Full outdoor reset with Conventional Boiler control strategies


Old 12-20-13, 02:32 PM
Thread Starter
Join Date: Mar 2013
Posts: 14
Full outdoor reset with Conventional Boiler control strategies

I currently have a regular Dunkirk CI boiler with fin-tube baseboards. I installed Beckett AquaSmart last winter and it has been working well. I'm in the process of acquiring CI radiators to replace the baseboards and at the same time I would like to convert my system to a full outdoor reset with constant circulation(having grown up with one, I really miss it)
I'm not necessarily looking to get any payback on the system. I like working with hydronics and would really like the increased comfort.

I'm going by this diagram: http://www.taco-hvac.com/uploads/FileLibrary/TD04.pdf
I plan to use injection mixing(mostly because I got a taco 00-VR circulator really cheap) So, as I understand this, the injection pump will provide a range of water temperatures to the system bellow conventional boiler's temps. Independently, the boiler reset will control the boiler's HIGH limit within the non-condensing range. Since the full reset is achieved, the system could be in constant circulation(I could either use a switch or just keep the thermostat a little higher)

So, what I'm confused about is, since the heat call is always on, the boiler will pretty much never shut down and will keep bouncing off the high limit. I imagine that if it is 55 degrees outside and I'm only needing 80 degree water, then it would take a while for the boiler to use up the stored water, until it would need to fire again. But since the heat call is always on, it would keep refiring at 130 or so. I understand the condensate problem, but if the boiler was allowed to cool down, it wouldn't be any worse than regular cold-start operation.
Is this just a compromise I have to accept(since I'm not using a modcon)? or is this a shortcoming of this particular control scheme? Or am I missing something(maybe I am not taking the time into consideration?)

I would also like to hear how other people are doing full reset with CI boilers and maybe some on narrative on how their system operates.
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Old 12-20-13, 03:45 PM
Join Date: Oct 2011
Location: canada
Posts: 568
I have built several systems using out door reset control. One system used a four way mixing valve that kept the burner off until the mixing valve was full open and the boiler water was the same temperature as that required by the reset control as low as 90f . The burner was allowed to fire until the mixing valve was fully closed or the high limit was reached, or the heat demand was lost. This was an oil fired boiler using a 0.40 nozzle In another system I used a Tekmar 256 which modulated the boiler water temperature to what ever was needed and no I had no issues with condensation and it was an oil fire steel boiler using a 0.75 nozzle . This is the easiest way to convert a boiler to out door reset.I never had the circulator on full time but would shut down on loss of a heat demand.
Old 12-31-13, 09:29 PM
Join Date: Dec 2013
Location: USA
Posts: 228
Time for some reading:
Tekmar -- Essays

All of them.
Old 01-01-14, 05:19 PM
Join Date: Apr 2011
Location: nj
Posts: 576
Just don't circulate low water temps through a cast iron boiler. I keep my tekmar at a minimum of 140* and work my curve from there.
Old 01-01-14, 06:30 PM
Join Date: Dec 2013
Location: USA
Posts: 228
Old 01-04-14, 08:19 PM
Thread Starter
Join Date: Mar 2013
Posts: 14
Maybe I wasn't very clear in my explanation. The boiler will not get cold water since I'm planning to use injection mixing, so the system water is not going to circulate through the boiler. I will be using taco 00-VR injection pump with built in controller. It also has boiler protection so it will shut off if boiler loop gets too cold.

I understand, that certain controls(like tekmar 356) have a boiler terminal and will automatically control the boiler, but it doesn't seem like taco VR controller does. Maybe I should call taco and ask what they think.
Old 01-05-14, 09:46 PM
Join Date: Dec 2013
Location: USA
Posts: 228
No Boiler control on the Taco, check.

The Tekmar 356 is the minimum that does it all but after playing with my 256 for 3 years
I can say that The Optional indoor sensor for room air temperature control is a must.

You can add a 256 to the Taco and turn off the Heat Manager load-matching energy saving algorithm on the Beckett AquaSmart and have a 356.

Checkout the 361 for a clean setup.

Good design template to start with.

Last edited by HeatWorm; 01-05-14 at 09:50 PM. Reason: Taco 00-VR is standalone

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