Pulse Width Modulation and low mass radiant flooring

moral212

It seems there are no modern thermostats that have a radiant floor function (floor sensor input and floor temp controls) that are not programmed with PWM settings, meaning you cannot use any of the floor settings without using PWM.

What are your thoughts on using PWM with low mass radiant flooring, say staple up transfer plates on subfloor with wood floors?

Are there downsides to this like boiler short cycling or loss of efficiency?

Thank you for your insight in advance.

Jamie Hall

The problem is a little more complex than that. Whether the system is low mass or high mass (or super low mass -- like forced air) the ideal approach is continuous analogue modulation which matches the heat input to the system to the heat required.

This is a good deal more easily said than done. However, many modern systems -- at the cost of some complexity -- manage a very reasonable compromise. Recognizing that the primary controlling parameter is the difference between the outside air temperature and that target indoor air temperature, and that is possible to modulate (and maintain reasonable efficiency over a remarkably wide range) they do measure the outside air temperature -- on an analogue scale -- and, using a transfer function suited to the application (the "outdoor reset curve") they modulate the burner output -- again, an analogue modulation -- to produce an output temperature from the boiler (or furnace) to maintain the target indoor temperature.

Now clearly this modulation isn't going to be perfect, since there are a number of parameters which it does not take into account, but by taking the one with the largest effect into account and aiming to slightly overshoot, they do very well -- using an indoor sensor (floor or air or both) only to occasionally reduce the output to avoid overshooting.

In principle, one could also use only the indoor temperature -- floor or air -- as the controlling parameter, except that there is a problem: neither will take into account the heat required. They only sense the result of the heat input. In a very low heat capacity system, such as forced air, this may not be a problem, as the lag between changes in heat input and sensed parameter is small, and analogue (or narrow pulse width) modulation could be used without too much overshoot and oscillation. However, in higher mass systems the problem of oscillation may be severe, unless there is a rate of change sensing element involved as well as a simple point in time element (which, oddly, is incorporated in "old fashioned" air temperature sensing thermostats with an anticipator).

Combining an air analogue air temperature sensing device with a modulating boiler would work -- but the sensed range would have to be very much smaller than that used with outdoor reset. The target floor temperature, however, is not the actual desired output parameter: the space temperature is. One really needs to have both to attain satisfactory control. Could this be done? Certainly. Would it be beneficial in terms of overall efficiency? Possibly.

Ideally, one would have the correct sensor suite to measure the input parameters and the deviation from the desired target parameter and control a fully modulating boiler or furnace system with them; the actual computer control to do that isn't all that complicated. There is much to be said, however, for the simplicity and reliability of less sophisticated systems which simply turn the heat on when it is too cold, and off when it is warm enough...

STEVEusaPA

Tekmar has 'simple' radiant floor thermostats with options for floor sensor, room sensor.
Not quite sure about short cycling. That has less to do with a proper thermostat than proper boiler and zone sizing/piping.
Tell us about your entire system-heat.

Edit: For some reason I posted this around 3:30 but it ended up in Drafts.

hcpatel78

I just installed Tekmar 519 with floor sensor. And it is also short cycles the boiler . It demands the heat even after it just satisfied 2 minutes ago. I noticed that my set temp is 77 and Tstat reads 78 and still it demands the heat. I collected data how and when and what temperature the TState demands the heat and it satisfied. After collecting straight one to two weeks of data still I don't understand the pattern. 
I have set up small camera facing TState to record all activities.

moral212

Jamie Hall said:

The problem is a little more complex than that. Whether the system is low mass or high mass (or super low mass -- like forced air) the ideal approach is continuous analogue modulation which matches the heat input to the system to the heat required.

This is a good deal more easily said than done. However, many modern systems -- at the cost of some complexity -- manage a very reasonable compromise. Recognizing that the primary controlling parameter is the difference between the outside air temperature and that target indoor air temperature, and that is possible to modulate (and maintain reasonable efficiency over a remarkably wide range) they do measure the outside air temperature -- on an analogue scale -- and, using a transfer function suited to the application (the "outdoor reset curve") they modulate the burner output -- again, an analogue modulation -- to produce an output temperature from the boiler (or furnace) to maintain the target indoor temperature.

Now clearly this modulation isn't going to be perfect, since there are a number of parameters which it does not take into account, but by taking the one with the largest effect into account and aiming to slightly overshoot, they do very well -- using an indoor sensor (floor or air or both) only to occasionally reduce the output to avoid overshooting.

