On Saturday, April 7, 2018 at 2:16:35 PM UTC-4, mike wrote:
On 4/7/2018 9:48 AM, trader_4 wrote:
On Saturday, April 7, 2018 at 12:57:07 AM UTC-4, mike wrote:
On 4/6/2018 6:10 PM, J.Albert wrote:
What I have:
Burnham gas furnace, hot water, model P205 WI (it's 30 years old but
still in good condition)
There's no stack vent (it had an old HyTemp which I've disconnected
electrically and locked open).
No fans of any kind.
Electric circulating pump.
House is old (but not large) and has big cast-iron radiators.

I was having some problems (furnace wouldn't start correctly), had my
plumber/heating guy come over. Checked the pilot/ignition, cleaned the
sensor.
He installed a new thermostat (old one must have been 40+ years old).
The new thermostat is a Honeywell "round" T87K1007.
There are only two wires from the furnace that connect to it.
No fans, no air conditioning.

The furnace still wasn't right afterwards, but I investigated on my own
and came to the conclusion that the intermittent ignition module (old
Honeywell S86H) had something inside that was failing. I replaced it
with a new Honeywell "universal" module (S8610U3009/U), and that seems
to have solved the problems with the furnace.

But I find that the new thermostat is quirky.
The "room temperature" indication (bottom pointer) seems to have no
relationship to the "desired temp" setting (top pointer that you set by
rotating the dial).

I checked the thermostat's DIP switches (it has these instead of the
"predictor" that the old one has), and found one that wasn't set
according to the manual, and reset it.

But... I've come to the conclusion that the thermostat doesn't do what I
want it to do.
Please keep reading.

I understand that a thermostat is intended to automatically maintain
room temperature within a slight variation from the desired target
setting. Perhaps within 1 degree +/-?

The result is that the furnace cycles on/off too much for what I want.

Emotion aside...
What is it that makes a stable temperature undesirable?
Can you put back the old thermostat?
The thermostat datasheet suggests that the jumpers might be setting
the number of cycles...but that's vague.

That is almost certainly what they are doing. I have a HW VisionPro
and that is exactly what it does, you set the number of cycles per
hour that you want the system to run. It will target that for when
it's maintaining the set temp. They also have guidelines like he
posted for different suggested cycles based on type of sytem, eg
gas, electric, etc.



You can emulate the predictor by putting a resistor in series with
the thermostat and placing the resistor proximate to the temperature
sensing element. You'd have to do a lot of experimenting to determine
the resistance and proximity.

I don't see how that solves anything. A resistor is like the heat
anticipator that old thermostats had. All it can do is cause the
thermostat to warm up quicker than the room and turn off the system
a bit early. The idea there is that there is residual heat that will
then bring the room up to the final set temp. He wants a 6 point
swing, which would mean you'd have to trick the furnace out in the
opposite direction, ie make it go way past where it would otherwise
turn off. He's need a cooler, not a resistor.

The intent was to emulate the anticipator.
I'd have to agree with your statements.
I was thinking that the thermal mass of the resistor would heat rapidly
when the furnace was on, but cool slowly by convection in air when the
furnace was off. You'd have to do a lot of experimenting...

I agree, you're right, there is potentially that effect, but the heat
would have to last long enough to keep the thermostat above room temp
so that it delays it coming on as the room drops. With thermostats
there isn't enough mass there for the effect to last more than what?
Ten, fifteen minutes? He wouldn't get his several hours, that's for sure.








Try bolting the thermostat to something with large thermal mass
and restricting the coupling between the air and the sensing element.

That would potentially create wider swings. But practically, IDK
how much thermal coupling he'd get as opposed to it reacting to the
air entering the thermostat.

I couldn't tell from the specs, but it sounds like the sense element
is a thermistor that could easily be coupled to thermal mass.

I would agree that it's a thermistor, but how you couple that to a mass
of any real consequuence
in the typical modern thermostat, IDK. And it would have to be a heck
of big mass to get hours of delay. I guess he could modify it, remove
the thermistor, put it on wires, put it between a large mass like two
bricks.....