antonyupward
New Member
"Overheating" The Heat Exchanger
That's a great point... but my understanding is that the boiler's controller is smart enough to stop firing if the boiler water temperature (i.e. the input to the DHW Heat exchanger" is likely to exceed the design maximum. Hence it seems the heat exchanger wouldn't overheat.
But perhaps I misunderstood your point?
FYI...One reason I'm not too concerned is that the newish recirc pump is not the only thing which creates a "low flow" through my tankless. It happens every time we take a shower, and since that situation has been there since the tankless unit was installed in 2003 and we've not had a failure of the DHW heat exchanger I'm not too worried.
Since there is an interesting story to this last point I'll explain in more detail...
-- I have a NYThermal Trinity T200 Combi boiler/tankless DHW.
-- All the DHW Recirc stuff described in the above post from Grundfos etc.
-- I also have a GFX Gray Water Heat Exchanger. This device recovers the heat from the waste water in the stack and pre-heats the water coming from the street. This is so effective that the pipe bringing "cold" water to the Trinity from the GFX can be warm to the touch (perhaps as high as 30C/86F) when the shower has been running for some minutes. In the climate here in the Toronto when the water arriving from the street in mid-winter can be as low as 4C / 40F this is an amazing temperature difference - i.e. of the energy in the 38C/100F water leaving the shower, around 70% of the energy in the waste water is being used to raise the temp of the incoming street water from 40F to 86F - without burning any gas in the boiler at all!
So... what's the connection to low flow during showers?
Step A: When someone starts taking a shower the flow switch in the Trinity sees the call for DHW since the flow is greater than 0.5GPM, and lights the boiler at the maximum firing rate 200,000 bth/hr.
Step B: Once the boiler reaches its maximum boiler water temperature (208F, i.e. the temp of the water entering the DHW heat exchanger), the boiler ramps down the firing rate to the minimuim required, varying the firing rate as needed to keep the boiler water temp between 170F and 208F. This can be as low as 45,000 bth/hr. Typically the DHW temp coming out of the DHW heat exchanger is around 160F-190F.
But, the temperature of the water coming into the boiler from the street via the GFX is also climbing as the waste water heat recovering unit starts to work - from 40F initially (i.e. normal temp straight from the street), to around 86F once the shower has been running for a while and all the waste pipes have warmed up.
As you know on the output of any tankess heater you need to have a tempering valve which mixes "cold" water from the "street" and the "hot" water from the tankless heater This is to control the maximum temp of the hot water being sent to the facets to avoid scalding. I have mine set around 120F - perhaps a little too high upon reflection.
So what happens is this...
Step C: as the temperature of the water coming into the tempering valve from the boiler reaches 160F-190F and the water coming into the tempering valve from the street via GFX climbs towards 96F the tempering value needs more and more of this pre-wamed street water to reduce the water coming from the boiler from 170-190F to be at the 120F I want. In other words less and less of the 160F-190F water from the boiler is required to raise the 86F water to 120F!
Step D: So now we have a situation where more than 0.5GPM is flowing, but the boiler's minimum firing rate, 45,000 bth/hr is higher than the level required to keep the boiler water temperature at 208F. So the boiler controller, sets the firing rate to 0 bth/hr - i.e. the boiler is no longer lit. This prevents any overheating of the DHW heat exchanger.
Step E: Now it gets really interesting. In reality, so little water is required at the tempering valve from the boiler to get the 98F "street water" to the required 120F, that the flow rate of the 86F water going into the boilter drops below 0.5GPM...so the boiler no longer sees a call for DHW at all (i.e. the flow switch opens)!
Step F: Some while later, usually 1-2 minutes, if the shower is still running, the temperature of the water coming out of the boiler has dropped to such an extent that the tempering value starts to call for more than 0.5GPM from the output of the boiler, and the boiler once again relights (back to the step A).
For the time between Step B and Step D the flow through the heat exchanger is gradually getting lower and lower - from ~3.5GPM initially to 0.5GPM. For a while betweeen during step C you can watch the display on the boiler controller changing the btu/hr rate down to 45,000, the boiler water temp reaches 208F, then the water temp starts to drop to 206F, 204F, 202F and the controller puts the btu/hr rate back up to 50,000 and the boiler water temp goes back up to 208F, etc. This process repeats until the conditions of step D become true and the boiler stops firing at all.
So...back to your legitimate concern. My impression of all this is that the Trinity T200 boiler's controller is smart enough to turn off the firing (Step D) if the boiler water temperature is likely to exceed the 208F. Hence it seems the heat exchanger can't overheat.
