tstat 'hydronic' setting appropriate for modern boilers?

R. Kalia

Our Honeywell thermostat has settings for hot water heat, forced air heat, electric heat, etc. Basically it is a question of limiting the maximum number of on/off cycles per hour to 3 for a boiler, vs. a much larger number (I think 12) for electric heat. I believe this is done by allowing the temperature to stray further from the setpoint for hot water than for electric heat.

I assume that the point of the hydronic setting (3 cycles/hour) is to reduce short-cycling in a traditional setup, where the thermostat turns the boiler on and off.

However, a modulating boiler with outdoor reset (say, a Munchkin or a WM Ultra) is basically running all the time, with water circulating through at least the primary piping, and the internal electronics responsible for reducing short-cycling. The thermostat's only job is to police the system by stopping the circulator if the temperature goes too high because of an imperfect reset curve or because of internal gains.

Under those circumstances, does it make sense to use the hydronic setting? Could one set the t-stat to the electric heat setting? The reason to do that would be to get a smaller temperature swing.

Weezbo

what model # t-stat are you refering to?

do you have air conditioning?

R. Kalia

why?

I am not sure why the model is relevant (it's a T8602, but other Honeywells have the same options).

Whether I have AC has no bearing on my question about hydronic heat (the AC cycle settings of the tstat are different from the heat settings).

Ted_9

R. Kalia, we are all used to asking these question. You may not thing a question is relevent, but it may be helpful to know those extra details.

You should also always provide model numbers when disscussing equipment.

R. Kalia

OK.

OK. I guess I figured Weezbo was making a joke of some esoteric kind. My question has absolutely nothing to do with air conditioning.

Weezbo

Ok .

I just read all that product lit and i can say that i have not used these thermostats so thank you. however ..I dont Know. #4 says 1 cph for radiant 3 for something else and 9cph for electric. so way i see it thats a good question. 7X9 =63 so about every 7 mins it makes an evaluation...perhaps as the responsive ness of radiant is say closer to 1 to4 degrees an hour it sees no benifit to regulate information that often. i honestly dont know however.theres a guy from honeywell that stops in his name is Bill maybe he can give you thier clearest understanding on the whys and where fores. convector heating ramps up and down more often than radiant,I have just installed some radiant that can out ramp my baseboard(convectors) although the goal is to keep the lowest temp and lowest temp margin supply and return so multiple readings per hour would seem the better answer to me also. Lets See what Bill@ Honeywell says next time he drops by. Thanks.

BillW@honeywell

If you have outdoor temp reset,

the benefits from a programmable stat won't be as pronounced as they are for a standard on/off system. In-floor radiant systems shouldn't be set back sharply at night, since they may take too long to recover, and neither should most steam systems. Outdoor temp reset allows you to use only the heating capacity that you need, based on the outside temp. In general, all the electronic thermostats from the basic T8400 thru the new VisionPro use cycles per hour instead of the old electromechanical heat anticipators. 1 cycle per hour is recommended for steam and many radiant applications. 3 cycles per hour is recommended for high-efficiency forced air, and most general hydronic heat. Cast iron hydronic baseboard and cast iron radiators may work better with 1cph, but that varies considerably. 6 cycles per hour is the factory setting, and is used for conventional forced air, and 9 or 12 cph is for electric heat. On the AC side, it's fixed at 3 cph, and can't be changed. Follow the directions in the installation manual for how to change the settings.

If you have outside air temp reset, use a non-programmable like the T8400 or the Round T8775, since it's only function is really as a temperature sensor. Don't forget that the house is a system, and how your heating components perform are directly related to how efficient your insulation, windows, ventilation, house orientation and landscaping are set up.

R. Kalia

but what about my question?

Thank you for your reply, it is nice to hear directly from Honeywell. I am aware of the problem with setback when using outdoor reset. My question, however, was about something else entirely. It had nothing at all to do with setback.

The thermostats you recommended, the 8400 and 8775, have the same cycles/hr choices as my thermostats. I don't have to buy new thermostats, I just have to set my current thermostats so that there is no setback. That is why I didn't post a model number originally.

MY ONLY QUESTION IS, how many cycles per hour should I use? Do I really need the hydronic setting (3 cyles/hr) given that the thermostat is not turning the boiler off and on as it might with a high-mass cast-iron boiler, but rather is only turning the circulator on and off? Can I use a higher number of cycles and thus get more even heat?

Unknown

Maybe I can help

The concept of reset competes with the concept of setback thermostats and the logic within them.

Pick one or the other.

There ISN'T a good number of cycles per hour on a system that uses a thermostat to monitor the system constantly and hopes to never turn the system off.

Said another way, reset tries to prevent cycling, and your thermostat wants to know how much cycling you want to build into the system.

Wrong control, as Bill said.

Noel

R. Kalia

did Bill say that?

> Wrong control, as Bill said.


Thanks for understanding my question! I agree that there's a clash of concepts, but Bill didn't say that. He recommended other Honeywell thermostats that work in exactly the same way! (They don't have setback, but this is irrelevant.)

I don't know how else to "police" a system that may overheat because the reset curve is not perfect or because of internal gains. TRVs for every radiator is WAY too expensive.

Setting a higher number of cycles causes the "policing" to kick in more often and reduces the allowed temperature swing. But it still won't be very often if there is outdoor reset.

