GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Polaris vs HTP Phoenix for hot water and radiant heat

mangler66 | Posted in Mechanicals on

Looking at using either of those products for both hot water needs and heating (with coil/air handler) in a 3000 sqft house (slab on grade, 2 story, in Southern Ontario).

It looks like 70000btu would be sufficient, but I am wondering about the pros and cons of each tank. already know most features of each product, so I am mostly asking about anecdotal reliability issues with each brand (e.g. igniter issues on the Polaris, which may have been fixed by now).



GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #1

    A primary difference between the Phoenix & Polaris burner is non-modulating, whereas the all versions of the Phoenix operate over a range of fire rates. The Polaris is only on & off, with a somewhat narrow temperature band between where it turns on & off, which makes it prone to short cycling if the radiation (or in your case, air handler) it's capable of delivering the heat at the full BTU rate of the burner.

    The smallest burner Polaris has a firing rate of 100,000 BTU/hr in, or ~95,000 BTU/hr out (if operating in the condensing range), which is probably more than 2x the heat load of a pretty tight reasonably insulated (not high performance) 3000' slab on grade house. The amount of cycling you'd get won't be terrible if your 99% heat load is say, 45,000 BTU/hr, and you sized the air handler to deliver (1.4 x 45,000 BTU/hr=) 63,000 BTU/hr with ~50-52C entering water temperatures in th air handler coil. But if the heat load is only 30,000 BTU/hr and you sized the air handler to deliver 40-45,000 BTU/hr it could be an issue. With the 130KBTU/hr and higher (they make 150K, 175K, and 199K versions too) the cycling would be more frequent, putting wear & tear on the ignition system, etc.

    The minimum modulated input of the Phoenix is either 35,000 BTU/hr or 40,000 BTU/hr (depending on model),and even the smallest burner has a range of 35,000-100,000 BTU/hr, which makes it pretty easy to set up with the appropriate output for an air handler right-sized for your heating load.

    The Phoenix Light Duty is preferable for houses with lower heat loads, since it can modulate down to as low a 25,000 BTU/hr.

    Key to designing a system that meets all your needs starts with an aggressively calculated heat load number, and up-sizing the air handler's capacity to cover that with some margin, but not more than 1.4x (ASHRAE's recommended ideal), and a water heater with a burner that has at least 25,000 BTU/hr of burner capacity to spare (for typical hot water users), and a tank sized for the biggest tub you need to fill. With the Polaris it's a trade-off- to avoid excessive cycling you have to up-size the air handler, yet you can't upsize it to the point that you won't be able to take a long shower with the air handler running, which means it always cycles when just the heating system is running, it's only a matter of how often. With the Phoenix it's possible to set it up so that it modulates the firing rate up when there is simultaneous space heating & showering draws, but it won't cycle the system on/off during space-heating-only conditions. The heat rate & temperature out of the air handler will drop a bit when there is a sustained hot water draw during a space heating cycle, but that's about it.

    Overall there would be far fewer ignition cycles with a Phoenix than with a Polaris, even with the designs of both systems optimized for the loads.

  2. mangler66 | | #2


    Thanks for the thorough response. I knew about the lower modulating features on the Polaris, but in real life it does not seem to be an issue. I read a Minnesota study where they ran all the common offenders in a water heater, heating coil combination for low cost home renos, and they said the gas consumption and short cycling were not an issue.

    Real life pros for the Polaris are much better insulation, ferritic stainless steel (a plus for the combustion stack, a minus for the tank) and simpler operation. Also easy to get 1000000 btus with a 1/2" gas line.

    Pros for the light duty Phoenix is better modulation for low loads, and austenitic stainless tank (may be a slight minus if they also used the same steel in the combustion stack).

    Is there a way to just cut the heating when someone is taking a shower? Seems logical and a good way to avoid oversizing the equipment.

    All in all they seem close, so i was hoping installers could chime in with warranty length and service call information to set them apart.

    I will likely have a 40000btu fireplace on each level so I'm ok with cutting it a little close with the air handler heat load.

  3. Expert Member
    Dana Dorsett | | #3

    The short cycling with air handlers & Polaris only becomes an issue if you have a low heat load and an air handler coil right-sized for that load.

