TT Smart 80 & Prestige Solo 110 Summer Gas Usage

Zman

Condensing

Attached is a presentation that should clear up the condensing question.



My point about the electric water heater is that if you multiply the wattage of the heater coil by 3.412 you will get the Btu output. Most heaters are around 5kw. I say this because people who install regular electric water heaters on there jobs love to make a big deal about oversizing  boilers for the DHW load when they do indirect jobs. It just does not make sense.



Back to your setup. You have a boiler with 110K/btu input. derated  for efficiency it will put out between 90 and 100 k/btu. At 18% firing, it will be putting out 18% of that number.



By the nature of  heat exchanges the return temp can never be lower that the tank temp. By turning down some settings, you should be able to keep the return boiler temps under 130.





Your indirect has a great deal of surface area relative to the boiler size. This means you do not need a 46 degree add. I would set the the add to about 20 degrees and the tank temp to 120. I would leave the on/off differentials alone. The anti-legenella should be turned on and the DHW timeout I usually set to 90 minutes. If you have a variable speed circulator on the DHW turn it to low.



Keep in mind all this is chasing approx 5% in efficiency.



By turning down these settings you may run the risk of the DHW cycle taking too long and short changing the heat load. Keep an eye on this.



Carl

JustinS

Thanks!

That presentation looks quite informative - I'm sure it'll help a lot!



Regarding setting the tank temperature to 120F, wouldn't this mean that I wouldn't have maximum hot water for a portion of the cycle (how ever long it takes to fall from 120 to 114 relative to the final temperature after heating is done)?



My dishwasher says that a minimum of 120F should be available to it...



As for the DHW Timeout - are you referring to the period of time that the DHW circulator pump continues to run to remove heat from the boiler? I think that's my DHW Post Pump Time... right now, it's only set to 1min...

Zman

Settings

If you need higher than 120 then so be it. I would play with the lower numbers and see how you like it. 120 is not a magic number for dishwasher sanitation.



The DHW timeout setting is not the same as post purge. It is designed to prevent the house from getting cold in the event of a very long DHW cycle. It will make the boiler switch back to heating mode after a set amount of time.

JustinS

Gotcha

I see that setting now - DHW Priority Timeout...



Would you increase the DHW Post Pump Time - seems like it would be a good idea as the boiler stays quite warm for several hours after a DHW call - might as well get that heat out of there as much as possible...

SWEI

Cause and effect

are inexorably intertwined here, and not really the point.



Let's assume for your particular location, weather, and fuel that condensation begins at one very specific temperature -- say 130ºF.  The efficiency difference between return temps of 129ºF (we'll call that condensing mode) and 131ºF (we'll call that non-condensing mode) would be almost immeasurable.  The curve is smooth and has a low slope.

JustinS

Right

I realize that the efficiency doesn't just jump to 95% or whatever at a sliver of a degree below the dew point... just clarifying that you can't condense at 150F or whatever above the dew point...

JustinS

Length of Piping

It's hard to say for sure since I don't know the path of the recirculating loop but it's probably about 70 ft of piping that's in the walls that I won't have access to in order to insulate...



There's maybe the same amount in the basement that I will be able to insulate...

JustinS

Recirc Pump Cycle Time

I re-enabled the pump this afternoon and was rather surprised at first to discover that it seemed to be running about every 5 minutes or so for about 90 seconds... over time, I noticed that the time between cycles was increasing from 5 min to 8 min to 9 min - the last time I measured it, it was 18 minutes between cycles...



What could cause it to vary so much? Not sure about the lower variations but the 18-min cycle was right after the boiler finished heating so I guess heat was diffusing up through the cold water inlet of the tank? Maybe?

Zman

Cycles

I have never messed with the post purge setting. I would concerned that it would create a delay between the DHW and heat cycles.The logic in the controller may overide.



You could observe how long it takes to equalize the DHW storage with the Boiler temp and set it for that amount of time.



I think your thinking about the recirc aquastat being effected  buy the tank is correct.



Carl

JustinS

Another Thing

Another thing that I noticed - I was fiddling with the temperature-sensing wire yesterday, I think I pushed it in order... later on, I noticed that the DHW cycle was occurring more frequently - maybe every 2 hrs or so? The historical curve was drastically different than before...



So I was wondering - what did I do?!?



I then remembered that I had touched the sensing wire and pulled it out some...



This morning, I noticed that the boiler only cycled once during the night and the curve looked much like before...



I tried looking in the SMART installation docs as to how far down the wire should be but didn't see anything obvious... putting it down until I can hear the sensor against the far end of the thermowell seems to shorten the cycle a lower - now, I know further down is cooler (especially with the recirc pump running) but really? That much of a difference?

