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Following the introduction of Boiler Plus to Building Regulations in England, David Iszchak, technical trainer at Vokèra by Riello, offers a personal comparison of the potential of the four additional energy saving options now compulsory when installing a combination boiler.
In addition to increasing the minimum ErP Space Heating Energy Efficiency threshold for any boiler to 92%, Boiler Plus offers the customer a choice between one of four mandatory options for improving overall efficiency when having a new combi boiler installed in any property other than a new build development. This can be load or weather compensating controls, a smart thermostat, or a passive flue heat recovery system. But how can installers best compare the energy saving potential of each measure?
Although it may seem open-ended, the ‘save up to’ caveat which often appears in statements relating to boilers and controls, allows for the fact that actual savings will depend on various factors. These include how the product is fitted, into what system and property it is installed, how the end user will operate it and what it is replacing. The choice of boiler/control might be based upon the price the customer is likely to accept, or maybe the installer’s personal preference or experience. Other considerations could be a particular manufacturer’s incentive or discount, user specification, or simply the overall operating efficiency of the finished project. All this can make recommending a solution that will deliver satisfactory performance and cost for a potential customer something of a ‘conundrum’ for installers.
It should be noted that the figures we’ve chosen to illustrate potential savings for each option are just examples. There are many other permutations that could also have been considered.
Before we get to the calculations, we need to remember that the biggest advance in gas boiler efficiency has been the adoption of condensing appliances over standard efficiency appliances, around 10 – 20% based on manufacturers figures no matter what the operating temperature.
The other thing to bear in mind is that each m3 of natural gas contains around 11.7kW energy gross depending on the constituent elements making up the gas (circa 10.73kW sensible heat and circa 0.93kW latent heat) so there is more than 10 times the amount of sensible heat than latent heat. Sensible heat is available all the time, every firing and up to around 94% can be captured with perhaps 4% lost through flue products and 2% radiated from the casing.
The latent heat is also always available, but only when conditions allow and perhaps 50% of that can be captured dependant on the efficiency of the heat exchanger in harvesting sensible heat and the return water temperature (if there’s a plume at the terminal not all the water vapour has been condensed where it’s needed).
We’ve used an average cost for natural gas of 55p/m3 but prices will vary by contract and consumption.
Example 1: ON/OFF timed thermostat
For a complete picture, we need to set a base level and consider an ON/OFF control with no load compensation or optimisation, simply a go/stop control system with no setback temperature. Whilst this option is not permitted in England, as it does not conform to Boiler Plus, it is still acceptable in Scotland, Wales and Northern Ireland.
Assume operation of 9 hours a day at an average of 5.5kW/hour, so 5.5 x 9 x 7 = 347kW.
The boiler will be running hotter and condensing less with the boiler heating selector set to the eco position (unless fitted with Vokèra’s SARA function or similar – see boxed section) and let’s assume it will be at around 80% space heating energy efficiency under ErP.
11.7kW – 14% reduction in efficiency
11.7 – 1.63 = 10.1kW/m3 natural gas.
347÷10.1 = 34.3 m3 @ .55p/m3 = £18.31p/week
Examples 2 & 3: Compensating controls
Operating a boiler with compensating controls (weather compensation or load compensation) for 24 hours a day at condensing temperatures with, say, a continuous average load for space heating of 2kW for seven days will require:
2 x 24 x 7 = 336kW/week
With space heating, assume losses from the gross sensible and latent heat potential (11.7kW)
94% ErP space heating efficiency; 4% flue losses – 2% irradiation losses – 4% latent heat losses) = 10%
11.7 – 1.17 = 10.53kW from each m3 natural gas
Therefore 336kW ÷ 10.53 m3 = 32 m3 @ 0.55p/ m3 = £17.60/week (a 4% saving against example 1)
Example 4: Smart Thermostat
In the same property using ON/OFF controls with the stipulated Optimum Start function but alternating between comfort and setback temperatures for one week with three comfort periods a day. Each comfort period saving an average of 15 minutes deferred firing to achieve the comfort temperature by minimising a programmed pre-heat period by 15 minute each session (optimised start) and taking the overall heating load over the week to be the same, just variable to the occupier’s preferences.
Comfort temperature load 4kW, setback temperature load 1kW
Nine hours a day at comfort temperature and 15 at setback = 36 +15 = 51 x 7 = 357kW/week
Less the optimisation savings:
4kW ÷ 0.25 (the average optimised 15 minute delay in firing to achieve the comfort temperature at the scheduled time) =1kW
1kW x 3 (optimised starts a day) = 3 x 7 = 21 so 357 – 21 = 336kW
As some of the firing may not be at condensing temperatures we can assume less of the gross heat potential of the fuel will not be available, so perhaps a boiler efficiency of 88% of the 11.7kW gross for space heating (2% irradiation + 4% flue + 6% latent heat lost/m3)
11.7 – 12% losses = 10.3kW so 336 ÷ 10.3W = 33.62m3 @ 0.55p/m3 = £17.94p / week (a 2% saving against example 1)
Comparing these figures, load compensation over optimisation rather than just optimum start (if enabled) appears to offer minimal efficiency benefits. Improvements in comfort are far more difficult to assess due to the property occupier’s personal requirements so what other benefits of load compensation are there?
It can be argued that low temperature operation prolongs the life of an appliance and reduces wear by limiting operating cycles; there’s logic to this but as high temperature ON/OFF boilers have lasted for 25 years plus, and longer in some instances (not at the same efficiencies of course), this is moot.
Excluding external load compensation (weather compensation) an internal load compensating thermostat employs a communication protocol between room control and boiler (necessary to communicate the varying temperature flow requirements); in addition to allowing the user to program the heating schedule and define temperatures to personal requirements, the control can bring the boiler PCB to the control display as well as taking command of the boiler operation. Boiler operating condition, performance indicators, alarm notification, history, even boiler reset all become available remotely on the control that may be located in a position more convenient for the user to access than at the boiler. It also means that if internet connectivity is included, the boiler condition will be visible to the boiler and/or controls manufacturer, as well as to the property owner while away from home, and the installation/service engineer for monitoring purposes – subject to relevant access permissions and data protection compliance.
Example 5: Passive Flue Heat Recovery
The fourth optional additional energy saving measure included in Boiler Plus is not a control. Passive Flue Heat Recovery uses heat remaining in boiler flue products to pre-heat the incoming cold water supply to a combination boiler, reducing the load upon the main heat exchanger. Results here are harder to assess as much depends on occupancy of the house and the amount of hot water consumed but manufacturers suggest ‘up to 30%’ of the costs of heating domestic hot water can be saved.
Say a property has an annual natural gas energy bill of £1000, the hot water portion of that might be £180/year. IF the full 30% saving could be made then around £60 a year might be removed from the fuel bill (should the 30% be actually achieved). A possible problem is that combination boiler hot water flow rates are rarely high enough for all users, better flows are usually desired, so what can happens is flow restrictions are adjusted to allow better delivery rates, this results in the same amount of energy being consumed and maintaining cost levels but improving performance. That of course may be the customer’s preference, rather than a reduction in running costs.
So to sum up, whether they use our figures and assumptions or substitute their own, we hope heating professionals will find our examples helpful or even devise their own comparisons when it comes to assessing the true energy saving potential of the four options.