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Thermostat failure

Posted by marknmt (My Page) on
Sun, Jan 27, 08 at 16:40

Recently we spent a chilly night until the service man came and replaced a thermostat in our 14 month old Climatemaster Heat Pump system.

The thermostat was covered under Climatemaster's warranty, but the service call was not. So we're paying $105 (includes $25 travel charge) without complaining. (I did feel like the service call should have been covered, since this company installed the unit, but that's that.)

But we're a little concerned about a thermostat that fails that quickly. Maybe just a one-time thing, and the service tech did comment that he doesn't see this problem very often, but we're hoping we don't start running into nickel-dime problems.

I'm interested in any comments, and I appreciate the generosity of those who take their time to help. Thanks!

Best,

M


Follow-Up Postings:

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RE: Thermostat failure

I am a service guy with many years of experience. I saw the original digital honeywell thermostat and other brands. Digital thermostats can easily fail. I have replaced many of them. They can have some good brands but the more they do the more they fail. I am especially discouraged with the cheapo's sold at HD and Lowes. They seem to fail often.I have a 1 tear old Trane thermostat that wholesaled for $350 that has a blown heat circuit. I use it as a thermostat. Trane said it was not under warrantee without a receipt. This came out of a warranty forclosure. I have worked with computers for many years and still perfer mechanical thermostats.


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RE: Thermostat failure

Thanks, Bob.

Could a mechanical thermostat do this job? The heat pump needs to know when to supply heat and when to supply cold. I don't particularly care if my thermostat knows what time it is and so on- I just want to be able to set the parameters and forget about it.

I appreciate your response- and I will be grateful if you have time to answer this one too.

Thanks.

Mark


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RE: Thermostat failure

In 2002, I replaced my old mercury bulb t-stat with a Honeywell programmable made especially for heat pumps. It failed in September of this year (had no display), so I got about 5 years out of it. It had a 1 year warranty. Pro stats like the Honeywell VisionPro have a 5 year warranty and cost about $140.

I replaced the failed stat with a Honeywell 7500 7-day programmable from HD for $99. A few things I don't like about the new stat:

- No vacation hold function
- Does not track compressor run time (how can I yell at my wife if I don't know how long the compressor has run?).
- Have to hit a button (temp up/down) for the backlight to light up.

It was easy to replace.

FYI: I don't like mechanical stats for heat pumps. If the heat pump can't keep up on a bitter cold day, the house has to lose 2 degrees for the aux to kick on. With a digital stat, it will see that the heat pump is losing ground and will cycle the aux on and off.

Best to you.


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RE: Thermostat failure

Thanks Gargyl. I appreciate the information.

I should pointed out in my first post that it's a ground source heat pump and the auxilliary is turned off. So far we've not needed it, even in some days of sub-zero Montana winter. We do expect to enable it if we leave town during the winter- so if the pump failed or the water discharge froze we'd still have heat to prevent broken pipes and dead plants.

Thanks again,

Mark


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RE: Thermostat failure

I bought a Hunter 44360 'stat for three years ago for 50 bucks at Lowes. It has so far worked without trouble.

After two years, the low-battery indicator came on. not long afterward, the display would fade whenever the backlight was lit. Replacing the battery fixed both issues.

While the Hunter thermostat is digital, it is not entirely solid-state. A micro-relay is used to switch the thermostst wires. The contacts within "click" when they close or open. I don't think that relay is driven solely by the two AA batteries within the 'stat. Instead, the batteries initially close the relay when heat is called for. After the circuit is made, the relay coil "steals" some current from the 24-volt wiring to keep it closed. The batteries wouldn't last two years without this "help".

This brings up a point. If the furnace is shut off from the fire switch, the thermostat can no longer take power from the 24-Volt transformer. This is no problem unless you leave the 'stat switched to "heat" when it's cold. In this case, the micro-relay would be powered by batteries alone. If you leave it that way over a winter vacation, the relay will remain energized calling for heat that never comes. As a result, you will likely be greeted with a couple of dead batteries upon your return. Brrr.

Some will argue that a relay has a limited life because its moving mechanical contacts will wear out. This is true; all relays will eventually fail. However, this usually takes a long time to happen.

Solid-state switches - usually triacs - are used in more expensive thermostats. These devices are completely silent and need little current to switch. Because a triac has no moving parts, it "should" outlast a relay. In the real world, however, they sometimes do not.

Triacs are far more susceptable to voltage spikes or momentary miswires. A nearby thunderstorm could leave a triac french-fried long before mechanical wear does in a relay. A triac fails usually as a short - so the heat never shuts off.

Where am I going with this? Well...I don't believe all "cheapo" thermostats are unreliable. With some attention to design, there's no reason why my 50-dollar Hunter should fail any sooner than a 350-dollar Honeywell.

In addition, anyone whose had battery issues might want to check if they left the thermostat switch in the "heat" position while the furnace is off. If your thermostat is similar in design to mine, this will kill your batteries a lot sooner.

Even the "new" digital thermostats use technology that's been around for years. Features aside, you shouldn't have to pay big bucks for one that works.

Peace.

Marco


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RE: Thermostat failure

Thank you Marco- very informative; makes it clear how much there is to know about this. I found your post helpful. Thanks very much.

Best,

Mark


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RE: Thermostat failure

If you have a hot water system and use the older Honeywells with the heat anticipator (T87F), you can expect a lifetime out of them. If you change something on the hydronic system and don't check the amp draw or change the heat anticipator setting, you can expect immediate failure.


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