Boiler size

johneym

hello Folks,

small bungalow and rads newly installed.I have worked out the rad output and added 10,000btu for water. Total required is 50,000 btu. So would I be correct in installing a boiler to set at 50,000 btu? Or should it be set to more that this?

Many Thanks

John

Dtp1979

johneym wrote: »

hello Folks,

small bungalow and rads newly installed.I have worked out the rad output and added 10,000btu for water. Total required is 50,000 btu. So would I be correct in installing a boiler to set at 50,000 btu? Or should it be set to more that this?

Many Thanks

John

Find out the heat loss calculations. Rad size must be considered too. But either way you'll be installing a 50/90 condensing boiler. Do not oversize a condensing boiler

John T Carroll

Dtp1979 wrote: »

Find out the heat loss calculations. Rad size must be considered too. But either way you'll be installing a 50/90 condensing boiler. Do not oversize a condensing boiler

Relation (another) is renovating a small house and the required heating (ignoring cylinder requirement) is 8 Kw, this apparently is taking heat losses into account and is based (as far as I know) on a ambient temperature of 0C.
What size boiler would you recommend in this instance and what is the maximum turndown in the modern gas boilers? and what effect does it have on the boiler efficiency.? Thanks.

Dtp1979

John T Carroll wrote: »

Relation (another) is renovating a small house and the required heating (ignoring cylinder requirement) is 8 Kw, this apparently is taking heat losses into account and is based (as far as I know) on a ambient temperature of 0C.
What size boiler would you recommend in this instance and what is the maximum turndown in the modern gas boilers? and what effect does it have on the boiler efficiency.? Thanks.

Not sure how it effects efficiency but the smallest turn down is 15kw for grant and 12 for firebird. Maybe if you spoke to the manufacturers there might be some room to adjust down further.

Wearb

@Dtp. Gas boilers mentioned.

Dtp1979

Wearb wrote: »

@Dtp. Gas boilers mentioned.

Sorry. Long day!!
Most common boilers installed are 24kw probably. After the temp rises they eventually modulate down to 5kw. Most boilers can't be set.
Ones that can are Ariston and Valliant. Maybe there's more but I'm not aware of any.
My own Ariston is a 24kw. I have it set to 9kw max output for the heating and have weather compensation too.
When hot water is needed it overrides the weather comp and ramps up to 24kw. I'd usually have 150L up to temp in around 15mins.

John T Carroll

Dtp1979 wrote: »

Sorry. Long day!!
Most common boilers installed are 24kw probably. After the temp rises they eventually modulate down to 5kw. Most boilers can't be set.
Ones that can are Ariston and Valliant. Maybe there's more but I'm not aware of any.
My own Ariston is a 24kw. I have it set to 9kw max output for the heating and have weather compensation too.
When hot water is needed it overrides the weather comp and ramps up to 24kw. I'd usually have 150L up to temp in around 15mins.

Thanks, I should have said modulation instead of max turndown ratio, so around 5:1, 24Kw/5Kw, which is what I might have expected. As the relations hot water requirements are minimal, the hot water zone can be be brought in earlier in the morning before the heating is required then it probably hasnt any big influence on the boiler output choice. The required heating demand is much the same as your own so maybe 15 Kw to 20 Kw would fit the bill?. A combi boiler would do the job as well but in order to give a reasonable shower flowrate of say 10 LPM one would need a boiler of around 30 Kw and as the heating requiremants at times will probably be as low as 2.5 Kw then the combi could be cycling on/off frequently but these boilers are very popular abroad so I wonder is there any particular reason why they are not too common here.

Dtp1979

John T Carroll wrote: »

Thanks, I should have said modulation instead of max turndown ratio, so around 5:1, 24Kw/5Kw, which is what I might have expected. As the relations hot water requirements are minimal, the hot water zone can be be brought in earlier in the morning before the heating is required then it probably hasnt any big influence on the boiler output choice. The required heating demand is much the same as your own so maybe 15 Kw to 20 Kw would fit the bill?. A combi boiler would do the job as well but in order to give a reasonable shower flowrate of say 10 LPM one would need a boiler of around 30 Kw and as the heating requiremants at times will probably be as low as 2.5 Kw then the combi could be cycling on/off frequently but these boilers are very popular abroad so I wonder is there any particular reason why they are not too common here.