Unknown

T 87

simple as that.

R. Kalia

OK, now same question different language

The T87 is certainly just as good as a digital thermostat in this application. But where do I set the anticipator so as to get the best temperature control? (This is the same question as "How many cycles?" for a digital tstat.)

Remember, the tstat is not connected to the boiler and so there is no current rating to read. It just serves as a relay to turn the circulator on and off based on the room temperature.

Unknown

set it for the relay current

You would set it for the current it senses, be it boiler, relay, or other control.

Set it as high as possible (1.2) if you want to take the anticipator out of the equation almost entirely.

Noel

BillW@honeywell

Which type reset control are you using?

Are you varying boiler run time, using a 3-way mixing valve or a 4 way rotary valve or injection pump? What kind of emitters do you have? Maybe I misunderstood, but you can use a T8602 as a conventional stat if that's what you already have, just set the stat to the temp you want, then hit "hold". The new electronic stats don't only cycle 1,3 or whatever times an hour, they will call for heat whenever the temperature drops below the setpoint, and shut off when satisfied.
Some hydronic systems need 1 cycle per hour, others will work with 3. If you want something that's manually adjustable, the old T87 has the mechanical heat anticipator thats set to the current draw of the gas valve or oil primary, and you can fine tune it by raising or lowering the setting until you get it to the level you want. The older stats, however, are less accurate than the new electronic models. You also can go to http://hbctechlit.honeywell.com, and type in the number of the control you want info on. You don't need the whole part number, just T8602 or T8775 or T87. This will get you the tech sheet on each product at no charge, and in a tech format, no marketing hype. I hope this answers your question.

R. Kalia

OK. I can do much the same thing with the digital tstat by setting it to the lowest possible cycles/hr (1 cycle/hr, essentially equivalent to no anticipation).

But I won't. I will set it to the highest possible cycles/hr setting (12) and see if I get more even temperature control. I won't be short-cycling the boiler since I'm not controlling the boiler, just the circulator. If I don't get any noticeable benefit, I'll lower the cycles/hr.

The digitals sense temperature somewhat more accurately than the mercury switches, so I won't use the T87.

Unknown

I see

You want the circulator to stop and start.

Won't that drive the control nuts, that is trying to find the water temperature that makes the place as comfortable and economical as possible while constantly circulating?

What do you gain, in comfort OR cost to run, by using the thermostat?

Noel

Unknown

Stop all the confusion

set it for three cycles per hour. This was determined a few years back as a good setting for the higher efficiency packages today.

If you are using a thermostat with an anticipator then determine the amperage draw of the primary control (zone valve, relay etc) multiply that number times 1.4 which is a factor the control companies (Honeywell, White Rodgers and Robertshaw) came up with, this will give you 3 cycles per hour at 50% of heating load.

Example:

Amp draw of primary control is .4 X 1.4 = .56 set anticpator to .6

Once the system is up and running you may have to fine tune it a little.

Unknown

Timmie

Would this be what you would do on a constant circulation system, with a reset control?

Noel

R. Kalia

You didn't read my question

> set it for three cycles per hour. This was

> determined a few years back as a good setting for

> the higher efficiency packages today.


You can't stop the confusion if you don't pay attention.

jackchips_2

A little testy

are we R?

I believe Tim was speaking to the whole thread and not to you personally but we'll see what he has to say.

Unknown

I was noticing the attitude, too.

Too bad, I thought old R was getting quite a bit of info, compared to how much R puts back into the Wall.

Noel

BillW@honeywell

Noel & Tim

Thanks,guys. Your input is truly appreciated.

Unknown

Noel, no I am talking about

a conventional system with direct control of burner and circulator from a call for heat to bring in zone valve or relay. This would also be a definite mid or high efficiency package. If it is an old vintage forced hot water system then set it to match the amperage of the primary control and then adjust for customer comfort later on.

I think this question being presented relative to a system using indoor outdoor or constant circulation needs a little more sophistication for control of temperatures both in the boiler and in the space. I think a pri/sec piping set up would solve the problem but do not have all the details of this gentlemans problem.

I also do not like his attitude but that is another discussion.

I am busy right now so I will not pursue this any further.

Thanks for asking Noel.

Unknown

R. Kalia, I was not directing

my comments directly to you but to the thread in general. It is also very important here on the Wall to be very polite and civil to one another. We are all here to help one another and sometimes if we do not have all the information it is hard to do that. Give us all the details of your system piping arrangement, controls involved, bolier types (makes a difference if it is a Munchkin or Ultra) etc and I am sure someone much more capable than I am will give you an excellent solution to your problem.

R. Kalia

How else?

How else do you prevent the house from overheating if the reset curve is not perfect or there are internal gains? The boiler controller doesn't care, it just tells the burners to fire if the water temperature appropriate for the weather has not been achieved. Th thermostat won't kick in all that (couple times a day?) if the reset curve is close.

In any case, how else but with a thermostat do you police overheating when there is outdoor reset? What other methods are there? I can't afford an all-TRV configuration.

PS Another, better method is to use a 3-way zone valve to "short circuit" the water circulation if the thermostat tops calling for heat. This way you get circulation absolutely all the time, but sometimes it is warm water from the boiler, other times the return water gets recirculated. That way you on't have start/stop noises. I got this from an article by Siegenthaler.

R. Kalia

Yes, sorry about that.