??'s about replacing a self-heating Tstat (for steam)

Precaud

OK, I've about had it with this Moneywell RTH230B thermostat. The reason: self-heating causes short-cycling when recovering from setback.
Example: woke up this am to 68F, bumped the control up to 70F to take the morning chill off. Boiler fires. Within a minute or two the temp reading is 69F. In another 6-7 minutes, it reads 70F. No heat has been produced by the radiators, but the tstat says it is 2F warmer. In reality, the main vents are still open, it's still warming up the mains. After a few minutes at the target temp, the tstat considers the temp target to be satisfied, and shuts the boiler off. Again, no heat has been produced. With CPH set to 2, it will be another 1/2 hour before it fires again. During that time the temp reading will settle back down toward 68F, which is the real temperature anyway. And then it will start another short-cycle.

My priorities for a tstat are: accurate, steam-compatible, uses AA batteries (I hate AAA's), non-programmable.

First question. Is this behavior typical?

Second question. Some brands use a CPH setting to accomodate steam systems. Others use a "temperature differential". Which is better, which do you prefer, and why?

I'm looking at an ICM SC1600L.
https://www.supplyhouse.com/ICM-Controls-SC1600L-Non-Programmable-SimpleComfort-Heat-Only-Thermostat-w-o-Fan-Switch-Single-Stage

Jamie Hall

No, that behavior of a thermostat is neither typical -- nor correct. Some thermostats have what is called an anticipator in them, which is simply a very small heating element which warms the thermostat -- slightly -- so that it will turn off soon enough so that the temperature of the space won't overshoot.

It is possible that that element is damaged in your thermostat in such a way as to be overenthusiastic, although that degree of overenthusiasm is highly unlikely. It is also possible that your thermostat is wired in such a way as to be switching far too much current. A normal 24 volt thermostat should not be switching more than 1 amp -- or less (line voltage heating thermostats can cope with 15 amps, but that's a very different beast).

I would check the wiring and see what the thermostat is actually connected to. If I had the test equipment, even better would be to actually measure the amperage in the thermostat wiring when the thermostat is calling for heat -- and if it's over half an amp or so, find out why.

Before you go out and buy an electronic thermostat, which excess current will simply fry.

EdTheHeaterMan

On older mechanical thermostats there is a heat anticipator that was adjustable (on most thermostats). Your current digital thermostat heat anticipator feature is not compatible with your steam system’s long operation cycle.

It looks like your choice is a good one because it has an adjustable differential.

Jamie Hall

OK took a look at that specific model thermostat. That model does NOT have an anticipator, but rather a cycles per hour setting.

That said, if the temperature reading on the face of the thermostat is behaving as you say, there are only two possibilities: either the thermostat is seriously defective in a most unusual manner or, much more seriously, there is a high resistance short or other source of heating in the wiring at the thermostat base.

You may have a serious fire hazard there. Double and triple check all the wiring.

Precaud

@Jamie Hall : Interesting, so you suspect the relay is generating the heat, possibly due to it switching excessive 24V current. I hadn't considered that. I'll check it out. BTW, the RTH230B IS a digital programmable tstat...

@EdTheHeaterMan : So you think the heat generated is intentional, mimicking the old 'heat anticipator' feature? The RTH230B manual doesn't say anything about that.

It sounds like you prefer an adjustable differential over a CPH setting for steam. Is there a reason?

HVACNUT

Set CPH to 1.
But check the resistance through the wiring and relay if you can like @Jamie Hall mentioned.

Precaud

The minimum setting for CPH on the RTH230B is 2...

Precaud

OK, the wiring is fine. And the relay is only switching 205mA of 24VAC, well below its 1A per output rating. Hard to imagine the relay generating heat under those conditions.

So the problem is on the tstat element or internal circuitry. My initial thought was, perhaps some of the circuit sits dormant until a call for heat occurrs, and this heat reflects its activity. Just a WAG.

Methinks I'll just replace it, given the relatively low cost of doing so.

So I'm interested to hear opinions/pros/cons of tstats that use "temp differential" vs "CPH" settings for steam systems.

ChrisJ

Precaud said:

OK, the wiring is fine. And the relay is only switching 205mA of 24VAC, well below its 1A per output rating. Hard to imagine the relay generating heat under those conditions.

So the problem is on the tstat element or internal circuitry. My initial thought was, perhaps some of the circuit sits dormant until a call for heat occurrs, and this heat reflects its activity. Just a WAG.