Did I misunderstand something?
That's a great point... but my understanding is that the boiler's controller is smart enough to stop firing if the boiler water temperature (i.e. the input to the DHW Heat exchanger" is likely to exceed the design maximum. Hence it seems the heat exchanger wouldn't overheat.
But perhaps I misunderstood your point?
FYI...One reason I'm not too concerned is that the newish recirc pump is not the only thing which creates a "low flow" through my tankless. It happens every time we take a shower, and since that situation has been there since the tankless unit was installed in 2003 and we've not had a failure of the DHW heat exchanger I'm not too worried.
Since there is an interesting story to this last point I'll explain in more detail...
-- I have a NYThermal Trinity T200 Combi boiler/tankless DHW.
-- All the DHW Recirc stuff described in the above post from Grundfos etc.
-- I also have a GFX Gray Water Heat Exchanger. This device recovers the heat from the waste water in the stack and pre-heats the water coming from the street. This is so effective that the pipe bringing "cold" water to the Trinity from the GFX can be warm to the touch (perhaps as high as 30C/86F) when the shower has been running for some minutes. In the climate here in the Toronto when the water arriving from the street in mid-winter can be as low as 4C / 40F this is an amazing temperature difference - i.e. of the energy in the 38C/100F water leaving the shower, around 70% of the energy in the waste water is being used to raise the temp of the incoming street water from 40F to 86F - without burning any gas in the boiler at all!
So... what's the connection to low flow during showers?
Step A: When someone starts taking a shower the flow switch in the Trinity sees the call for DHW since the flow is greater than 0.5GPM, and lights the boiler at the maximum firing rate 200,000 bth/hr.
Step B: Once the boiler reaches its maximum boiler water temperature (208F, i.e. the temp of the water entering the DHW heat exchanger), the boiler ramps down the firing rate to the minimuim required, varying the firing rate as needed to keep the boiler water temp between 170F and 208F. This can be as low as 45,000 bth/hr. Typically the DHW temp coming out of the DHW heat exchanger is around 160F-190F.
But, the temperature of the water coming into the boiler from the street via the GFX is also climbing as the waste water heat recovering unit starts to work - from 40F initially (i.e. normal temp straight from the street), to around 86F once the shower has been running for a while and all the waste pipes have warmed up.
As you know on the output of any tankess heater you need to have a tempering valve which mixes "cold" water from the "street" and the "hot" water from the tankless heater This is to control the maximum temp of the hot water being sent to the facets to avoid scalding. I have mine set around 120F - perhaps a little too high upon reflection.
So what happens is this...
Step C: as the temperature of the water coming into the tempering valve from the boiler reaches 160F-190F and the water coming into the tempering valve from the street via GFX climbs towards 96F the tempering value needs more and more of this pre-wamed street water to reduce the water coming from the boiler from 170-190F to be at the 120F I want. In other words less and less of the 160F-190F water from the boiler is required to raise the 86F water to 120F!
Step D: So now we have a situation where more than 0.5GPM is flowing, but the boiler's minimum firing rate, 45,000 bth/hr is higher than the level required to keep the boiler water temperature at 208F. So the boiler controller, sets the firing rate to 0 bth/hr - i.e. the boiler is no longer lit. This prevents any overheating of the DHW heat exchanger.
Step E: Now it gets really interesting. In reality, so little water is required at the tempering valve from the boiler to get the 98F "street water" to the required 120F, that the flow rate of the 86F water going into the boilter drops below 0.5GPM...so the boiler no longer sees a call for DHW at all (i.e. the flow switch opens)!
Step F: Some while later, usually 1-2 minutes, if the shower is still running, the temperature of the water coming out of the boiler has dropped to such an extent that the tempering value starts to call for more than 0.5GPM from the output of the boiler, and the boiler once again relights (back to the step A).
For the time between Step B and Step D the flow through the heat exchanger is gradually getting lower and lower - from ~3.5GPM initially to 0.5GPM. For a while betweeen during step C you can watch the display on the boiler controller changing the btu/hr rate down to 45,000, the boiler water temp reaches 208F, then the water temp starts to drop to 206F, 204F, 202F and the controller puts the btu/hr rate back up to 50,000 and the boiler water temp goes back up to 208F, etc. This process repeats until the conditions of step D become true and the boiler stops firing at all.
So...back to your legitimate concern. My impression of all this is that the Trinity T200 boiler's controller is smart enough to turn off the firing (Step D) if the boiler water temperature is likely to exceed the 208F. Hence it seems the heat exchanger can't overheat.
Did I misunderstand something?
