Vintage thermostat relays and temp controller scale
anyone here with knowledge of yesteryear , 1960 Honeywell temp sensor with scale and copper probe bulbs
I believe theses are tied into a radiant electric ceiling heat system , can someone explain how the wall thermostats communicate with the relays and how they both communicate with theses scale differential temp controllers ???? I really don’t understand there function they are Honeywell part number T415A
Other parts involved in this system
Wall thermostats
Relays
Comments
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The wall thermostat looks like a normal line voltage electric thermostat. My best guess is that the controllers with the sensor bulbs measure the floor temp and turn the radiant elements on and off to maintain the floor at the set temp and the wall thermostat turns the system off if the room overheats/turns it on and off on the room temp.
The only place I have seen the letter type of differential scale is on modulating controls where it sets the rate of change as the temp approaches the setpoint but it seems unlikely that those would be modulating controls unless they modulate by turning the elements on and off like an electromechanical infinite range surface element control but I doubt the contacts would have lasted 60 years if that were the case. Unless they use a mercury switch.
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the copper probes are actually in a attic mounted in a way it appears they are trying to sense the outside temperature from the soffit in the roof
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if you get the part number the manual is probably out there somewhere. sounds like some sort of outdoor reset but i'm not sure exactly how. the radiant temp or power being proportional to the outdoor temp makes sense but i don't know how you do it with 1960's controls. what do the labels inside the covers say?
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what purpose does it serve though , if every room has a thermostat sensing room temperate, which tells the heating element to supply heat or not supply heat . what is the point of the system sensing outside temperate ? Has is it relevant to controlling interior room temps
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Honeywell T415A is a 2 wire series 40 (line voltage) temperature control . The control makes contact on a temperature fall.
Not a modulating control which would be a series 90. Not an outdoor reset which would require two sensing bulbs.
You are going to have to pull the covers off the relays to figure it out.
The only thing the T415A can be is a low limit
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what do you mean by “ low limit “
Thanks for the responses everyone
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or it is warm weather shutdown. or shut down of part of the system.
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in this old white paper https://cdnsciencepub.com/doi/pdf/10.4141/cjps58-018
it says the T415A Honeywell was used to automatically sense freezing temperates to automatically warm farm crops in a experiments that’s all I can really find about this part on google
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Depending on the model they made the controls in different temp settings. Yours on the scale it goes from 20-90 degrees.
Low limit means the control contacts make contact on a temperature fall, same as a heating thermostat.
I was thinking it might be wired in parallel with the thermostat but doesn't look like you have enough wires at the relay for that.
If the bulbs are sensing attic temp and with a temp setting on the scale of 30 degrees, it has to be a low limit can't be a warm weather shut down which would be 55-60 degrees.
Your going to have to trace some wires and take the cover off the relays to get some help.
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That would be freeze protection. It runs the circulators if outdoor temperature drops below the setting.
All those old controls are really cool, but the whole thing might like a 21st century upgrade.
Can we see the boiler and manifolds? You say each room has its own thermostat?
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Electric radiant.
Maybe the thermostatic controls act as a 2 stage control, some of the panels/loops are connected to that switch so on a heat call from the t-stat if the outdoor temp is below a certain temp it energizes all of the elements, if it is above some of the elements are switched off.
The relay doesn't make sense because judging from the wire size, line voltage thermostats are available that can switch that load. Maybe the original t-stat was 240v but low current.
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I THINK YOUR ON TO SOMETHING WITH THIS, let me explain why
in the largest room of the house, i have tested the radiant ceiling heat with a thermal camera,
in this room if the temperate in the room is up to set temperature only the heat cables around the perimeter of the room are visibly glowing on and heating , IF I WAS JUST starting the system for the first time, and lets say the rooms physical temp. was 60 degree and i turned the thermostat on to 72 to call for heat, the whole ceiling will glow and all the heat cables will be activated in the grid until it reaches set temp, then afterward only the smaller section of heat cables on the perimeter of the room are glowing and producing heat… i assume to maintain since there is a lesser demand for heating
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I was thinking it might be wired in parallel with the thermostat but doesn't look like you have enough wires at the relay for that."
well the 415a scale temp. differential probs, do lead to a junction box of wires that appear to be tied into the 3 Honeywell relays on the 2x4 walls
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In the building I work in which was built in 1980 someone got the brilliant idea that all the heat loss is at the outers walls of the building so the only heat they installed was fin tube around the outer walls. The thermostats in the interior of the building at least in theory just turned the chilled air supply on and off. It was always cold in the offices in the winter. When the building was renovated it was retrofitted to a more conventional system.
Your electric meter will tell you pretty quickly if the perimeter heat is always on when it is cold out regardless of call from the room t-stat.
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Perimeter heat with a VAV air system is/was a very common system with office buildings. The perimeter heat around the outside walls offsets the heat loss. The interior heat that comes from electric lights, copy machines, people is usually enough to make all the interior zones a cooling load year-round.
We used to find that in the summer for cooling we ran the discharge temp off the ac at 55 degrees. If we ran 55 year round people would complain about cold air blowing on them in the winter (especially with overhead distribution and low ceilings) so in the winter we would raise the discharge temp to 60 degrees.
The 55 or 60 deg air is usually off roof top units supplied by refrigeration or and economizer in the winter.
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the biggest problem was that the VAV boxes and controls weren't maintained or even fixed properly when you'd complain so the were stuck on in most places. oh and the chillers were always broken such that they weren't working at full capacity most of most summers.
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thanks everyone for the reply’s
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