Weather Compensation #1: Installation

 

In summer 2023 we had the weather compensation addon installed on our Ideal Logic+ S30 system boiler.

 

Background

My interest was peaked when I saw various articles about how reducing the flow temperature of a gas condensing  boiler would make the boiler operate more efficiently:

• https://www.nesta.org.uk/report/salford-energy-house-boiler-flow-temperature-testing-initial-report-october-2022/
• Boiler Tweaks To Tackle The Cost Of Living Crisis. AKA Lower the flow temperature on a combi boiler

 

However, much of this advice is targeted at combi boilers with instant hot water (as opposed to system boiler with stored hot water).

We have a system boiler (ie with a hot water tank), so apparently the water in the hot water tank must be heated to 60degC to ensure that bugs do not grow. From previous posts you will see that we reduced the flow temperature from 70degC to 65degC, but could we go further without risking unhealthy hot water?

Some Googling revealed the existence of weather compensation boiler addons, including one for our particular model of boiler. The operation is:

• When hot water is being heated then the flow temperature is set to 80degC for both hot water and radiators.
• When the hot water is not being heated then the flow temperature is set by an outside air temperature sensor, and is typically much lower, in a range where the boiler efficiency is increased.
  
• The boiler low temperature control is now used to set the curve used that relates outside air temperature to inside air temperature; when it is warm outside the flow temperature is reduced and when it is cold outside the flow temperature is automatically increased.

 

The Installation Process

The first problem was to get my plumber to agree to fit weather compensation. At first his reaction was that it would not work, but in the end I convinced him and we agreed that I would fit the sensor on an outside wall and take the cable to the boiler and he would wire up the boiler.

 

My Ideal Logic+ S30 System Boiler	Weather Compensation kit: a thermistor, plastic box, screw,  instructions, a sticker for the boiler and a screw connector.

 

Installing the cable was reasonably straightforward. I watched a couple of You Tube videos on exterior cable installation and left plenty of cable for the engineer to connect to the boiler. The cable had to run the full width of the house as the boiler is against a South facing wall and the sensor had to be fitted on a North facing wall so that it measures air temperature and is not affected by direct heat from the sun.

 

The engineers job was straight forward except for the existing rat's nest of wiring in the junction box adjacent to the boiler. The issue is the need for an additional hot water demand signal so that flow temperature is raised when heating the hot water. There was a spare core in the cable between the junction box and the boiler, but the junction box was such a mess and, in the wiring process another core came lose so that he was left with a floating core and having to workout where to re-attach it.

 

The cost of the installation was approximately:

• Weather compensation kit: £60 (expensive for what it is)
• Cable, clips, etc: £25
• Heating engineer's time: £120

 

Operation

Good news, the heating engineer was a great sleuth and deduced correctly where to re-install the detached core...

...weather compensation worked straight away.

 

Ideal Logic+ Weather Compensation Graphs: you select the curve (from 12 to 30) using the flow temperature control on the front of the boiler - as you turn the control if shows the graph number for a few seconds and then reverts to showing the actual flow temperature.

 

The user guide included a set of graphs. The graph names confused me to start with as they are not really related to room temperature setpoint, as, of course, the room temperature setpoint is controlled by the regular thermostat (in our case the Honeywell Evohome system). However, with a higher the graph number then a higher the room temperature setpoint will be achievable, but this will also be a function of insulation, air volume, radiator size, etc...

...finding the best setting/graph number is a trial and error process.

 

I started with a high graph number (25degC) and nudged it down over time:

• The higher the graph number then the quicker the rooms heat up
• Lowering the graph number gives a lower flow temperature and so should be more efficient.

We are currently at graph number 22degC and, for our house this is working well; I may try still lower in the future.

 

Results

Weather compensation worked as advertised; you can see and feel that the flow temperature is higher on colder days and, when the hot water is being heated the flow temperature is 80degC, and the radiators get really hot. On warmer days flow temperatures in the 40's and 50's degC are common (See graph "22", ie 22degC Room Temperature Setpoint ringed in red above).

 

The system was installed in August and apart for a quick test we did not have the heating on until September. How would it perform in real life? Would it be possible to reduce flow temperature and keep the house comfortable? My summary:

• Compared to before weather compensation was installed, the boiler is on more often, but with a lower flow temperature, the radiators do not get really hot, other than on the coldest of days and first thing in the morning when the hot water is being heated.
• As a result the house temperature is more even and feels more comfortable.
• We have the hot water come on at 6am, it typically takes about 30 to 45 mins to heat the tank. We have the heating off over night and so we get a short blast of hot radiators to warm up the house 1st thing in the morning - this works well.
• During the day it does a good job in maintaining a steady temperature; in my view better than without weather compensation.
• One draw back is that if the heating has been off for a while (eg we have been away), then it is more important to warm up the house for several hours before we get home (via the Evohome app) - warming up the house from cold takes much longer with a low flow temperature.

 

On warmer days, once or twice, we had the case that the Honeywell Evohome system demanded hot water for a radiator but the boiler was off - if the outside air temperature is too high then the boiler will stay off, even if heat is demanded. This was not uncomfortable (ie the house did not feel cold), but, at first, it was confusing when I could see that heat was being demanded and the boiler was off.

 

Prior to installation I did have a worry that the Honeywell Evohome self-learning logic would not cope with varying flow temperature. This did not seem to be an issue...

...perhaps someone from Honeywell could comment.

 

And does it save money? Based on the Salford University results I am optimistic, given that, other than 1st thing in the morning, the flow temperature is much reduced...

...I plan to post data for the year to September 2024 compared to September 2023.