i want to buy an oversized boiler

timmer3

hi. i got some documentation last week from SEI about the home energy saving scheme. they make a point about boiler sizing "an oversized boiler can be very wasteful of energy". this is fair enough i can understand that. but it raises a question for me about the model of boiler i was thinking of buying. i basically want someone to give me a good reason *not* to buy a 42KW gas boiler, because i can't see any. i've read other threads here that say it will affect the future energy rating of the house. if someone had the time to read this post i'd be really grateful if they could point out one good reason not to buy this boiler (except the few hundred quid it costs more than a lower powered one).

first a bit of background on the house.. by next year i'll have upgraded the insulation in the whole house, it's a 3 bed end of terrace 1940's house in dublin 12 with mass concrete walls. i'm putting in 90mm kingspan thermal boards on (the inside of) all external walls. 300mm attic insulation, roof insulation, and under-floorboard insulation as well downstairs. and double glazing. so it won't ever be super energy efficient but it might get a C rating or something like that. i don't really believe in putting in gzillions worth of effort and money into a house that will never be top rated anyway. i think i've set a reasonable target for energy efficiency.

currently i have miniscule hot water requirements. but that could change in the future if a family ever happense etc. it's a small house and to free up space i want to get rid of the cylinder and attic cold water tank. this will also drastically reduce the amount of pipe-work that runs rings around the house, wasting hot water every time i use a tap. i've fairly made my mind up about this. at the moment i heat the cylinder twice a day to have enough hot water to wash the dishes but it's a pure waste of money because i use so little. i really like the idea of only heating the water you use, on demand, from a condensing gas boiler.

so... when looking at gas boilers i paid a lot of attention to the DHW flow rate that can be delivered, which is important for running a mixer shower (i don't want an electric). so i naturally went for the most powerful one, a bosch greenstar 42 CDi, 42KW. the reason i think this boiler is great is because you can independently set the DHW from 9.8 - 42kW, and the central heating from 9.4 - 30kW (33,437 - 102,360Btu). i worked out my BTUs for the house from radcalcs.com at 33000. so surely this is a great choice if my needs will grow in the future? i can run it as economical as i like now and turn it up if the requirements increase. i don't want to buy a 'junior' boiler with a 'junior' DHW flow rate (i.e. bad shower), only to replace it as soon as my requirements increase. i probably will never need to increase the CH above the minimum setting, but even if i do surely the only change is that the house will heat up quicker and the TRVs will kick in and the gas will stop being used, so no wasted energy.

i'm a complete novice on these issues but this makes a lot of sense to me. i'd be glad of an expert opinion. thanks.

davidoco

timmer3 wrote: »

i basically want someone to give me a good reason *not* to buy a .........

The incoming supply from the mains (water) not being able to deliver 17 ltr pm let alone 10 ltr pm


The Council may strive to supply a 10 m head but in reality this may not be achieved. A bit like broadband.

sydthebeat

Putting an over sized boiler is a dwelling that doesn't need it is akin to a yummy mummy driving her ikkle darlings 2 miles to school every morning is a 4 litre SUV.

One reason alone not to put one in is the initial cost alone.

Your biggest issue is getting the boiler properly sized. Theres a distinct lack of responsibility in the industry regarding sizing, the suppliers will tell you your plumber should size it, the plumbers will tell you your supplier should size it.

as a general rule of thumb:

Section A3.5 of the DEAP manual refers


A3.5 Estimating the required boiler size While DEAP should not to be used to size the heating system (this is usually up to the system designer or heating system supplier/installer) the following method can be used to estimate the required heat system power output. Note that sizing the heating system is not the responsibility of SEI and is entirely between the client and the assessor or system designer. Maximum recommended dwelling heating load: Expressed in kW. For dwelling heat load, the maximum space heat demand is calculated by multiplying the heat loss coefficient by the desired maximum temperature differential (deltaT usually -1 to 21 degrees C). The heat loss coefficient is the heat loss in W/K of the dwelling when accounting for fabric and ventilation losses. The heat loss coefficient is called the "Total Heat Loss" in the DEAP. 2 to 4 kW should be added to the space heating demand for water heating depending on occupancy or floor area. The total of space and water heating requirement should then be multiplied by 1.1 to account for distribution losses giving the following maximum recommended dwelling heating load: Dwelling space and water heat requirement (W) = dwelling space heat loss + dwelling DHW requirement = Total Heat Loss*22 *1.1 + 4000*1.1

timmer3

davidoco wrote: »