Methinks I'll just replace it, given the relatively low cost of doing so.

So I'm interested to hear opinions/pros/cons of tstats that use "temp differential" vs "CPH" settings for steam systems.

I found the green face Visionpro 8000 series I have seems to heat it self when the backlight is on. I had endless issues with it and finally gave up. It was a top of the line one that can do 2 or 3 stages etc, not the standard 8110 whatever it is.


CPH vs differential...

CPH attempts to predict the behavior of the system and work around it. I.E. when it gets it right, it may be able to keep the temperature closer than 1/4 of a degree. Of course changes in conditions etc throw it off so it needs to "relearn" things.

Under most conditions a Honeywell CPH thermostat will be tighter and steadier than a differential thermostat.

The higher the CPH you set, the tighter it tries to keep it. 2 CPH doesn't literally mean 2 CPH.

Precaud

That's interesting, @ChrisJ . Makes sense the backlight would gen some heat. So, by "gave up", do you mean, you're still using it regardless?

Honeywell uses AAA batteries in the current models (like the Pro1000), so they're out of the running for me. All the other simpler non-programmables that use AA"s have differential, like the ICM above and this one from Braeburn:
https://www.supplyhouse.com/Braeburn-1025NC-Single-Stage-Heat-Only-Thermostat

ChrisJ

No, I bought a Prestige and put the other one on the shelf

mattmia2

I'd pull another wire and power it from the boiler if I could.

Maybe the relay contacts are burned and creating heat?

Is the voltage to the t-stat reasonable stable between when it is call vs not calling, either from the boiler or from the batteries, however it is powered?

My mom has an 80's electronic white rodgers t-stat that when running off a generator would shut off then whatever happened to the generator when the furnace shut off would cause the temp it was reading to go down and immediately start another cal for heat. I don't know if it was the frequency going up a little without load or the voltage going up, but one or the other was changing the calibration of it.

Precaud

mattmia2 said:

I'd pull another wire and power it from the boiler if I could.

Not easily done... I think the wire was laid in before the walls were lath and plastered.

Maybe the relay contacts are burned and creating heat?

That's what was inferred above. But there's only 5W passing through it.

Is the voltage to the t-stat reasonable stable between when it is call vs not calling, either from the boiler or from the batteries, however it is powered?

I'd have to check that. Battery life with the unit has not deteriorated. That wouldn't be the case if the battery was nbeing loaded down.

Precaud

Update: I ordered the ICM SC1600L tstat that I mentioned in the first post, and received it yesterday. This is the heat-only, non-programmable, no-frills version of this series, though it does have a backlit display which activates when a key is pressed. Cost is less than 23 quatloos from Supplyhouse.

Last night I set it next to several other digital thermostats I have here to compare its reading, and it's temp readout was no different. The manufacturers seem to have that aspect of production down pretty well at this point.

One difference from the Honeywell is that the ICM's thermistor is mounted standing off the PCB, right next to the side ventilation slots, whereas the Moneywell's is attached via short leads and glued into a slot on the plastic cover. I assume they did that to stabilize the readings from drafts or whatever. Given its temp rise when the relay is actuated, said stabilization didn't seem to have worked...

Mounting holes are standard 3.25" spacing, same as the Honeywell, so I installed it this morning.

Compared to the specs and data sheet on the Supplyhouse.com webpage, this appears to be an updated version. The configuration functions are now accessed by holding both buttons down with the heat switch in off position. This is much more user-friendly than the previous funky arrangement, which required that you remove and replace the batteries and then gave you 10 seconds to change the settings.

Speaking of configuration functions, besides offering five temperature differential settings from 1 to 5 degrees, it now also has a "Heat Time Delay" function. When set to On, a 4 minute time delay is inserted between a call for heat and turning on the relay. This could be useful to help deter short cycling.

The LCD display has nice large numbers with very good contrast, easy to read even when the backlight is off. The only thing I see that I don't like is the lack of a heat-on indicator in the display. (yeah, I know... double negative...)

So, initial impressions are good. With daytime highs in the mid-60's, my boiler has sat idle all week. An Arctic blast is due to arrive tomorrow, lasting through Tue-Wed, so I'll have a chance to live with this thing for a few days and will report back.

ChrisJ

Are you sure it's using a thermocouple?

