Home>Discussions>INSULATION & HVAC>Programmable Thermostat for hot water radiator system
11 posts / 0 new
Last post
Programmable Thermostat for hot water radiator system


Another newb here, and here is my first question.

I just moved into a home built around 1930's. I don't know what the original heating system is, but it currently has a natural gas fired hot water boiler, that feeds cast iron radiators throughout the house.

It has a manaul thermostat and I want to replace it with a programmable one. When shopping for a programmable thermostat, what should I be aware of one choosing one, since I have a hot water boiler? Can I just go ahead and buy any cheap programmable thermostat?

Also, what would you recommend for the pressure of the boiler and the temperature for the boiler?


Re: Programmable Thermostat for hot water radiator system

Take a look at these and see what you think. Good Luck!!! :)


Re: Programmable Thermostat for hot water radiator system


That sounds like an excellent heating system---cast iron radiators, once heated, stay hot for a long time, increasing comfort and home enjoyment---most poeple can do without a programmable t-stat unless the house is going to be empty of people for a large part of the day, or a weekend---in such cases, a programmable t-stat is indicated.

Boiler pressure is usually set at 12 psi to 15 psi when the boiler is cold, & should rise to no more than 20 to 25 psi when the boiler is hot---there is a PRESSURE RELIEF VALVE in the piping that will open at 30 psi, spilling water onto the boiler room floor---this is a safety valve that prevents higher than normal pressures that could damage the system.

There are 2 little dials inside the aquastat that usually set the water temp at 160 degrees low limit and 180 degrees hi limit (sometimes set to 200 degrees).

The low limit of 160 degrees differs in some boilers if you get your hot water from a tankless coil inside the boiler---in such a case the low limit is set to 140 degrees to keep the domestic hot water hot enough for bathing, dishes, etc.

Re: Programmable Thermostat for hot water radiator system

Sorry for getting back so late. The last couple of weeks have been very busy and hecking, both at work and at the house. At the end of December, I will have been in my new home for 2 months, HOORAY!

I'm still learning about my house, from researching online and from trial and error. So far, nothing has blown up yet, which is good :D

Hi Sten,

Thank you for the link, but that item just seems overly expensive. I has hoping for something more reasonably priced. But I guess it does not matter what kind of programmable thermostat, that I get. I just want a simple one that turns on the boiler in the morning, turn off when I leave, turns on before I get back home and turn off overnight. I know radiators need 2 hours to crank up, but that will be compensated in the programming of the time. Natural gas prices are insane.

Well not really. I used about 106 ccf and it costed me $63, BUT another $63 for distribution, customer charge, balancing, taxes, etc. WTF?! And that was only for the month of November, I haven't cooked or use gas oven yet.

My boiler has a motor pump that pumps the hot water through the system. I guess any cheap programmable thermostat would not be an issue?

So far, I only get half the windows covered with plastic film and some caulking done.

Hi NashuaTech,

My boiler is at 18 psi and 112 degrees. I have my thermostat set to 55 degrees. I keep it at that temp 24 hours / 7 days. It just seems cheaper to maintain a constant temp at 55 degrees, until I get a programmable thermostat. However, what confuses me is that the wall thermometer, some cheapo from Walmart shows 68 degrees. Either the thermostat is wrong or the wall thermometer is wrong. Do I need to buy a new thermometer and figure out which one is wrong?

I have a separate hot water heater that is not part of the boiler.


Re: Programmable Thermostat for hot water radiator system


Please see the other post I addressed to you under the Navin entry.

I would recommend you hold off buying a programmable t-stat for the time being until you resolve some of the no heat issues with the rads---also as noted in the previous entry, your water temp is WAY TOO LOW for a radiator heating system---the water temp should be approx. 180 degrees to heat the rads---this will dramatically shorten the heat-up time for the rads to within 10 minutes, rather than the long time you have to wait for heat now---boiler water temps that you have now can cause condensation & corrosion in the boiler combustion chamber & forces the boiler to fire forever in order to satisfy the heat requirements of the rooms-that's why you're burning so much gas.

