How Low Does Your Boiler Temp Drop When a Zone Is Calling?

Posts: 81
Joined: Sun. Sep. 11, 2011 1:51 pm
Stoker Coal Boiler: AHS S260
Location: Oneonta NY

Post Mon. Feb. 13, 2012 8:54 pm

LsFarm wrote:I have my supply/return to my shop slab as a primary loop, I have a mixing valve that can be turned down to 85" in the secondary loop, which is the floor system.. there is a Taco 007 pump running 24/7 circulating mixed water to the slab, so it stays at roughly 55* depending on how cold it is and how much heat the shop is losing to outdoors..
the primary loop runs constantly, and is therefore also at a steady delta-T.. so there is no shock of 40 gallons of 80* water hitting the boiler, when the slab wants heat.. the slab is sipping heat all the time..

This constant small load on the system also acts like a timer on the boiler,, about every 30 minutes the boiler runs and brings the water up to 160*..

Yes I do have a few pumps running 24/7, but they are .78amp, so about a 100 watt expense..
Probably better than your 'tankless' DHW heater trying to help out the AHS260,,

And YES that tankless is NOT plumbed right,, it should never be in series or in any way be able to 'help' bring up the boiler water temp, only bring up the water temp going to the taps in the sinks and showers/tubs/laundry.

Greg L
Hey Greg, could you explain the delta t thing to me? Ive thought about running the pump continuously but in my mind the boiler would be running all the time. The highest return temp I've seen is 95* from the slab and that mixing with 180* water is shocking the system and dropping tithe system down as low as 145*. Thanks for the help.

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Post Mon. Feb. 13, 2012 9:45 pm

You really need to post a diagram of your system for recommendations. Picture worth a thousand words type of thing.

I am not sure what you have based on your current description, but attached is one diagram of a radiant system. In this diagram the pump is cycled by the thermostat and the mixing valve protects the boiler from a low delta t. Very simple arrangement, but has it's limitations.

Posts: 81
Joined: Sun. Sep. 11, 2011 1:51 pm
Stoker Coal Boiler: AHS S260
Location: Oneonta NY

Post Tue. Feb. 14, 2012 8:50 pm

heres a pic of my plumbing for the garage zone. The circ pump pushes out the 1" pipe and comes back on the 4-1/2" pipes. The pic doesn't show it well but behind the upper right side thermometer there is a shut off ball valve that im guessing bypasses the mixing valve or atleast lets the majority of the water to run through the baxi and then back to the boiler. What would happen if I closed this valve 3/4 of the way? Wouldn't this limit the amount of cold water dumping back into the system? The way im looking at it this would only allow small amounts of hot water in and small amount of cold water to get back to the boiler. Is this the correct line of thinking or am I backasswards? Thanks to everyone for all the help. Every problem is a huge learning expierence and I couldn't do it without all the help.

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Post Tue. Feb. 14, 2012 9:56 pm

I had those temps on return when I had a system that tried to put out the heat when the thermostat called for heat. Now I have approximately 10* diff between the slab temp and the air temp. The return fluxuates a little as the warm water pulses through. I have saved ungodly money by putting in this injection system. And I do not get the big swings in temp. When your slab is coming back at 95*, and the outside weather warms up, it is to hot inside! I checked mine today and it is an older 350 series controller, it is now discontinued but I think they still make one that will do the same thing. The only drawback is running a circ 24/7, but this evens out my temps as I have inadequate piping in my slab. My runs are to long so it transfers heat evenly when it is circulating without fresh hot water being injected. I don't think I have seen temps above the mid 80's with it brutally cold outside since I changed over. Off course this depends on your heat loss. Read up about better controls and consider constant circulation.


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