Relationship between setpoint and differential

Discussion in 'Boiler Forum' started by Montalvo, Dec 11, 2009.

  1. Montalvo

    Montalvo New Member

    Messages:
    77
    Location:
    California
    Two years ago, with much help from this forum, I replaced a 100 gallon commercial HWH with an 80 gallon Phoenix NG modcon water heater from Heat Transfer Products. This unit heats water for one side of the house and services the radiant floor heating for the whole house (7K sq ft) here in No. CA.

    During a recent cold snap (temps down to 26 degrees...REALLY cold for us), the radiant heating wasn't able to maintain the thermostat setpoints in rooms set at 69 degrees (dropped to 66 overnight), despite having half the house's thermostats set at 50 degrees (closed rooms we only use when we have house guests). The installer told me I'd just have to move the setpoint on the Phoenix higher during cold weather. It's already at 145 degrees and the differential is at the factory default of 7 degrees. Next Christmas, when we have a full house of visitors demanding warm rooms AND hot showers, I'm concerned that even with the setpoint at 180 degrees I couldn't keep the house warm and/or deliver DHW. There's a mixing value to prevent scalding so my biggest concern is whether this unit can simply meet the demand.

    When cold weather sets in, will it be better for me to a) raise the setpoint or b) lower the differential or c) some combination of the two? Thanks for any suggestions you can offer.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,139
    Location:
    New England
    Does the boiler support an outdoor reset control? If it does, it will adjust the setpoint to the outside temperature and the return water temp automatically - making the water hotter as it gets colder outside and finding the return is cooling off too much.

    What are the supply and return temperatures of the radiant loops? Depending on the type of floors and how the radiant pipes are installed, there is a limit on how hot you can make the water - you don't want to damage the flooring or have the beginning of the loop so hot you can't stand on it in bare feet! But, you may be able to increase it.
  3. Montalvo

    Montalvo New Member

    Messages:
    77
    Location:
    California
    Here's some additional info, Jim...

    I didn't find any indication in the Phoenix manual regarding installation of an outdoor reset control so I'll check with the manufacturer on that. Regarding the radiant system, the floors are limestone or w/w carpeting over gypcrete or concrete slab. And, as I understand it, the heat exchanger thermostatically limits the radiant output to 85 degrees, i.e, no danger of overheating. But I don't know if that limit can be adjusted or what the system implications of doing so would be.

    Can you help me understand the implications of raising the boiler's setpoint vs. lowering its differential?
  4. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    Radiant systems should not be using an outside temp reset as it's too difficult for the system to recover. I assume this is a staple up system? If not then someone installed way too little tubing because 145 is way way hot for in floor. anyhoo, I suspect that even raising the temp to 180 will not do the trick. You probably need some supplemental baseboard
  5. Montalvo

    Montalvo New Member

    Messages:
    77
    Location:
    California
    Need more info, Peter.

    First, the PEX tubing is in the gypcrete and slab, not stapled below the sub-floor (we have some slab, mostly raised foundation, but all the tubing is encased, not stapled). And as my post to Jim stated, the heat exchanger for the radiant has an output temperature of 85 degrees, even when the Phoenix water is at 145.

    Peter, what tells you the system can't deliver? When we had the 100 gallon water heater, we had no heating or DHW deficiencies. Are you saying that this new boiler simply doesn't have the horsepower to do the job?

    The last thing I want to do is add baseboard heating. One of the primary reasons we installed radiant is because we have high ceilings and radiant ensures that the heat stays at floor level. Adding baseboard heaters would create the air currents that permit all the hot air to end up at ceiling level.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,139
    Location:
    New England
    You can probably increase the water temp of the mixer. If the set point can't be maintained in the boiler, then you'd need to raise the supply temperature. Having a narrow differential is likely to cause the boiler to short cycle, increasing the wear, and decreasing the efficiency.

    Do you have thermometers on the supply and return to the radiant? What about the return to the boiler? Do you know what those are when things are okay, and when it can't maintain the temp?

    I don't know if all outside reset controls are very smart. As I read it, on mine, it uses that as just one input in deciding where to run the boiler water temp. It monitors the return water temp in addition to the outside to then determine if it should raise the supply. In your case, it won't work as what you really need is to adjust the mixer temp. Those are available, but may not be economically retrofitted.
  7. frenchie

    frenchie Jack of all trades

    Never mind.
    Last edited: Dec 11, 2009
  8. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    when it comes to heat output, what it is is what it is. If you have to run 180 or so to get the house temps where you want then that's what you need to do. Unfortunately, 180 degree floor temps are bad for the gypcrete and also pretty uncomfortable on the feet. As to the added hot water load, unless it has changed the output temp of the water to the radiant loops it should make no difference. /Even if the tank is set for priority, it should not be operating long enough to cause that problem. BTW, radiant or baseboard, the heat still rises. Did anyone do a heat loss calculation on the structure before the radiant was installed? Are your floor carpeted?
  9. Montalvo

    Montalvo New Member

    Messages:
    77
    Location:
    California
    Here's more info...