SWEI

Sensor locations

are significant, as you have just witnessed firsthand.  Now that it's picking up an earlier signal, you might try increasing the differential a bit.

hot_rod

always use heat transfer paste

many times the sensors are not controlling accurately due to bad thermal conduction.

JustinS

Location

So should it making contact with the bottom of the thermal well?



Seems like would be the most accurate reading of the water temperature since it's in direct contact with it...



Where should the heat transfer paste be placed? Down into the thermowell?

JustinS

Recirc Piping Temperature

The other day I attached a digital thermometer lead to the copper piping around my recirculating pump... I noticed that the rate of change seems to vary a good deal but often the temperature falls by appreciable amounts (as much as 0.5F) over a 5 second interval.



It seems like the answer would be yes but I am wondering - is the water in that pipe really cooling that quickly? I still need to insulate that pipe but wow, that seems really fast...

JustinS

Lowered Storage Temp

OK, so I've lowered the storage tank setpoint to 126F with a differential of 6F...



While the return temp wasn't less than 130F once the boiler got going (more like 140F), its firing rate was rather low - it even spent a good part of the cycle at 0%...



What exactly does 0% firing rate mean? Does that mean it's truly not firing OR is at the bottom of its modulating range (i.e, 30K BTU/hr)?

Zman

0%

I believe that 0% is off. You could confirm by looking in the glass window on top of the exchanger.

What is the supply and return temp when it is at 0%. It sounds like the flow rate is not right

JustinS

Temps

Supply was right around set point of 146F and return was maybe 6-10F below?



What is the glass window? Look at what in it?



Why do you say flow rate is not right? What flow rate?

Zman

Window

Inside the cabinet, on top of the heat exchanger is a window where you can view the flame.

The boiler will sometimes get confused if the delta t is too tight between supply and return. Yours sounds pretty normal. Do you have a multi speed circ?

JustinS

Gotcha

I'll take a look at that next time it's in a cycle...



Yes, the DHW circulation pump controlled by the boiler is a 3-speed Taco pump - it's currently set to Medium - I remember you suggested setting it to Low, haven't had a chance to do that yet...

JustinS

No Window?

Tried looking for a window while the boiler was on but I didn't anything like that, nor did I see any flame...

SWEI

Window is on the top of the heat exchanger

You need to remove top jacket access panel (the rear half of the top cover) or find a moderately long dental mirror in order to see it.

JustinS

Turning Off Recirculating Pump => Significant Impact

So I turned the recirculating pump off about 4 weeks ago and the result is quite significant...



The boiler went from running every 4 hrs or so to only 2-3 times per day... There is generally 12-14 hrs between firings now...



Part of my problem, I think, is that the piping isn't insulated in the walls...but I am also wondering if perhaps the fact that the DHW return piping merges with the cold water inlet to the tank before the scalding valve...



Earlier people had said that the scalding valve should only let minimal amounts of hot water from the tank out, enough to bring the DHW return back up to 120F... Seems to me that this would be the case initially but once hot water leaves the tank, it needs to be replaced and this would be with a mixture of DHW recirc and cold water from the street...



What do you think?

Zman

Cold water

You are correct that the heat you are loosing is through the uninsulated pipes in the house.



As far as your concern about cold water entering the system. Water is essentially incompressible right? You can only fit a finite volume of water any piping system. Unless someone opens a water valve somewhere in the house, it would defy the laws of physics for the cold water to enter the system. It would have to displace the the water that is already there and it cannot.



There are formulas that will estimate the heat loss through insulated and uninsulated pipe. What you are witnessing is consistent with uninsulated pipe.



An difficult but important concept to get your head around.



Carl

JustinS

Yeah

That makes sense... Thanks for the explanation... I guess I was getting confused by the fact that the recirc water is going both through the scalding valve and into the tank...



Disappointing that the copper piping around the pump still loses so much heat even after I insulated it...



I guess that the recirc pump is putting a lot of cooled water back into the tank... Too. Bad

SWEI

That 5ºF differential

on the aquastat is a big part of your issue.  You might look at a demand-based trigger to start, then allowing the aquastat to turn the pump off once the water arrives.

JustinS

Demand?

What would the demand be based on?

SWEI

Demand start of pump

Can be triggered by a button press (on the way into the bathroom or kitchen) or using an occupancy sensor (easiest way is to power the pump from one of the light circuits and use the occ sensor to do both jobs at once, but there are other options as well.)  This way the pump only runs when you need it and shuts off once the aquastat trips.