mod/con boiler indirect heating concept

[JHZR2]

Hello,

As many know, we got a modulating/condensing boiler with indirect water heater. It is all in and working great for the most part. Ive been trying to think through the logic of how best to operate the water heating though since that will be used year-round and so efficiency is important.

The indirect has an aquastat, like any old boiler. You turn the dial to the setpoint, it works a set of contacts to tell the boiler to fire and send water through that zone to heat incoming water.

The boiler logic has a boiler water setpoint of 180F. The water heater is currently set at 125, and has a blending valve ensuring the water stays around 110-115, I believe going to the house.

The water tank temperature is set to prevent bacteria growth. In that it may even be a bit too low. The blending valve setpoint is to prevent scalding, and the boiler water temperature to ensure water heating performance.

My issue is that 180F is getting warm when it comes to operating in high efficiency "condensing" mode. Operating with heater feed water of 130-140F would allow a lot more of a condensing mode operation and still provide some delta T for the water heating operation. I imagine some of this will be just plain trial and error, but what is the smart approach for water heating? 180F factory default? Try to push lower to a more condensing mode of the boiler for efficiency? Is there a water heating (boiler water) temperature that could be too low and help boost the chance for bacteria growth? For example, if the HX surface is a scalding 180F, there will be some localized areas in the domestic water that will be >160F, and therefore the chance of bacteria growth will be much lower?

Maybe Im overthinking this... Our issue is low water pressure and relatively cold water overall with the shower on max temp upstairs. Want to maximize comfort while minimizing gas use and optimizing operations. Any thoughts on this long-winded post?

 

[duaneb9729]

As some one that does this for a living, i believe your hot water heater temp is set to low, as i understand it, they dont want the water temp to be over 125 at the fixture, not the heater.

I personally run my hot water portion warmer than that near 140 degrees.

Although, I have no kids in my home. I would run my water temp lower if I had young kids.

Do you have cast iron radiators?

What brand boiler did you purchase?

 

[duaneb9729]

I would like an answer myself to see if the boiler is more energy efficient with the water temp turned down, I suspect not from an AFUE stand point. Just a shorter run time which would translate into a lower fuel bill.

 

[JHZR2]

Duane,

The boiler is a peerless purefire modulating/condensing boiler, with a second "zone" that is an indirect water heater.

I do have cast iron radiators. The OM and preset logic with the unit has a max temp when not in boost mode of 160F for cast iron.

Cast iron traditional boilers need some minimum temperature to preven condensation, rust and destruction. However, since mine is designed for condensing, I want to optimize it as much as I can. It is going to be different with a vented water tank, etc. Since I can condense and remove extra energy from the gas, I can run a bit differently.

The temperature mixing valve recommended 110F at the closest hot water outlet at a given flowrate. Apparently above 119F can cause scauding. Apparently 120-122F is minimum water temperature in a heater tank to prevent Legionnaire's bacteria from growing. Preferable is 140F. Not a big deal to me since I ave a blending valve, but the higher my tank temp, the higher my heating water loop has to be to heat the water, and the less condensing I can possibly run.

 

[duaneb9729]

In more conventional boilers its recommended to install a bypass valve to prevent the cast iron block, from sweating.

Over 180 degrees cast iron radiators can harm kids under two years old by burning them.

so as i understand it, if you raise your hot water temp, your boiler temp will also have to go up?

The nice thing about your boiler is the alum instead of cast iron block, which takes colder return water temps much better.

Is this a green boiler with a removable front and top, sort of a type of plastic?

 

[meangreen01]

I don't know your living situation, but unless you're a single guy running 140-150 degree water through the indirect is gonna kill your recovery time. Not an issue if you shower at 6 AM, do a load of laundry and fill the dishwasher at 7 PM. But, if your household includes having a kid taking a bath, followed by wife showering, while doing a load of laundry, plus running dishwasher, you'll soon forget the pocket changed saved by running the boiler at that low temp as your family will be [censored] about no hot water.

