Phoenix Evolution space heating efficiency

Gordan

I get the benefits of the appliance, but here's one thing that baffles me: in the typical situation where space heating accounts for a much greater portion of the output than does domestic water heating, does it make sense to heat the water to a higher setpoint (domestic) only to then, basically, mix down to a lower setpoint for  radiant supply? It seems analogous to using a high-temp boiler as a heat source for radiant, albeit to a much smaller degree. It just doesn't seem like a recipe for ultimate efficiency, theoretically; a boiler would still seem like a better choice for low-temp circuits. How does it work out in real life? 96% for space heating during shoulder seasons just seems... hopeful.

heatboy

Isn't that what......

.... happens with a standard condensing boiler and indirect?  Not having seen one yet, I would think boiler temps will only ramp up on a call for DHW?

hot_rod

I've found

even at 130- 140 setpoint mine still condenses when it runs. The key to high efficient operation is allowing them to condense and squeeze additional energy from the condensate.



True, lower operating temperature allows more condensate, but I think you will find they are more fuel efficient than a typical cast or copper boiler that does not operate, successfully in condensing mode.e.



Running at outdoor reset and providing heat at the lowest possible temperature, then boosting for DHW seems ideal. As of yet I've not seen a tank style, like you mentioned, able to do this. I'll bet there is a way, however it might not be cost effective to produce.



The other huge advantage is the built in, insulated buffer "capitance" that a tank style heater provides the heat side. Multiple, small zones really benefit from this and cycling, especially with a modulating burner is greatly reduced.



hr

Jean-David Beyer

what happens with a standard condensing boiler and indirect?

I do not have any mixing valves in my system. The boiler and its associated controls produces water at the temperature needed at the time.



With my mod|con, there are three thermostat inputs. These inputs have three different priorities, and you can hook any thermostat to any input. However, normally the DHW is connected at Priority 1 (most likely to be serviced), and the other two may be used for other things (such as heating the house).



Each priority selects which circulators are to run, and what reset curve is to be applied. For DHW, there is no reset curve: it puts out whatever temperature you set it for. Default for mine is 190F, but the recovery rate is so fast I reduced it to 160F in hopes of possibly getting more condensing. (I am not sure it makes much difference.)



For my system, Priority 2 is to supply my radiant slab zone. Default is 80F to 120F, but I set it down to get more condensing and so the circulators run more of the time. It runs between 75F and 120F depending on how cold it is outside. I do not believe it has gotten over 110F because it never went below 14F outside this year.



For priority 3, which heats my upstairs with finned tube baseboard, the temperature range is from 110F to 135F. Here, too, I do not believe it actually went over 132F this year. I believe the default is 130F to 180F.

NRT_Rob

I agree with you

with very low temp design and appropriate cycling control/sizing you should be able to run a mod/con boiler in the high 90's fairly often. I think.



so I think of it like this. The lower your design temp, the better a mod/con can do in comparison. the lower the zone count or the more water/emitter mass you have, the less the water volume of the evolution will help.



I do have to say though for a lot of installs out there... 120 to 140 degree temps, no thought for cycling control.. the evolution is pretty cool.



Also, it's pretty hard to argue with the speed of install. that's got to be a good cost saver. good enough that it would impact any payback calculations...

HDE_2

HTP Phoenix

Condensing is a direct result of return water temps, not really setpoint temp. Something to consider on all similar applications.

Does the Evolution publish a CA-AFUE (combined appliance) rating?

If not it should that number tells the true combi use efficieincy.

Gordan

You have a point.

Since this thing modulates the flow through the source side of the FPHX using a VS injection pump, then you'd expect the return temps to the source to be not too far above the system return temp. Perhaps that difference is negligible to the system efficiency?



I guess it matters whether the combustion gases pass through the coldest part of the heat exchanger first (they do) or last (they do.)

CMadatMe

AFUE

Maybe this chart could help out. Even though it may be from Viessmann I'm sure it relates to all mod/cons. Its on page 2 of the document. I couldn't snaphot the graph I'm not that computer savey. The graph give boiler efficiency as it relates to load % in and water temps