    The title of this thread is "Polaris vs HTP Phoenix for hot water and radiant heat", which is deceptive, since there is no radiant heat described, only the air handler. In radiant heating situations the short cycling can be much more pronounced, particularly (but not exclusively) with low-mass radiant such as staple-ups, less so with radiant slabs. There are some standard (warranty violating) hacks that people have done to the control board for the Polaris, giving it a higher high/low differential to avoid the short cycling.

    One cheap & easy way to stop the heating system from running during shower is to use a dumb aquastat on the air handler plumbing to inhibit the air handler's blower from operating whenever the water temp drops below 110F-115F or so, guaranteeing that the shower gets the full burner output. I've done this at my house, with a heating system designed around a reverse-indirect water heater with a modulating tankless as the boiler. I also have a ~50% efficiency drainwater heat exchanger on the shower to allow the modulation level at min-mod to be within range of the design heat load yet still deliver continous showering performance when needed.

    A 40,000 BTU/hr fireplace can overheat a place pretty quickly if running full-bore. Some also represent rather large air & heat leaks in an otherwise tight house- select the models carefully.

  4. mangler66 | | #4

    Thanks Dana,

    I suspect it would help if I added the correct details :)

    I will be running an air handler coil as a "main" method of heating (and cooling). (I suspect hydronic heating with aircoil/handler would be more appropriate?) My climate requires a/c, and my wife does not want the look or potential temp delta of mini splits, so ducted it is.

    I will be putting in radiant pipes in the slabs of both the house and the garage, if only to have the option at a later date. I was toying with the idea of running a secondary loop after the air coil, to lower the temp of the water before I return it to the tank. It seems silly to own a condensing unit that may never condense with the return temps directly form the coil. I was even considering heated towel bars in the bathrooms as a secondary loop. I don't think I would mind the slightly higher temps in the bathroom during the heating months, and that would definitely bring the return water below condensing temps. Not that I have had much luck finding radiant towel bars to date. Other option is "overheating" a slab loop downstairs that have the guest bedrooms. Not ideal, but i would get indirect heat for the living quarters upstairs. I like your aquastat idea btw, a cheap way to get it done.

    Looking at the drainwater heat exchanger, at least for the main second story ensuite shower. They used to be terribly priced here (1000 dollars can), but they are coming down to more "reasonable" levels now.

    I was looking at the mantis high efficiency fireplaces, so I would not feel bad running them vs the main heating system. You are correct on installs, in my current house they figured they did need to insulate the cavity/bump out, because, you know, you can just run the fireplace. I basically run the "always on" pilot all winter, so I get a warm glow instead of a cold draft for the fireplace. I will do better on the new build...

  5. Dana1 | | #5

    The temperature of the return water from the hydro-air coil is controlled by design, and would require fairly hot storage temperatures to NOT be in the condensing range! The hydro-air zone at my house operates at an entering water temperature between 125-130F (depending on what other zones might be calling for heat when it's running), delivering 112-115F return water temperatures, WELL into the condensing zone.

    With a new design you start with the heating load calculations, and select an air handler that can deliver the specified amount of heat with water temps no greater than 135F or so. There is some wiggle room adjustment in the heat delivery based on the water pumping rate- slower pumping rates delivers cooler return water, but also a lesser amount of heat. At water temperatures much less than ~120F the exit air temps at the registers can be a bit tepid- in the 90s F (as I have measured when someone is taking a long shower, pushing the water temp in the buffer to 115F or lower), but that still gives a bit of range to play with in the design that both condenses and delivers comfortable "warm summer breeze" air rather than scorched air or tepid autumn draft air out of your ducts.

  6. mangler66 | | #6

    You seem to have figured out part of the equation, by using lower inlet temps for your coil. Again I should have been more clear: I want to go beyond dew point, I want max efficiency for my boiler. While it is true that the condensation point is around 130F, you need to go much lower to squeeze out the max efficiency from your boiler:

    If I am paying high dollar for a 96+ efficiency tank like a HTP or a Polaris, I don't want to run them at 87% efficiency if I can avoid it.The air handler I am looking at will likely need 160F inlet temp to provide 700000+ btu as reasonable air flow rates (quiet). Don't want to go below 140F tank temp anyway to avoid any risk of legionaires.

    So at 160F inlet (max heating days) my return temp by design will be around 120F, far from ideal for max efficiency.See EN7X model:

    Ideally I'd want to bring my return temp to ambient if possible, but I would settle for <90f to max out the 96% efficiency condensing water heaters.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |