hydro air

gary_28

i'm installing a new boiler that will provide heat by heating a coil in an air handler. this system will NOT providing a/c too, the system will be covering about 1100 square feet and my calculations call for about 35000 btu's. the installer say's the coil that he is using is approx 90,000 btu's because it has to be matched to the new air handler.

1) isn't that over kill?

2) does that mean i need to have a boiler that is atleast 90000 btu's dedicated to the coil and more for the baseboard that will also run off the boiler? he seems to feel i can provide less then 90000 btu's of boiler power and be ok.

3) he also wants to step up to a air handler with a variable speed fan. i was told that only really comes into play with a/c running thru an air handler. that you need a special thermostat and humidity sensor for the variable speed fan to work properly and that i'd be paying more money for something that doesnt come into play with heating.

finally...

4) i've read where some ppeople set the fan on a air handler (not a variable speed one) on low so that the hot air runs almost continuously at a low speed providing a slow constant heat for the house which is supposed to equal out areas better and provide more comfort with a hot air system. wouldn't that add to fuel costs running the boiler for longer periods. some say yes and other say no because the boiler doesnt cycle as much.

what do you think????

thks
gary

Dekstrous

I used to sell for an a/c company for a short time so take this for what it is worth. Perhaps others will chime in.
questions 1&2: Yes its overkill if your specs are right. all that means is you have the ability to provide 90k btu's if you only send it 35k with a modulating boiler you will only get 35k from the coil but have the option for more if needed. I believe your return temperatures will be lower also with the larger coil or you can increase your gpm.
Question 3&4: there are 3 ways to control your room temperature. Vary the water temperature and constant blower speed, vary blower speed and maintain constant water temperature, or vary both. Best and most complicated, vary both. Advantage and disadvantage to the other two methods will be cost to operate and comfort. Variable speed air handlers are very nice. Much more efficient, granted you wont need the variable speed to condense moisture on the coil for humidity removal but from an efficiency and noise standpoint it still may be worth the added expense. If you choose to go fan on all the time and modulate the coil temperature I would definatly go the variable speed fan due to the energy savings in the long run. Many variable speed units are rated in watts while many standard blowers are rated in amps. Ask your installer what the ratings of his equipment are may help with your decision.
As far as running a boiler for longer times costing more. A BTU is a BTU. The longer the system runs to provide enough heat to heat the space the more comfort you will perceive. You are providing the same amount of heat at the same amount of cost but over a longer time period at a lower temperature. Now that factor can change if you account for cycling of a high mass cast iron or such boiler oversized for the application. with such a low load you are going to want to have a storage tank for a buffer and or a modulating condensing boiler. It is very important that whoever is designing and installing this system know what they are doing to ensure you will be happy with the end result.

gary_28

ok, so lets say i need 35000 btu's for the hot air heat in parts of my house and 20000 btu's for the baseboard areas. if they use a 90000 btu coil do i need a boiler that can put out atleast 110000 btu's for both the baseboard and heating coil or can i in theory use a 60000 btu boiler that would cover the 20000 needed for baseborad and supply 40000 for the coil?

Robert O'Brien

Size

the boiler to the heat loss,the larger than necessary radiation will allow you to operate at lower temps,perfect for a condensing boiler.Or ODR if it's oil fired.

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gary_28

i wont be using a condencing boiler. just to get this straight. ur saying i should size the boiler more towards the actual 55000 btu's i need. not towards the actual coil size. also are you saying i can lower the temperature that the boiler is being heated too? am i understanding you right??

thks for the input

Robert O'Brien

Yes

there is a benefit to having more radiation than you need,there's no benefit to having more boiler than you need.If the boiler is not a mod/con,then at least use ODR

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Dekstrous

just a reminder and a formula

What are you going to be using as a heat source. This application sounds very appropriate for a Mod/Con boiler but due to the size may not be cost effective. Try and use a storage tank if you can to minimize cycling of the boiler. Fluid Power Energy makes a nice low temp valve for condensing boiler protection. It is also sold by lochinvar for their boilers. If you need to size a storage tank this will help you. Take your desired run time(Wet Stack time + a little, usually ten minutes is adequate) x Minimum boiler input minus minimum system load taking into account outdoor reset divided by Boiler Temperature Rise x 8.33 (pound of water) x 60 for minutes in an hour. your total will be the size of system volume you will need to prevent short cycling. Calculate your system total in piping, boiler storage capacity, and then you will know how much you need for additional storage in the form of a tank.
You can also reverse the formula and figure out how bad a boiler is cycling.
Often a tech is the first one to figure that equation out when he is there for a service call on a 5 year old boiler that looks like its 50.

gary_28

the budget is limited and my current setup is very wastefull. to make a long story short the house is primarily forced hot air from a 30 year old furnace but i also have a 50 year old boiler that is supply a small amount of baseboard and my DHW so i am running two complete systems and using alot of oil to do so. this was there when the house was purchased. we will be going with what keyspan is offering a discount on which is a Cast Iron Natural Gas Burnham Boiler. i don't have the model handy at the moment. we will be doing a seperate stand alone gas hot water heater. i know a indirect system with a storage tank is more efficent but once again the budget is limited and there is a significant added upfront cost for that tank but i am still looking into it. i'm trying to put together a working system within a budget. thanks agin for everyones help. this site is awesome.

Paul Fredricks_5

Maybe an easy way to think of the coil. It has the potential for 90,000 BTU's at design conditions. But if you give it less hot water of slow down the air air going across it. it will put out less heat. The coil can only use what you give it.

Kind of like a car on a drag strip. Maybe the car has the potential of doing the quarter mile in 12 seconds, but only if you give it full throttle. Give it less gas, it won't go as fast.

Yeah, size the boiler to the need of the house. Your contractor wants to use a bigger coil because one with the right size BTU rating would be to small for the ductwork.