Thus, when you make these simple adjustments, you may not feel the need for a new t-stat---as noted, most people can get away with turning down the t-stat when they go to work or retire.

As noted in the previous entry, the previous owner was perhaps trying to turn the heating system into a radiant-type system---but I would recommend you first concentrate on getting the rads to heat up, add some insulation to the exterior walls (R19) and attic (R40), improve the tightness to the windows & perhaps decide to have an indirect HW heater installed before thinking about trying to make this a radiant system.

Getting a new thermometer is a good idea--but these cost little.

When you eventually buy a t-stat it should be designed for only heat (not cooling), have a manual heat anticipator that can be set between .10 amps to 1.20 amps to better control the heat temp overide & allow for zone valve installation, if indicated at a later date---zone valves allow a 2nd floor, or rarely used part of the house to be on a separate t-stat, which can be lowered to save a lot of fuel.

It's regrettable that the previous owner installed a gas water heater for the domestic water supply---this is totally unnecessay with a hot water boiler---the standard hookup is to install a companion to the boiler, which is an indirect 40 gallon hot water heater---these DO COST $700 for the indirect & $300 labor for approx. $1k, but they don't use any additional gas fuel because they get their heat from the circulatng boiler water & will save you lots of $$$ in annual fuel bills.

Please post back with your progress report on raising the aquastat water temp & getting the rads to heat up.

Re: Programmable Thermostat for hot water radiator system

I just moved into a 1903 built house and am loving it. Still trying to work out the kinks in the steam heat system. I have replaced all the air vents inthe radiators, but have a couple of busted radiator valves that are clanking. Working on getting them replaced. I am struggeling with my programmable thermostat right now. I called the boiler manufacturere to see if they coulfd recommend a programmable Tstat for their furnace. They were no help. I am currently using a Honeywell programmable tstat, but it has a smart technology that is driving me crazy right now. Instead of turning on at the time that it is programmed for, it turns on earlier, some high tech algorithim programming that anticipates how long it takes your heating system to reach the desired temperature at the programmed time. With Steam heat it takes several hours for my house to heat up, so my system is turning on at 4:30AM to reach the desired programmed temperature by 7:00. It seems that all of the Tstats that I have looked at are really meant for newer furnaces and not steam boilers. Has anyone come across a simple programmable Tstat for steam boilers that turns on and off when you want it to? The smart technology does not work well with steam heat.

Any input is much appreciated.

Re: Programmable Thermostat for hot water radiator system

The Honeywell you have has the ability to turn off the 'Intelligent Recovery' option. I have done the same. I do not know what model you have, so get a hold of the users and installers manual to find the key combination to enter 'Installers Setup' There you can adjust many aspects of the stat.


Re: Programmable Thermostat for hot water radiator system

Hi NashuaTech,

Please see my reply in the post from Navin, Bleeding did help the attic (waiting for the air to warm up), but the second floor bedroom and first floor summer kitchen. After bleeding, the system all by itself adjusted itself to 20 psi and 120 degrees with my adjusting any of the controls. I do not know how to do that on my system.

Sparch is having problems with his programmable tstat. That is why I asked about buying one first. Most programmable ones seem for forced air and I did not want anythign expensive and complicated. Just raise temp up and down and programmed times.

So far, I am keeping my tstat at 65 degress 24 hours, 7 days. The reason being when I come home and manually adjust it, it just takes so long and I am guessing that I am burning a lot of fuel. I figure, if I try to keep a constant temp, then I am not burning as much fuel going from 55 degrees to 70 degrees. Is this logic correct, or am I wrong?

So far, I have invested in the plastic sheets for the windows and caulks. I have lived in homes from 2 years old to 10 years old, and I am surprised how newer homes are just as drafty as old homes. As for my old home, wall insulation is out of the question due to cost and having to tear down the walls or drill through the brick exterior, I would hate to do that to my house. At some point, I will get the kitchen re-done, saving all the original windows, woodwork and trim. Tear down all the plaster to studs, then properly seal the windows, insulation for the walls and drywall it (cheaper than plaster).