    Jim, there aren't thermometers on the heat exchanger pipes for the radiant heating. I just went down to look at the Phoenix and the temperature read 143 with a setpoint of 145. The radiant wasn't running so I went upstairs and turned up the thermostat in an unused bedroom that was 60 degrees. Within five minutes, the Phoenix's temp dropped to 113. I used my laser thermometer on the four pipes entering the heat exchanger on the radiant but they seemed to shift quite a bit. The two lines to and from the Phoenix were about 95 and 110 and to and from the floors were 105 and 99. What kind of readings should I be seeing and are my laser readings on the pipes an accurate way to measure the temperatures of the water?

    Peter, my floor coverings are about 60% limestone and the rest w/w carpeting. I did quite a bit of research about radiant before building the house. Contrary to what you said, radiant heat doesn't rise like baseboard because the entire floor is evenly heated and cold air can't get underneath that mass of heated air to allow it to rise. The 19' ceiling in my living room is cooler than the thermostat reading in the room. By contrast, baseboard heating generates air currents up the wall and cool air is able to then come down to replace the air being heated. The warm air, once raised, stays above the cooler air.

    Also, the radiant association says that the pile of your carpeting does little to interfere with radiant heat transfer. But the PAD is a MAJOR consideration. We installed a rubber pad, said to be among the best for heat transfer.
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,139
    Location:
    New England
    If the input to the mixer can't keep up, adjusting the mixed output higher probably won't help...sounds like the heat source may not be big enough. The larger the temperature difference between the room and the water in the pipes, the more heat you can transfer to the room. Keep in mind you have a large thermal mass, and it can take awhile to stabilize. Radiant isn't great if you turn it on and off (or set back)...it works better if you let it run. It could take a long time to stabilize if the temperatures are low and the amount you need to raise it is large. There's a limit on how hot you can make the water before it becomes uncomfortable or damages things like a wooden floor (or your feet!). Not sure what the highest you could make the output of the mixer. Does it have a separate control, or is it done with straight valves? On mine, I have a thermostatically controlled mixing valve. I run the water to my floors at a max of 130, but could probably go a bit higher if I needed more heat without damaging things (but not much higher). With the pipes embedded in the floor, you don't need (normally) higher temps. If you're using staple up, then you need considerably higher temps. the real thing is peak floor temp...What is the floor temperature?
  11. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    If you got that info from the radiant association then they are just plain wrong. Yes, pad makes a difference but carpet does also.

    I have done hundreds of radiant installations in the past 15 years. Believe me when I tell you that if you can't get the temp up in the room, you either don't have enough tube in the floor or you need to jack the outlet temps way up past where it's comfortable.
  12. Montalvo

    Montalvo New Member

    Messages:
    77
    Location:
    California
    Thanks, guys...

    Jim, I haven't been able to identify a mixing valve on the radiant system. As I recall, the Phoenix installer put a mixing valve on the DHW, with a bypass valve allowing me to disable it if needed. From what you're telling me, it looks like I'm going to have to live with what I've got, which may not be too hard to do. It dawned on me that, because the house has forced air heating as well as radiant, I can use both if the radiant system is overtaxed. And while I'd rather not use the two in tandem, we've got enough thermostats (14 radiant, 6 F/A) to use the radiant in some zones and the F/A in others. But despite that, I think I'll put a call into the radiant installer to see if he has some recommendations for getting the maximum heating output from the system.

    And Peter, the radiant association referenced a study that computed R-values for various carpet types/thicknesses and the same for pads. The study concluded that while the carpet does impact efficiency, its impact is a small fraction of the impact of the pad. As for radiant heating not rising as baseboard or F/A does, that's certainly been my experience and I found it cited in virtually every source that I consulted on the trade-offs in home heating alternatives. I have an unheated catwalk above our living room and it's positively chilly up there overlooking the living room, despite the living room being warm as toast. The cooler air creates a thermal inversion layer through which the heated air from the floor can't penetrate. By contrast, the air currents inherent in baseboard and F/A disrupt the potential for any inversion layer. In any case, we sure love the radiant heat for making those limestone floors warm to walk on barefoot and keeping our extremities warm, even when the thermostat's set at 69 degrees.
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