Central heating questions

System

Phil3822 wrote: »

This is what I want however with a hive thermostat my system will just do the on and off thing, I don't believe the boiler will modulate around the thermostat set temperature. I will await stats installation response. Thanks again.

Your boiler will still modulate but not as much as it would with a TPI thermostat. From the Honeywell website:

Modern controls are designed to ensure that you get the best energy efficiency
possible from your heating system. TPI (Time Proportional & Integral) is a feature
in your room thermostat that ensures that your boiler fires for the shortest possible
time in order to reach and maintain your room at the comfort level selected:

How it works:

Firstly, there are two things you need to understand about your room thermostat:
It has a simple memory included in its software and it ‘learns’ how the room
temperatures are achieved and maintained. It also has a basic calculation
capability so that it can work with the learned data and current temperatures to
predict how long the boiler needs to be fired for.

TPI in action:

A TPI room thermostat works by continuously measuring the room temperature
and calculating a difference between measured and target temperature. When the
difference is greater than 1.5 degrees C (known as outside proportional band) it
fires the boiler continuously. When the room temperature gets within 1.5 degrees C
of the set point you have chosen on your room thermostat, the TPI software in the
thermostat comes into action. It performs a series of calculations where it
calculates how long the boiler needs to be fired to reach and maintain the set
temperature. The calculation is based on the previously learnt characteristics of
the room in which the controller is placed and the current measured temperature.
This ‘smart’ firing of the boiler will mean that the boiler is fired just long enough to
achieve the set temperature and no longer, thus increasing energy efficiency.


In normal operations you will not notice any difference in how your heating system
functions. You may, if you listen carefully, hear the boiler firing for shorter and
shorter periods as it approaches the correct temperature. When the room set point
is reached, you will notice that the boiler is fired for short periods in order to
maintain the temperature. The thermostat is calculating the rate at which the room
is losing heat and topping up with the precise amount required to counteract it.
There is normally an overshoot of around 0.5 degrees C when first reaching set
point. This is because the thermostat tries to reach the set point as quickly as
possible, after that it should settle at exactly the set point. The thermostat learns
the room’s heat loss characteristics over the first week of operation and you will
find that the initial overshoot is higher during that period. It will quickly learn the
characteristics and become more accurate.
Why is this different to non TPI thermostats?
In non TPI thermostats (commonly older devices), the boiler will continually fire
until it gets past the set point and then when the temperature drops to a specified
amount below the set point the boiler is then fired again. This creates greater
temperature fluctuations and less control of your comfort levels.

What you may notice:

In normal operations you will not notice any difference in how your
heating system functions. You may, if you listen carefully, hear the boiler
firing for shorter and shorter periods as it approaches the correct
temperature. When the room set point is reached, you will notice that
the boiler is fired for short periods in order to maintain the temperature.
The thermostat is calculating the rate at which the room is losing heat
and topping up with the precise amount required to counteract it.
There is normally an overshoot of around 0.5 degrees C when first
reaching set point. This is because the thermostat tries to reach the set
point as quickly as possible, after that it should settle at exactly the set
point. The thermostat learns the room’s heat loss characteristics over
the first week of operation and you will find that the initial overshoot is
higher during that period. It will quickly learn the characteristics and
become more accurate.

Why is this different to non TPI thermostats?

In non TPI thermostats (commonly older devices), the boiler will
continually fire until it gets past the set point and then when the
temperature drops to a specified amount below the set point the boiler
is then fired again. This creates greater temperature fluctuations and
less control of your comfort levels.

As you can see, Honeywell's fuzzy logic will fire the boiler at maximum flow until the proportional band is reached. It is only when the system is in this band that the boiler demand will fall and the boiler will get into a condensing mode. Looking at Hive FAQs, it would appear that they have no immediate plans to build TPI into Hive.

Finally, rather than concentrating on convenience and economy, you also need to think about failsafe options for wireless controls. For example, if my controller fails when I am away from home on a cold day, the Opentherm Bridge will default to a cycle of firing the boiler at set intervals to prevent freezing.

Edit: As your Atag boiler is Opentherm - capable, then this link may help you decide whether the Atag One controller (or Nest) might work for you.

https://theevohomeshop.co.uk/content/21-What-is-OpenTherm-and-why-use-Honeywell-evohome-with-it

Phil3822

Thanks Hengus, you have been invaluable in helping me learn about the atag system and options.

Looking through the boiler readings I am curious why the return flow temp alters. If the boiler output is at a consistent temperature why does the return temp go up over time closer to the output temp. I assume because the heat cannot be expelled from the radiators quick enough. Thanks all.

Raxiel

Phil3822 wrote: »

Thanks Hengus, you have been invaluable in helping me learn about the atag system and options.

Looking through the boiler readings I am curious why the return flow temp alters. If the boiler output is at a consistent temperature why does the return temp go up over time closer to the output temp. I assume because the heat cannot be expelled from the radiators quick enough. Thanks all.

Yes, as the house warms up, the water in the radiators will loose less heat to them.

System

Raxiel wrote: »

Yes, as the house warms up, the water in the radiators will loose less heat to them.

And as TRVs close, then the heating system effectively gets smaller. As I said earlier, all systems therefore need an open radiator without a TRV or an automatic bypass to prevent damage to the boiler.