a big draw like a shower or bath or convectional washer

ethicalpaul

Yeah, that's what would drive me batty, the boiler coming online after a shower or washing machine load.

JohnNY

ethicalpaul said:

To me it is. It would drive me absolutely batty to know that after I used some hot water my steam boiler (400-600 pounds of cast iron plus some gallons of water) would have to heat up from room temperature in order to heat the water back up in my tank.

You're just wrong about this. I know you keep saying it but it just doesn't....hold water.

SuperTech

ethicalpaul said:

Yeah, that's what would drive me batty, the boiler coming online after a shower or washing machine load.

Yeah, you are definitely wrong on this one. My boiler fires up a couple of times a day for domestic hot water with an indirect tank.  Lol, it doesn't affect the temperature of the basement and I don't use very much fuel oil from April to October.  It doesn't take much to keep a indirect tank 140 degrees.  Standby losses? Who cares if the boiler cools down after the tank is satisfied? Its going to be several hours until the boiler needs to fire again anyway.  I had an electric water heater,  I wouldn't go back to one. I know you like your heat pump water heater, but indirect tanks aren't as bad as you think. 

ethicalpaul

OK guys I respect your opinions but it sure seems like we get a lot of people having problems with them around here!

JohnNY

• The combustion efficiency of a gas-fired water heater is about 20 points less than a standard efficiency steam boiler.
  
• The standby losses of a water heater connected to a chimney compared with a boiler with an automatic vent damper (nearly every modern boiler) is vastly different.
  
• The standby losses of a sealed, insulated indirect water heater are next to nothing.
  
• Connecting an indirect DWH to a steam boiler is a reliable and sensible way to make hot water (granted, just not in a power outage).
  
• Let me not miss the low hanging fruit in this conversation. Heating an indirect water heater from a steam boiler in winter recovers valuable fuel that would very likely be lost without it.
  
• Pulling hot water from an oversized steam boiler is a great way of bringing the heating demand closer to the installed supply.
  

ChrisJ

JohnNY said:

• The combustion efficiency of a gas-fired water heater is about 20 points less than a standard efficiency steam boiler.
  
• The standby losses of a water heater connected to a chimney compared with a boiler with an automatic vent damper (nearly every modern boiler) is vastly different.
  
• The standby losses of a sealed, insulated indirect water heater are next to nothing.
  
• Connecting an indirect DWH to a steam boiler is a reliable and sensible way to make hot water (granted, just not in a power outage).
  
• Let me not miss the low hanging fruit in this conversation. Heating an indirect water heater from a steam boiler in winter recovers valuable fuel that would very likely be lost without it.
  
• Pulling hot water from an oversized steam boiler is a great way of bringing the heating demand closer to the installed supply.
  

Hi John,

How do atmospheric tank heaters with vent dampers and the thicker 2" insulation compare in the grand scheme of things?

ethicalpaul

That's a good list, and I thank you for it! I value so much your and others' patience when reading and responding to my posts which are no-doubt often lacking some insight and wisdom.

I will say this poster has oil so his list might also have these items on it:

• The efficiency of an electric water heater is 100% and a hybrid is 300-400%
  
• The standby losses of an electric or hybrid water heater are just as good as an indirect tank
  
• If the boiler goes down, at least there is still hot water, and vice-versa
  

Now that I have a couple radiant floors coming off of my steam boiler, I definitely feel the benefit of pulling heat out of my boiler in the shoulder seasons and putting it into my floors/living space instead of just letting it seep into the basement. This is similar to one of your points above.

JohnNY

ChrisJ said:

JohnNY said:

• The combustion efficiency of a gas-fired water heater is about 20 points less than a standard efficiency steam boiler.
  
• The standby losses of a water heater connected to a chimney compared with a boiler with an automatic vent damper (nearly every modern boiler) is vastly different.
  
• The standby losses of a sealed, insulated indirect water heater are next to nothing.
  
• Connecting an indirect DWH to a steam boiler is a reliable and sensible way to make hot water (granted, just not in a power outage).
  
• Let me not miss the low hanging fruit in this conversation. Heating an indirect water heater from a steam boiler in winter recovers valuable fuel that would very likely be lost without it.
  
• Pulling hot water from an oversized steam boiler is a great way of bringing the heating demand closer to the installed supply.
  

Hi John,

How do atmospheric tank heaters with vent dampers and the thicker 2" insulation compare in the grand scheme of things?

Hi Chris,
I hope you are well. As I understand it, vent dampers add 2% seasonal efficiency and only larger water heaters have them. Commercial gas-fired water heaters are not the kind of appliance or application where efficiency is of much consideration but it's an interesting question. I don't like to talk like this because there are too many opinions about mechanical things in a world where performance-over-time should take the place of preferences, biases, or speculation. But....it seems to me like sidewall jacket losses would always pale in comparison to a flue connected to a chimney. I've never touched a water heater in my life that was hot to the touch. Heat really doesn't pass through the jacket of a water heater. It rises out the flue at a much faster rate. A vent damper will always help but that advertised 2% isn't encouraging. I'm sorry I didn't answer your question.

Jamie Hall

To the comment that the electric resistance water heater is 100% efficient. Please, @ethicalpaul , you are a lot more intelligent than that. I know you are. Yes, in terms of the energy arriving at your house, but no, in terms of fuel efficiency. Assuming that at least a third of your electricity is coming from fossil fuels -- and it's likely to be a much higher fraction than that -- the total fuel efficiency is worse. Much worse. Since even the best high pressure steam plants are lucky to get 40% efficiency, and then you have transmission loss bringing it down to the 30% to 35% range.

Which, when you do the arithmetic for a hybrid, does bring you close to 100% fuel efficiency for the hybrid.

Now if you live in Quebec or Washington or Oregon, where the electric folks have dammed every river in sight (some ecologic problems there, but we'll not worry about that, eh?) the equation is different...

ethicalpaul

There are certainly inefficiencies in every delivery system. Natural gas is carried on boats sometimes! I find I am primarily concerned with the inefficiencies that occur after I have paid for the thing. The other inefficiencies will be worried about by the people who have to carry that cost.

The thing you should have hammered me on is that resistive electricity even at 100% efficiency is more expensive than natural gas at 80%