Yes 15 kW would be loads.
Combos aren't common here because of poor mains pressure, therefore tanks and pumps make the install much more expensive.
Limescale in very plentiful in Irish water too and this will destroy secondary heat exchangers.
And finally, combis are really only suited to apartments, ideally where there is only one bathroom and kitchen sink.
When fitting a combi, you should always consider the worst case scenario where all hot draw offs could be on at the same time.

You can fit a 38kw combi which can run two taps at once if the pressure is there and adjust the heating load down to the minimum setting(if you wanted too) great hot water performance and a heating output that could heat a couple of rads.

Gas boilers can be adjusted to match precisely the heating requirements which usually has very little negative effect on efficiency.


The flow rate for a 38kw combi is around 15-18 l/min as opposed to 10-12l/min for a 24kw combi.

John T Carroll

John T Carroll wrote: »

Relation (another) is renovating a small house and the required heating (ignoring cylinder requirement) is 8 Kw.

Heating requirement would be approx 11 Kw (max) if cylinder heating included.

Dtp1979 wrote: »

Yes 15 kW would be loads.

Some of the 15 Kw gas boilers seem to have a minimum output of 4.5kw/5 kw but the 20 Kw Vokera mynute I20 claims a modulation ratio of 10:1 or 2.0 Kw minimum, is this realistic? and if so, would it also be suitable and if so any views on their reliability etc.

John T Carroll

John T Carroll wrote: »

Heating requirement would be approx 11 Kw (max) if cylinder heating included.



Some of the 15 Kw gas boilers seem to have a minimum output of 4.5kw/5 kw but the 20 Kw Vokera mynute I20 claims a modulation ratio of 10:1 or 2.0 Kw minimum, is this realistic? and if so, would it also be suitable and if so any views on their realibility etc.

A 15kw will do the job.

I like keeping things simple, I would match the boiler output to a little over the required heating/hotwater requirement(unless it's a Wplan), stand back and see what movement I have by monitoring the boiler output.

I don't see the modulation ratio as really a factor for me as most heating system I work on have sluggish movement and can't take advantage of modern boilers intelligence that have been designed with good system movement in mind, also at times boilers that modulation down to such a low flame can have ignition issues which boiler with a higher minimum are less likely to suffer from.

John T Carroll

gary71 wrote: »

A 15kw will do the job.

I like keeping things simple, I would match the boiler output to a little over the required heating/hotwater requirement(unless it's a Wplan), stand back and see what movement I have by monitoring the boiler output.

I don't see the modulation ratio as really a factor for me as most heating system I work on have sluggish movement and can't take advantage of modern boilers intelligence that have been designed with good system movement in mind, also at times boilers that modulation down to such a low flame can have ignition issues which boiler with a higher minimum are less likely to suffer from.

That makes sense, a combi boiler would theoretically seem to have a huge mismatch in that regard as any reasonable hot water flow requires 30/35 Kw whereas the heating demand might only be 8 Kw max.

John T Carroll

John T Carroll wrote: »

That makes sense, a combi boiler would theoretically seem to have a huge mismatch in that regard as any reasonable hot water flow requires 30/35 Kw whereas the heating demand might only be 8 Kw max.

If I was fitting a combi I'd go as big as the mains water and gas supply would allow(taking into account suitability) and then set the heating output to a few kW above the heating load which helps prevent elephant into a rabbit hutch heating scenarios.

Tom44

Wearb wrote: »

@Dtp. Gas boilers mentioned.

NO mention of gas from original poster, JOHNEYM :cool:

One mention and yer all off

Dtp1979

Tom44 wrote: »

NO mention of gas from original poster, JOHNEYM :cool:

One mention and yer all off

I was replying directly to John t Carrolls post. I thought that was fairly obvious