outdoor reset w/ hyrdo air

Unknown

I am installing a 3 zone system in a home with
1) indirect water heater
2) air handlers (2) @ 75,000 btu's
is outdoor reset a consideration? and if so what would be the lowest boiler water temp?

steve_26

min temp

150 degrees min or the air will feel to drafty,To cool.

steve_26

try constant circulation, cycle the fan, it works great..
comercial and res...

Wayco Wayne

I agree,

Hydro air is not a good place for outdoor re-set. If you get too cool the air movement will turn into a low velocity wind chill. On the other hand the hotter the air the more bouyant it is. Then your'e fighting stratification problems. Change design to a radiant floor. WW

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BillW@honeywell

A home control that does just that...

The PC8900/W8900 Home Control system offers a programmable fan feature, that allows the fan to run independently from a call for heat or cooling. It also controls the humidifier, monitors outside temp, tells you when a filter change or cleaning is due. Available from any Honeywell distributor.

hydronicsmike

I have done...

...reset on Fan Coils in the past with a minimum of 140°F. The lower the water temperature, the longer the fan cycles are. I have never heard complaints about blowing cold air.

I am interested to hear from the pros though. I always thought that you wouldn't get the cold air blowing feeling until the water temp drops below 120°F. Am I wrong?

Thanks,

Mike

kf_2

Hydro-Air w/reset

In my experience most fan coils are rated at 140 degree entering water temp with a 20 degree delta T.

180 to 140 is a pretty good reset.


kf

Bill NTSG

I find......

the lower temperatures much more comfortable. When my boiler [was] up to high limit 185°F. the air from the registers was stifling hot and the temperature swings in the room were very noticeable. My new boiler has outdoor reset and the 135° and up water makes it much more livable and the swings are not as wild. I have aquastats that will not let my blower come on below 120°.

You have to experiment. 130° is maybe as low as you want to go. It depends on the house and the system. You would need a steeper curve than a hot water system. My boiler resets for panel radiation but works ok with hydro air because I only use it early fall and dead of winter when the out door temp falls below 20°.


Will it save money?? maybe yes, maybe no. But ......... it is about comfort. And comfort, until you have experienced it , it is hard to put a price tag on it. Beauty is in the eye of the beholder.

Boilerpro_3

Another perspective

While I do not claim to be a forced air expert, I do have some forced air systems that utilize full reset, right down to 70F supply water. Two of these systems were designed and installed by a very knowledgeable contractor/ engineer in the 1950's using either Barber Coleman or Honeywell proportional thermostats or outdoor reset controlling motorized mixing valves.
The one is in a very high end home, and the comfort is exceptional. When I first visited the home on a freezing day, the air leaving the heating coil was only slightly warmer than the return and the water feeding the greatly oversized coil was barely above room temp. The other is in portions of a church and there have never been any complaints of discomfort.
Both of these systems utilize commericial air diffusers placed high on the wall with very low air velocity. I suspect that this allows cool air to be introduced into the space, preventing drafts. This probably also allows superior coling performance over typical forced air ductwork systems with supplies placed low.
The use of full reset also tends to keep the blower on constantly. If you are providing ventilation through the same air handler, than you will be providing much better ventilation than with a unit that is cycling on and off. IMHO the need for ventilation is much higher in milder weather than severe in the winter because natural ventilation due to air leakage accelerates as temperatures drop. Also, if a room is filled with people ventilation requirements increase. However a room full of people will satisfy a thermostat due to internal gains and shut off the air handler...providing zero ventilation when it is needed most.
I understand that the industry recommends not resetting water to coils below 140F or so. However, in a ductwork system designed to handle low temp air, it appears fully reset water supply temps are an excellent idea.

Boilerpro

Joel_3

reset

We have reset hydro air well down under 140f for over 10 years. If your ductwork is well done it is not a problem and doesn't feel to drafty we have never had a complaint. People actually comment how well the systems work ithink the longer fan cycles increase comfort just like keeping the radiation on longer.

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Steve Ebels

BP You hit the nail on the head

" greatly oversized coil" is thw key line. That is exactly how we do all of our hydro-air installs. The minimum that I will use has to be rated 20% over on btu's at 140*. We then use the lowest fan speed and if it's not a retrofit, design the duct system for a low static pressure, low velocity. We get a ton of compliments on these systems and we reset 'em all the way down to the 80* range, Discharge air temp, as you noted is only about 10* above room temp. Be careful with your register location and air velocity and it'll work great.

Boilerpro_3

Steve,

> " greatly oversized coil" is thw key line. That

> is exactly how we do all of our hydro-air

> installs. The minimum that I will use has to be

> rated 20% over on btu's at 140*. We then use the

> lowest fan speed and if it's not a retrofit,

> design the duct system for a low static pressure,

> low velocity. We get a ton of compliments on

> these systems and we reset 'em all the way down

> to the 80* range, Discharge air temp, as you

> noted is only about 10* above room temp. Be

> careful with your register location and air

> velocity and it'll work great.

Boilerpro_3

Steve,

It's nice to recieve support for my ideas that often run contrary to industry opinion, especially in an area where I have little practical experience. As you have probably seen, I often tend to not run with the crowd....even the exceptional group of folks at this forum!

Boilerpro

tim smith1

minimum water temp

find out what your coil puts out at say 130 degrees at 68 entering air, then divide that by your cfm and that will give your appx temp rise across your coil at the lower temps. The 75,000 btu is probably at 180 degrees. The other part of the question is if this is reset with mix valve or injection or just straight boiler reset. If it is boiler reset then minimum return temps will dictate the lowest water temp also.