The incoming supply from the mains (water) not being able to deliver 17 ltr pm let alone 10 ltr pm

currently i get 15 ltr pm and i'm told by the council that in the next year or so the water supply to the area is being upgraded which will give better flow and pressure. even if this never happens it's quite a good flow. when i moved in last year i had a new pipe put in to the house and the difference was unreal.

timmer3

sydthebeat wrote: »

Putting an over sized boiler is a dwelling that doesn't need it is akin to a yummy mummy driving her ikkle darlings 2 miles to school every morning in a 4 litre SUV.

hi syd. thanks a lot for the reply. i appreciate that it is a little more expensive to buy a bigger boiler, compare €1350 for the 42CDi vs €900 for the 27 KW version. but €450 when you factor in the installation cost is not a huge sum of money if it is future proofed against increased requirements.

i accept the point about overkill, but if the central heating is operated at 9kw (which appears adequate based on my complete layman's calculation, i wouldn't have the expertise to work it out based on DEAP), and 9KW is hardly overkill, then what is the problem? if the boiler is intelligent enough to cut out when the desired temperature is reached, then where is the wastage? why wouldn't someone want their house to heat up quickly?

similarly with water, if you only 'heat' what you 'use' then the fact that it is heated at a faster rate (using more gas over a shorter period of time), why should that lead to wastage?

the SUV analogy doesn't seem to quite fit because if it got you to school twice as fast, and if it could operate in a 1.2 litre economy mode when you don't need the power, then it would be a great choice. any thoughts?

sydthebeat

timmer3 wrote: »

hi syd. thanks a lot for the reply. i appreciate that it is a little more expensive to buy a bigger boiler, compare €1350 for the 42CDi vs €900 for the 27 KW version. but €450 when you factor in the installation cost is not a huge sum of money if it is future proofed against increased requirements.

i accept the point about overkill, but if the central heating is operated at 9kw (which appears adequate based on my complete layman's calculation, i wouldn't have the expertise to work it out based on DEAP), and 9KW is hardly overkill, then what is the problem? if the boiler is intelligent enough to cut out when the desired temperature is reached, then where is the wastage? why wouldn't someone want their house to heat up quickly?

similarly with water, if you only 'heat' what you 'use' then the fact that it is heated at a faster rate (using more gas over a shorter period of time), why should that lead to wastage?

the SUV analogy doesn't seem to quite fit because if it got you to school twice as fast, and if it could operate in a 1.2 litre economy mode when you don't need the power, then it would be a great choice. any thoughts?

Assuming your dwelling requires a 9kw boiler, then by definition you would need to build 300% the size of your dwelling in an extension to come close to the maximum boiler output... do you intend doing this?

even if you built an extension the same size and to the same standard, youd only require an 18kw boiler....

i fail to see the logic in installing a boiler that has a maximum output over 4 times what you need. Every boiler has a setting at which it runs most economically, what is that setting with the 42CDi?? If its anything above 9kw, again assuming thats what you need, then it will always be running inefficiently.

timmer3

i won't ever dramatically increase the space heating requirements of the building. the reason i have selected the higher output boiler is to get higher output for DHW, better shower, faster filling up a bath, sinks etc. the lowest spec boiler can deliver 11 litres/min compared to 17 for the 42KW. even the lowest spec boiler has a central heating output range of 7.7 to 26 KW which is still 'overkill' for my 9KW space heating requirements (3 times overkill vs 4 times overkill). i can't find out what the 'sweet spot' for these boilers are, but i do know that they start up with minimum output and increase slowly as needed, and they are all A rated 90.1% efficient boilers.

you could say that i should get a low-powered central-heating-only boiler and a separate water heater but to me it makes sense to get a combined system, it's more space efficient and simpler to install and maintain, even if it costs a little more and even if i only use 25% of the CH capacity. i just wanted to clarify if it was actually going to waste energy to run a bigger boiler on a lower setting, or if i was going to be penalised from a BER perspective.