I don't have the tables in front of me, but you seem to be well versed in what's going on and the info is out there. You're indirect tank should have stats on recovery time relative to incoming boiler water. The most common ratings are based on 180 degrees coming in. So say the specs state a 50 gal tank at 180 recovers in 30 min, but at 140 it takes 55 min. What will be enough to satisfy your household demand?

I'd also guess your indirect tank is zone 1(priority over others) and you should verify if your current controls have the capability to provide that low of a temp to the priority zone. To my knowledge, most don't unless you were to set the boiler limit that low which would seriously impact your space heating on cold days.

If it were me I'd leave the indirect temp at 180. You're still condensing at that temp anyway. But, if it's just you, why not experiment? Me? My wife and kids would be [censored] in no time over no hot water, LOL!

FWIW, I've never taken into consideration bacteria growth in HW piping/tank. Maybe it's because we have city water that's treated/chlorinated or we're alway using it. IDK, maybe your situation is different.

 

[Shannow]

Remember always that the latent heat of evaporation is about equivalent to all of the sensible heat between freezing and boiling...so 10C might reduce the thermal efficiency on the gas side by a few percent (tops), while increasing your thermal storage by 10 percent or so (hot water temp minus ambient/inlet water temp).

Maybe add an ammonia bottoming cycle to remove heat from the flue gasses to the inlet air ?

 

[JHZR2]

Originally Posted By: duaneb9729

so as i understand it, if you raise your hot water temp, your boiler temp will also have to go up?

The nice thing about your boiler is the alum instead of cast iron block, which takes colder return water temps much better.

Is this a green boiler with a removable front and top, sort of a type of plastic?


No, the setpoint for the working fluid water temperature for producing domestic hot water can be and is different than the temperature when heating/maintaining house temperature. Ive seen situations where the setpoint for water temperature for heating the house is 110F based upon the outdoor reset, while when there is DHW demand, the water is always heated to 180F.

The HX is stainless in this newest purefire model, I believe. Yes green/grey, plastic front.

 

[JHZR2]

Originally Posted By: meangreen01

I'd also guess your indirect tank is zone 1(priority over others) and you should verify if your current controls have the capability to provide that low of a temp to the priority zone. To my knowledge, most don't unless you were to set the boiler limit that low which would seriously impact your space heating on cold days.

If it were me I'd leave the indirect temp at 180. You're still condensing at that temp anyway. But, if it's just you, why not experiment? Me? My wife and kids would be [censored] in no time over no hot water, LOL!

FWIW, I've never taken into consideration bacteria growth in HW piping/tank. Maybe it's because we have city water that's treated/chlorinated or we're alway using it. IDK, maybe your situation is different.


Ill look for the curves in the manual. It is just my wife and I so we can experiment... though most of our demand is at night, so your point is well taken.

Our controls allow us to set a separate DHW setpoint from the operation of the other zone which is heating. The controls work this way:

-If heating demand is called for, the boiler will run until this is complete. At this point it will run the DHW circuit for a minute or until the supply and return temperature is within 5F.

-If house heating is operating and hot water is called for, it will have priority and the boiler will heat to the hot water production setpoint (right now DHW will heat the boiler water to 180F).

-If hot water is operating and there is a call for heat, the hot water production will continue for 30 minutes, upon which the house heating will operate for some time.

The boiler design setpoint, etc for heating are all independent of the hot water production. The only way they would cross is if I set the boiler max temperature for, say, 175, and the hot water setpoint temperature for 180F. Then I guess the boiler would only go to 175.

But I experienced last night the operation for heating per the outdoor reset, the system going into boost mode becaue the house was very cold from setting it low while away, then a call for DHW, then more home heating, etc. Every time the setpoint for the boiler water changed. You can set the heating setpoints independent of thehot water setpoint.

 

[meangreen01]

What brand zone control panel and reset control? Sounds like that bad boy is pretty much set up for max efficiency. Bet it set you back a pretty penny!

 

[JHZR2]

Not sure, it all came together as part of the peerless purefire system. The thermal control on the WH is a honeywell. A lot of the other stuff it seems that peerless recommends is honeywell too.

Definitely max efficiency was the key. I believe we are at 95.3%. It was a good chunk of change, but we get $1500 on taxes and $400 check from NJ.