All of the digital thermostats I've pulled apart used thermistors. The green screen Visionpros used surface mount thermistors soldered directly to the PCB. So they essentially use the entire PCB as a sensor. But thats bad if any components happen to warm it up.

Precaud

Ooops, I wrote one thing while thinking another. I'll correct the post, thanks.

Yeah, I agree, a surface mount thermistor sounds like a bad idea for ambient temperature sensing...

mattmia2

I think that 3.25" spacing is to fit a standard 1 gang nema electrical box.

Precaud

Now that makes sense...

Precaud

The artic blast arrrived on time last night, so I started using this thing, and I can tell already, I do not like it. It doesn't "self-heat" like the Moneywell, but it overshoots like crazy. Its minimum differential setting of 1º does not mean "within 1º", but is "+ and - 1º". Ex: set it for 70, it turns on at 69 and off at 71. That is way too wide a working window for a high-mass heating system like steam. A few minutes after turnoff the temp has already risen to 72º. Fifteen minutes later, it is 73.

Not sure whether to keep it as a backup, or just send it back.

ChrisJ

Precaud said:

The artic blast arrrived on time last night, so I started using this thing, and I can tell already, I do not like it. It doesn't "self-heat" like the Moneywell, but it overshoots like crazy. Its minimum differential setting of 1º does not mean "within 1º", but is "+ and - 1º". Ex: set it for 70, it turns on at 69 and off at 71. That is way too wide a working window for a high-mass heating system like steam. A few minutes after turnoff the temp has already risen to 72º. Fifteen minutes later, it is 73.

Not sure whether to keep it as a backup, or just send it back.

I mean, it's not like everyone on the forum told you it was a mistake or anything.

Precaud

Huh? Noone said anything of the sort.

Besides, the problem is that how the differential behaves is not accurately described in their promo material or manual. If it were actually 1 degree, the outcome would be entirely different.

PMJ

The fact is there is no way of really knowing exactly what the algorithm inside any commercial unit is and or what it is doing. You can count on that the instructions won't tell you either because the person writing them doesn't know.

I recommend a straight on/off cheap Tstat with the tightest differential you can get. Then simply put a delay off/delay on timer in series with the tstat so you can set your burn and your wait times to achieve whatever CPH you want during all calls for heat. As long as the net burn time per hour will heat on design day you are always covered. 1.8 CPH, 2.3CPH? It is all up to you. You will have fine CPH adjustment capability well beyond any commercial unit. And, you will never experience a short cycle (or pressure) again.

SteamingatMohawk

Where is the tstat located in the room? What is nearby, like windows, doors, kitchens, etc. that could affect how it operates.

You mention setback at the beginning. Everything I have read that talks about using setback in a building as old as yours seems to indicate it is incompatible with the original design, where boilers were running all the time, but if my memory is correct, that applied to coal fired systems, not something else. Is your system a conversion? I know this may seem irrelevant, but one never knows what one might discover by engaging in "irrelevance".

I have some of the old Honeywell round thermostats. If you don't want setback, don't like batteries I will mail you one or two just for the heck of it. Just be aware I think every one I have has a mercury switch in it.

On the lighter side, your system info says you are at 7000 ft, could the air be too thin up there to transfer the heat? Just kidding.

Precaud

PMJ said:

The fact is there is no way of really knowing exactly what the algorithm inside any commercial unit is and or what it is doing.

Generally true, but having watched this one cycle three times, I can say with confidence it uses integer temperatures above and below the set temp. So a diff setting of 1º is actually a 2º differential.

I recommend a straight on/off cheap Tstat with the tightest differential you can get. Then simply put a delay off/delay on timer in series with the tstat so you can set your burn and your wait times to achieve whatever CPH you want during all calls for heat.

Well that's exactly what I thought I was getting with this one, and why I got it; a 1º diff, with a delay that can be enabled or disabled, would have probably worked great.

Tstat makers really need to describe more explicitly exactly how their units behave relative to the settings they provide.

ChrisJ

I'm about to install two of these for my boss.

Imo it's the best money can buy.

Precaud

SteamingatMohawk said:

Where is the tstat located in the room?

Thanks, but location is fine, not an issue here.

I have some of the old Honeywell round thermostats. If you don't want setback, don't like batteries I will mail you one or two just for the heck of it. Just be aware I think every one I have has a mercury switch in it.