I did not know that you can combine radiator heat with hot water. If that were done, then I could be saving a lot of money.

Re: Programmable Thermostat for hot water radiator system

Hi folks,

Lots of great info here--I have a similar cast Iron hot water radiator system and went through the same issue with thermostats not too long ago--my older thermostat came on one summer night and wouldn't shut off. I was able to buy a new thermostat and swap the wires over from old to new and was good to go. I wound up with a Honeywell that I got at Costco for ~$30. 7 day programmable.

I have a related question to this type of system that I am hoping a few of you may be able to shed some light on.

I also live in an historic home, circa 1860, that was converted to apartments, then condos in the 1980s. Based on the condo docs and a survey of the basement, the entire first floor of the home (which is now divided into 2 units) was once one heating zone for this hot water radiator system. When the units were divided (essentially front and back of house with a hallway between), a new heating circuit was run under the back unit, while mine was simply disconnected from the radiators in that unit, but still runs the entire periphery of the basement.

Essentially, my heating pipes run around the entire basement, though they feed only half of the first floor. I've asked two professionals that service our boiler if it would be worthwhile from an efficiency perspective to shorten the circuit (literally by 50%) but they both claim that there would be a negligible effect on efficiency.

I'm not an expert, but it seems to me that if there is 50% less water to heat in the lines, and the lines don't needlessly circumnambulate the basement, undoubtedly radiating heat and losing efficiency in the process (though I've insulated them with foam insulation), wouldn't the effort involved to make a few cuts and sweat a few pieces of pipe be worthwhile? Not to mention, there is likely over a hundred linear feet of copper pipe that I could bring to the scrapyard?

I appreciate any input/guidance.

Re: Programmable Thermostat for hot water radiator system

this old back:

There are several issues that crop up with your question:

Since you have a condo agreement, what does the docs say about you doing diy work on the heating system???---most agreements would prohibit you from doing such work.

From what you describe, there would be some, but very little fuel savings by shortening the copper tubing vis a vis the plumbing cost---especially from a pro (who would charge top dollar to do it---it would take you several years to recoup your investment)---however, if you know how to shut down the boiler, drain the system, & cut & solder copper tubing, the consensus is that you would save approx. 5 gallons of oil per heating season---in addition to the salvage value of the copper tubing.

There are numerous "heat loss" formulas to calculate the loss from copper tubing in a heating system, but since the water inside heating pipes travels at approx. 3-4 gallons per minute, it doesn't spend much time in the basement & the heat is dispersed thru the cast iron radiators.

This is even with UNINSULATED PIPES; since you have done the correct thing in INSULATING the pipes, the loss is GREATLY REDUCED in such a case---in fact, the remedy for heat loss in basement heating pipes is NOT to re-arrange the pipes, it's to INSULATE the pipes.

The Siegenthaler calculation of typical situations like running 75' of UNINSULATED 3/4" copper tubing thru a basement that has a temperature of 50 degrees will result in a water temperature drop of only 3 degrees by the time it hits the rads---this will convert to a loss of 5,600 btu/hr--remember, this is UNINSULATED PIPE, and 5,600 btu/hr is a considerable loss, thus the remedy is to insulate the pipes.

If the pipes are insulated, the heat loss drops to perhaps to 500 btu/hr., or approx. 600,000 btu per heating season, or 4.28 gallons of oil per heating season---approx $12/heating season---if you have gas instead of oil, the numbers should be similar.

As noted, the salvage value of the copper tubing & the fact that no additional materials need be purchased make it a worthwhile project if it is done as a diy job.

For further info on copper pipe heat loss & formulas to calculate such, Google "heat loss of uninsulated copper pipes" or "Estimating heat loss from copper tubing" or "Heat loss of uninsulated copper pipes"

Re: Programmable Thermostat for hot water radiator system


Thanks for the Tstat info. I went on to the Honeywell website and downloaded the setup manual and adjusted all of the settings. I was able to turn off the smart response. Thanks so much!


TV Listings

Find TV listings for This Old House and Ask This Old House in your area.