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Honeywell T8400B Thermostat Display Intermittently Blank

Two of the 4 thermostats in our house (for water circulating radiant heat) have on occasion had blank screens for several hours, then spontaneously came back on. Turning them off and on or changing the temperature had no effect. Is this a known problem> Should they be replaced?

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I have the same problem. It's because your domestic hot water (dhw) is on the same system as your radiant heat. In these systems, dhw is given priority over floor heating. That is, when your dhw needs heat, power to your Tstats are shut off until your dhw is satisfied. (Makes sense: cold showers no fun) Your T8400B's have no battery, so they are dead unitl power to them is restored by your zone control. It's really stupid that they use these Tstats, because you can't even SEE your room temp or settings let alone change them. I will be replacing my Tstats with battery powered soon; hopefully that will allow me to see and adjust during dhw cycles.

Posted on Nov 07, 2009

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Most likely a bad tstat i would replace I recommend honeywell focus pro 6000

Posted on Aug 29, 2009

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Posted on Jan 02, 2017

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I have 2 wires coming from the wall, 1 white and 1 blue and want to wire a new honeywell 5-1-1 day programmable thermostat.


likely one wire to TH, the other to W if the thermostat operates on batteries. If the thermostat requires a 24 volt power source it is not compatible with your system.

Oct 09, 2017 | Honeywell Heating & Cooling

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Radiant Heating - Why do You Feel Warm? The Best Most Efficient Heating System


What makes you feel warm on a sunny day in the summer? Radiant heating or radiant energy is usually recognized as the oldest form of heating. It provides the best comfort and is the basis for all types of heating systems. Radiant heating is "radiant energy" being emitted from any heat source. Radiant heating is the effect you feel when you can feel the warmth of the sun or a hot stove top element from across the room.

Radiant heat is widely known as the most comfortable and efficient heating method you can find. Some of the advantages of radiant heating are: it is usually more efficient than baseboard heating and almost always more efficient than forced-air heating because no energy is lost through ducts and other transfers. This technology can be used for both heating indoor and outdoor areas. These heating systems work by supplying heat directly to the floor, to panels in the wall or to the ceiling of a house. Because the radiant heating will make you feel warmer when you are in a room, you can have the thermostat at a lower temperature and still feel warm.

The feeling of warmth is the absence of heat being pulled from your body. If the walls, floors, and ceilings around you are warm then they are not pulling heat from your body. Then you feel warm. Your body is not giving up heat to them.

Because of this radiant heating systems can provide massive energy savings of more than 10-30% in most residences, and sometimes up to 60% in shops, hangars and warehouses. Radiant heating provides even warmth with no cold spots within a room. That provides a comfortable environment that cannot be matched by any other type of heating system.

New radiant systems are currently designed to run cooler than they did in the past. The water supplied to the systems will generally range from 90 to 140 degrees. Remember radiant heat can travel in any direction and is unaffected by gravity. Radiant heating systems will allow you to be more comfortable at a lower thermostat setting. Radiant heat will warm the closest and coldest mass, which then radiate heat throughout the space evenly. Radiant heating will give you the most consistent room temperatures from floor to ceiling when you compare it to any other heating system.

on Dec 12, 2009 | Heating & Cooling

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Underfloor Radiant Heating for Your New Sunroom


You want to add a brand new sunroom onto your existing house. The problem is that you are not sure how to heat it. Electric heating with baseboard heaters will spoil the look that you want from your new room. Hot water baseboard is going to present all types of problems getting hooked in to the existing system and piping it. And, if you have hot air heating system then you will have the problem of getting ductwork to the room. There is another way to heat this room easily and efficiently.

Radiant heating has been one of the most comfortable and efficient ways for heating for many years. The problem has been in the past to find a good heating source for small radiant floors that will be affordable, effective and efficient.

This problem has been solved now, with the availability of small new electric combination heater- circulator units. These units use small electric heating elements together with a small circulator pump to give you a all in one heating source and circulator pump.