In principle, one could also use only the indoor temperature -- floor or air -- as the controlling parameter, except that there is a problem: neither will take into account the heat required. They only sense the result of the heat input. In a very low heat capacity system, such as forced air, this may not be a problem, as the lag between changes in heat input and sensed parameter is small, and analogue (or narrow pulse width) modulation could be used without too much overshoot and oscillation. However, in higher mass systems the problem of oscillation may be severe, unless there is a rate of change sensing element involved as well as a simple point in time element (which, oddly, is incorporated in "old fashioned" air temperature sensing thermostats with an anticipator).

Combining an air analogue air temperature sensing device with a modulating boiler would work -- but the sensed range would have to be very much smaller than that used with outdoor reset. The target floor temperature, however, is not the actual desired output parameter: the space temperature is. One really needs to have both to attain satisfactory control. Could this be done? Certainly. Would it be beneficial in terms of overall efficiency? Possibly.

Ideally, one would have the correct sensor suite to measure the input parameters and the deviation from the desired target parameter and control a fully modulating boiler or furnace system with them; the actual computer control to do that isn't all that complicated. There is much to be said, however, for the simplicity and reliability of less sophisticated systems which simply turn the heat on when it is too cold, and off when it is warm enough...

Thank you for the detailed insight and explanations, very interesting.

moral212

STEVEusaPA said:

Tekmar has 'simple' radiant floor thermostats with options for floor sensor, room sensor.
Not quite sure about short cycling. That has less to do with a proper thermostat than proper boiler and zone sizing/piping.
Tell us about your entire system-heat.

Edit: For some reason I posted this around 3:30 but it ended up in Drafts.

The system has one whole house radiant zone controlled by a tekmar 563 on the upper level of the house and Runtal radiators in the finished basement. All heated with an IBC HC series boiler.

The Tekmar does seem to run fine with the floor sensor option off however I have no control over any floor settings as it turns off all radiant settings when off. When on however, it seems as though the boiler is short cycling when room temp is close to the set temp. I am guessing this is to do with the PWM settings for the radiant where run times are cut down to a percentage of the time when within a certain range of the set temp. I wonder if this would be the same if the floor was high mass or not?

Slightly irritating because it would be great to be able to dial in the floor temp setting without having to sacrifice the longevity of my boiler!

moral212

hcpatel78 said:

I just installed Tekmar 519 with floor sensor. And it is also short cycles the boiler . It demands the heat even after it just satisfied 2 minutes ago. I noticed that my set temp is 77 and Tstat reads 78 and still it demands the heat. I collected data how and when and what temperature the TState demands the heat and it satisfied. After collecting straight one to two weeks of data still I don't understand the pattern. 
I have set up small camera facing TState to record all activities.

Interesting, have you tried with the floor sensor off? See if it still short cycles? Seems to be what I am experiencing. Please do share your findings.

hcpatel78

With floor sensor off its behaving same way.

MikefromMn

I've got a Tekmar 561 controlling my high mass radiant floor. When it was set to radiant heat it would run the boiler for maybe a minute - then shut off, then repeat 5 minutes later. Was driving me nuts, so I turned off the radiant setting. Now it under and overshoots - but at least it's not short cycling all day long. There's got to be a better way.

moral212

MikefromMn said:

I've got a Tekmar 561 controlling my high mass radiant floor. When it was set to radiant heat it would run the boiler for maybe a minute - then shut off, then repeat 5 minutes later. Was driving me nuts, so I turned off the radiant setting. Now it under and overshoots - but at least it's not short cycling all day long. There's got to be a better way.

Agree! Even asked the manufacturer if there is anyway to turn of PWM, answer was no. Not 100 % positive its due to PMW but it does seem that way. Also seems like all other radiant Thermostats also use PMW.

jeff4444

I have the same set up with a staple up radiant floor and a tekmar stat with room and floor temp sensors. 

In my case, I only have the stat cycling the circulator pump off and on. You would allow the boiler to cycle on and off based on its internal hot water temperature. 

With that setup the pulse width modulation feature works very well and actually modulates the floor temperature setpoint by how far the room is away from setpoint.

The one thing the Tekmar stat is missing is an outdoor air reset also. If you could integrate the outside air temp to raise or lower the min and max floor temps it would be pretty much perfect. So, when there is a radical change in temperature outside, I have to readjust the min and max floor temp settings.