Outdoor reset Options

Christopher Haley

I am looking to use outdoor reset on my boiler at home. I have an 3-section boiler (~130,000 BTU) with some old cast iron radiators. A trick I learned for doing a cheap and mechanical means of outdoor reset is to take a Honeywell T991A1194 (a proportional temperature controller) and stick a bulb outside and one the main. It makes the contact from the control to the burner based on the outside air temperature. It actually works pretty well but I was wondering if anyone had any other means of acheiving an outdoor reset that might be better without having to get an electronic control involved. Thanks

Brad White_9

What you are describing

is for primary boiler reset control only. It has two inter-related drawbacks:

Firstly, it limits the boiler HWS output to the low-limit temperature of the boiler and/or

Secondly provides no protection for the boiler from having too low of a HWR temperature.

It only serves the production half of the equation; where the rubber meets the road, your radiators and rooms, is only along for the ride.

Basically you miss out on the deeper reset temperatures that a decoupled primary/secondary system will give you.

Say you are firing oil in a cast iron boiler. You will have a low limit temperature of about 130F.

For all you know, your boiler and radiation may be oversized relative to your actual heat loss; You may need only 130F water or less on the coldest day (maybe). Therefore where can you modulate to? You are stuck on minimum.

For cast iron boilers you have a few options but all have electronic features- swallow the notion and go with them:

4-Way rotary mixing valves or true P/S with injection would seem to be the best. Taco has a nice "i-series" valve and controllers. ESBE has a mixing valve with OD reset built-in. All have adherents and detractors on this site and there are other solutions also. Just giving you an idea of what is out there and of the limitations to "boiler-only" reset.

Brad

BillW@honeywell

We've come a long way since then...

The T991 trick has been used for years. Brad is right, and we have a new reset control that will operate the rotary valve or injection pump he's talking about, it's the AQ775. Go to www.customer.honeywell.com, scroll down to the lower right corner, then type in the AQ775 and you'll get the complete spec sheets. No password required, and no charge. These controls can be ordered from any Honeywell distributor.