Now you just have to lay your tubing in the floor of the room you are adding on to your house. Then you run that tubing into your existing basement or other room in your house. The tubing and the electric are attached to the heating unit. Then, wire a thermostat to the unit, and you are now ready to make heat. This really makes heating a new addition room very inexpensive and easy to do.

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on Dec 05, 2009 | Heating & Cooling

1 Answer

My Honeywell programmable thermostat's display is blank and I do not have any heat?


The thermostat, depending on the model, normally does not get powered by batteries, they are for backup power. You might be losing 24 volt from the furnace, causing the thermostat to run on batteries.

Apr 09, 2014 | Honeywell Programmable Thermostat Heater

1 Answer

I need to talk with someone re using Radiant Pro software. Keeps coming up with wrong numbers


Can you name the company to which you'd like to speak? You posted your two questions under Watts Water Technologies. There are several software packages that include the words "Radiant" and "Pro" in the name.

Can you bring up the "About" box in the program?

Sep 18, 2013 | Watts Water Technologies Radiant Heating...

1 Answer

My boiler heating is working but hot water is not coming. i want to know how to set hot water 24 hours 7 days


As I understand your question, you have a boiler with a 'summer/winter' hookup for domestic hot water.

Your boiler should run all the time, i.e. not be shut off by you. Your boiler provides hot water for baseboard heat in your home ... possibly through several 'zones' each controlled by an individual thermostat and circulator. Yours may be a steam system. If this is the case, you have radiators not radiant baseboard heaters and no circulators.

Your summer/winter hookup provides a constant supply of domestic hot water. It does this by taking cold water from your water main and passing it through a copper coil which sits inside your boiler and then to your hot water main in your home. Since the coil sits in the hot water at the top of the boiler, it is constantly being heated. This coil may be in a deteriorated condition in your case or it may be too small for your needs.

Several years ago, I did a small upgrade to my mid 1950's era American Standard boiler. The summer winter hookup in my case was mounted on a 4 inch cast iron boiler plug. The coil was 12 feet long (folded up to a package about 1 foot long). I was very afraid when the plumber came in with what amounted to a 10 foot long pipe wrench. My fear was I would have a pile of broken cast iron at the end of the day. All is well that ends well. He got the old one out and replaced it with a coil that consisted of 20 feet of copper tubing 3/4 inch in size (the folded tubing was about 20 inches long and fit nicely into the boiler). We now have all the hot water a household consisting of one guy and three gals would need in all but the extremest of times..

I think you r answer is here ... replace your summer/winter coil with a new, bigger coil.

Something else I did. My kitchen is 60 feet (pipe wise) from the boiler. It takes a long time to get hot water there. I put in a small electric hot water heater just under the kitchen. I put a timer on it so it runs for a couple hours in the morning and a few hours in the afternoon. The hot water line from the boiler serves as the cold water input to the heater. I now enjoy the convenience of quick hot water in the kitchen with the relatively low cost of oil heated water from the boiler as a relatively small cost of electricity.

My winter settings are 160 - 200 and my summer settings are 120 - 150 which seems adequate for our needs.

Thanks for your question at FixYa.com. I hope I have been of assistance to you today.

Feb 05, 2011 | Heating & Cooling

2 Answers

I have a 1999 isuzu rodeo it recently started over heating and the heat is not working. I replaced the thermostat, backflushed the radiator and pressure tested it and it is fine. so i broke down today and...


Hi,
Is this petrol or diesel?
If it is petrol check the following: 1. Ignition timing.
Diesel: Leave radiator cap off, run the engine until it is hot and see if the water is circulating, if it is you will see it running along past the radiator cap neck, once the thermostat opens.
If this is not happening then it can only be the water pump or a blockage in the cooling system somewhere which you will need to get chemically cleaned.
Let me know how you get on.
Thanks
Jason

Jan 28, 2011 | Cooling System for Isuzu Rodeo

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