Reset Ratio/Parallel Shift Settings

Alan(CaliforniaRadiant)Forbes

New Viessmann Vitodens 11-44 boiler to replace the old cast iron boiler in a 4,000 [] house near San Francisco.

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Alan(CaliforniaRadiant)Forbes

High ceilings,

lots of glass (all single-glazed) and a central atrium that acts as a chimney to pull the heat out. Stand next to it and you feel cold.

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Alan(CaliforniaRadiant)Forbes

Framing

The house sits suspended over a ravine on a steel framework. No underfloor insulation.

Our design temperature here in the Bay Area is 36°.

The existing system is copper tube radiant in the slab floor. I have no idea what the spacing is.

Given this information, what would your reset ratio and parallel shift settings be?

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jerry scharf_2

Heating calcs

Alan,

I assume you did a heat loss calc on the house at design temp. Since you have a pretty good guess at the radiant surface, you should be able to figure out the water temp required for the design day. Do the same thing with a design temp of 55, and that should give you a first approximation of the reset curve.

jerry

hydronicsmike

IF it was a tekmar....

I could help you out, but.........

Just kidding. Give me a call and I'll help you anyways.

Mike

hydronicsmike

Formula to calculate

Heating Curve Or Reset Ratio = [Design Supply Temp - Room Temp (usually 70°F) ] Divided by [Room Temp (usually 70°F) - Outdoor Design Temp].

Example:

180°F Dsgn Sup - 70°F Divided by 70°F - 20°F (??) Out Dsgn = HC of 2.2


Hope this helps.

Mike

Alan(CaliforniaRadiant)Forbes

I wanted to

see what you guys thought the numbers would be based on the characteristics of the house. All of the rooms were between 35-40 BTU's/[] and the hardwood floors didn't seem as though they would hinder heat transmission into the rooms.

The original settings I used were a reset ratio of 1.0 with a parallel shift of 5° C. This wasn't able to keep up with the heat loss; I'm now up to 1.2 (reset ratio) and 15° C (parallel shift) and the boiler now seems to be able to get a grip on being able to heat the house.



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Mike T., Swampeast MO

IMHO ratio is too high and the system hasn't had time to stabilize. You're going to need quite a bit of parallel shift.

Go back to 1.0 ratio with shift at your current setting. Set ALL of the t-stats ABOVE the desired room temp and let the room temp setting of the boiler rule for at least two solar cycles. See what happens and I bet you can figure out the original layout/balance of the system...

If the cantilivered sections still prove too cold, I'd flatten the curve and add more shift. If you steepen the curve I have a sneaking suspicion that the system return temp will try to climb.