query on which boiler

[cully]

I have a new house requiring heating and DHW installation. 600 mtr sq. Underfloor & rads heating and 400ltr DHW cyl.
The DHW will require around 26kW and the central htg. requirement (from sap) will be around 4500kWh in January so I reason that a 26/ 30kW boiler will suffice for both heating requirements with priority heating to the hot water.
The question I have is what boiler fuel to use? There is no mains gas so the option is LPG or oil.
I am thinking about the situation when the boiler demand is low, one / two circuits requiring heat, and the boiler cycling. LPG boilers modulate down, oil boilers don't so this suggests using LPG but oil is a cheaper fuel.
I appreciate your thoughts
Thanks
Cully

 

[bernardgreen]

Option 1..... Have two boilers, one at 26kW for DHW and the second at 4kW for heating.

Option 2..... Have just one boiler at 4kW and have a hot water cylinder.

 

[Dan Robinson]

FFS Bernard - how many times do you have to be told your ideas on system design are pathetic or at best misguided.


OP....IF your heat loss is that low then think of two separate systems - maybe ASHP for the heating and a dedicated LPG water heater such as a Rinnai possibly with a buffer tank. You could get RHI on the ASHP then.

Why such a big cylinder? ASHP could also heat the hot water and be topped off with an immersion for L8 compliance. Not sure of the economies of that with a 400 litre store.

Hot water priority is fine, but not with such disparate kW requirements.

 

[Razor900]

http://www.daikin.co.uk/minisite/hybridheatpump/

 

[cully]

Thanks to all for the response,
Dan, the house mentioned is my son and wife's and they want to utilize either oil or gas (hence the question on which fuel "oil or LPG")
The reason for considering a 400l cyl. (not cast in stone yet) is basically down to number of bathrooms (five) but it will probably end up being 300l when we resolve the boiler size / recovery time issue.
We have talked to three different heating engineers who all have differing opinions about the way forward, but one did give your response about a boiler for each system but we thought this was going "over the top" but is it?
We need to re-visit this idea.
Thanks
Cully

 

[Dan Robinson]

Unless the house is fully occupied must of the time,storing that amount of water is not the greatest way.

Hence my suggestion of a dedicated water heater life the Rinnai.

Why do they want to lock themselves to a specific fuel type? If the heatloss is that low then oil is going to be useless without a buffer tank.

ASHP is ideal if it is ufh.

 

[cully]

Dan, the problem I can see with the Rinnai is in this house the distance from the boiler to the farthest hot water outlet is around 20 mtr plus bends etc. with the others outlets between. we were going to run a pumped secondary circuit from the cyl.to give immediate hot water to all outlets. It isn't easy to place the boiler in the middle of the circuit to alleviate this
Looking at the ASHP suggestion the reason we were hesitant with that was because we got a quote for a "GSHP" system of £30,000
But doing a quick "google" on the air source pump it seem a lot cheaper, so we will luck at that.
Thanks
cully

 

[Dan Robinson]

You can still use a secondary return with an instant water heater.

 

[cully]

Thanks