Ha. I think that's what was here before I got the Honeywell digital in the mid-2000's. I didn't pay much attention to how the system performed back then. It reminds me of the 2-speed automatics Chevy used to put behind their V8's. PowerGlide, I think they called them. Real dogs.

On the lighter side, your system info says you are at 7000 ft, could the air be too thin up there to transfer the heat? Just kidding.

LOL. No, but it does seem to impact my reasoning ability from time to time

mattmia2

The round honeywell will have a real anticipator with an adjustable resistor....

Jamie Hall

mattmia2 said:

The round honeywell will have a real anticipator with an adjustable resistor....

If it's one of the old ones, anyway. They work just fine, every time.

SteamingatMohawk

Be careful, my first car was a two door, turquoise and white 55 Chevy BelAir 265 V8 with PowerGlide. I wish I didn't junk it in 1968 for $25 when I got out of college. That was when you could climb into the engine compartment and adjust the points.

If you change your mind, my offer still stands, at least until I clean out that part of the basement where the spares are located.

I swapped it out for a Honeywell wireless thermostat with WiFi capability, so I can monitor the system from my phone and don't have to leave the basement if I want to change the thermostat setting. It's in a rental property and I live 8 miles away, so it's great. $300, but worth it already.

PMJ

Precaud said:

Tstat makers really need to describe more explicitly exactly how their units behave relative to the settings they provide.

Dream on. The only reason I would use one is for the scheduling ability. Maybe some will turn out to the the best way to do data logging/ and if you need remote access.

But for straight up control....I prefer to know actually what is happening. There is no way any of these will provide close to the ability to actually fine tune the cycle rate as what I described.

ChrisJ

> @SteamingatMohawk said:
> Be careful, my first car was a two door, turquoise and white 55 Chevy BelAir 265 V8 with PowerGlide. I wish I didn't junk it in 1968 for $25 when I got out of college. That was when you could climb into the engine compartment and adjust the points.
>
> If you change your mind, my offer still stands, at least until I clean out that part of the basement where the spares are located.
>
> I swapped it out for a Honeywell wireless thermostat with WiFi capability, so I can monitor the system from my phone and don't have to leave the basement if I want to change the thermostat setting. It's in a rental property and I live 8 miles away, so it's great. $300, but worth it already.

As good as a 55 Belair looks the 1950s were not a good time for Automatic transmissions. Terrible comes to mind.

Hap_Hazzard

As I recall, they continued to suck clear through the 60s and on into the early 70s.The PowerGlide was a 2-speed, am I right?

ChrisJ

> @Hap_Hazzard said:
> As I recall, they continued to suck clear through the 60s and on into the early 70s.The PowerGlide was a 2-speed, am I right?

I have respect for the TH 350 and TH 400 from GM. All others are pretty sad until later.

SteamingatMohawk

Automatic transmissions were in their relative infancy back then.

The first wheel was a square on an axle. Only by thumping it along did it become round.

SuperTech

> @Hap_Hazzard said:
> As I recall, they continued to suck clear through the 60s and on into the early 70s.The PowerGlide was a 2-speed, am I right?

My auto mechanic friend always said the powerglide transmission was almost indestructible.

ChrisJ

> @SuperTech said:
> > @Hap_Hazzard said:
> > As I recall, they continued to suck clear through the 60s and on into the early 70s.The PowerGlide was a 2-speed, am I right?
>
> My auto mechanic friend always said the powerglide transmission was almost indestructible.

They're a solid design as far as reliability.
But terrible for performance. They have only two gears.

Hap_Hazzard

I didn't work on much Detroit iron, but when I saw that they were putting in torque converters, ATF coolers, modulator valves and all that other crap just to change two gears, I thought that was the stupidest thing I ever saw, except for the Austin America.

Precaud

Hap_Hazzard said:

As I recall, they continued to suck clear through the 60s and on into the early 70s.The PowerGlide was a 2-speed, am I right?

Yes. I remember we cold take my 62 Impala up over 60 in first!

Precaud

This unit has the tightest differential control I've seen yet:
https://www.supplyhouse.com/Pro1-IAQ-T701-T701-Digital-Non-Programmable-Thermostat-1H-1C

from ±0.4°F to ±2°F in 0.1º steps.

No programmable delay, though.

Their description suggests that cycle rate, differential, and anticipation are interchangable terms.

ChrisJ

They don't have anything cheaper?

Precaud

Do you enjoy being a jerk?