Questions of Recoverability

Wayco Wayne_2

is not yet occupied. We installed a Boiler and a radiant floor, suspened tube, and hooked up to an existing radiator loop with cast iron radiators. The heat loss calc was 61000 btu's and we installed a boiler with an 85000 output. The electrician working over the weekend turned the power off to the boiler and when we arrived on the job at 10 the house was cold. About 40 degrees inside. It was 33 degrees outside temp. I turned the boiler on and monitored it through the day. It did not turn off all day and was only at 170 at 4:00 when we left. That's 6 hours without reaching the aquastat setting of 180. I realise there is a lot of thermal mass to bring back up but I find it unsettling that it shoud take so long to recover. How much extra capacity should one put into a job to have good recoverabilty?

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Boilerpro_3

Not surprising

As you said,lots of mass. Also, what is the output capacity of all that old iron? I bet if you look at the radiation capacity, you'll find its way bigger than you need for the load, especially when assumning 170F water...150 btu/Sq ft EDR. I suspect you'll find that all you need for design day is about 140 to 150F water.

In my own home the heat load is now 52,000 btu/hr at design and the original radiation was about 900EDR. This works out to about 58 btu/ Sqft EDR. With this much excess radiation, we're talking about 100F water temps on design days, about the same as most concrete slab radiant systems. With that much radiation, you'l be hard pressed to see that water temp ever reach 180F with a boiler sized to the heat load, as it should.


Boilerpro

hr

How did it perform

before you added the radiant? How much radiant, load wise, did you add? Anything else changed?

hot rod

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Mike T., Swampeast MO

"...suspened tube, and hooked up to an existing radiator loop with cast iron radiators..."

If you mean that literally I think you're in for some big-time problems in the suspended radiant.

PJO_5

This happened to me...

Two years ago, we where stuck in the big snowstorm that hit the northeast...near some ski resorts! :-) Best skiing I ever had in the Poconos.

When we got home two days later, the boiler had tripped off because the vent was blocked by snow. The house was 42 degrees, and it was about 15 outside. The CO detector that was in the basement (about fifteen feet from the boiler) read an alarm of 137 ppm, so the boiler did the right thing...

I cleaned the vent out, and fired the boiler up. The upstairs baseboard zones got above 65 or so in about three hours, but the downstairs radiant combined with the garage slab didn't satisfy the t-stats until the next morning...about 12 hours later.

Take Care, PJO

Duncan_13

Boiler output might not be the problem

Could be the suspended. The subfloor (and floor coverings, eventually) is an insulator which slows heat transfer. If the boiler's got the output to do the job, it might be the blanket (subfloor) over the tubing that's slowing it down. Just needs time to catch up.

Wayco Wayne_2

No

not literally. They are separated by a primary secondary set up with VSI on the radiant floor. WW

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Wayco Wayne_2

Yes the radiators

are from an old gravity system so there's lots of cast iron to warm up before the temps can rise. I think it will be OK once warmed up but starting cold and being there to watch and wait was killer.

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Wayco Wayne_2

Thanks

That sounds similar. It would be nice to have a second boiler back up for such situations. WW

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Wayco Wayne_2

I never stood and

watched it. It never failed to warm up the house I just don't know how long it took. I replaced it with a new gas fired Buderus that is actually smaller than the original hence my angst. I did a heat load calculation and I'm always nervous until it proves to me it can work. Hard to start with an ice cold house though. WW

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Mike T., Swampeast MO

Thank heaven!

Was the space temperature at a reasonable level after that run time?

From a cold start like that (and with iron rads and radiant panel floors) you have an INCREDIBLE amount of mass to heat IN THE STRUCTURE before you even think of heating the air...

Weezbo

suspect the recovery is around 1 degree every 20 min or so.

time it. see how long it takes to come up...then give it a few hours and see if you can figure the exact time...thats what i did today was off by 7 min

Wayco Wayne_2

I hope that's the case

The radiant floor has a Tekmar 361 control that won't even turn on the variable speed circulator until the return boiler temperature has reached 130 degrees. It took forever to fill the cast iron radiators to that point. We're running out of cols weather to test this thing. I don't want to wait until next winter to find out if I have a capacity problem. WW

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Weezbo

WW

try this right quick....tape your sensor on the return on the primary just after the mixer for the radiant. that way soon as the boiler primary reaches temp its working then and there.i use a bucket of different tricks to get the system up and hey its like 35 below zero last week.i use fixed temp variable speed ,modulating temp ,fixed mix , fixed mix variable...i have a funny type by pass i invented:) i call it a speed heat by pass:) it works like a bypass around a fast fill or a by pass at a boiler or remote header except when i crank the speed heat open it zipps boiler temp around my :"mixer": sorta of a manual high temp modulator on a fixed mix system.