Boiler size

R

roomtolet

Refurbishing a 9 bedroom bed & breakfast, bedrooms have a bath/shower in each, central heating uses two C55 Keston boilers, hot water will be three 300 litre unvented cylinders supplied via their own boiler/s.
Could you please give me a rough idea on the size of boiler to supply these cylinders?,would like to heat them within 30 minutes.

Thanks.
 
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You can not heat 3 cylinders that size in 30 minutes with a domestic boiler.
For a project that size, it is even more important to find a good RGI that can advice you, than it is on average size jobs.
 
M

monoxide62

900 litres for a recovery rate of 30 minutes @ a deltaT of 60* would require a boiler output of around 125kw , not knowing coil rating i would hazard a guess 28kw? , in this case boiler output should be around 84kw , with this in mind you could install a couple of 40kw boilers operating on a lead/lag arrangement , no problem.
 
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gasafengineer

Some time soon you will get the google guys on here telling you 6kw will be plenty enough to heat those tanks. :LOL: :LOL:
 
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Don't recall mentioning domestic or commercial.
You are right of course, you did indeed not provide enough information to give informed advice.
Never heard of somebody doing a commercial project asking for advice on a diy site, hence my response about domestic boiler.
 
R

roomtolet

Don't recall mentioning domestic or commercial.
You are right of course, you did indeed not provide enough information to give informed advice.
Never heard of somebody doing a commercial project asking for advice on a diy site, hence my response about domestic boiler.

Then please do not assume.

What information did i fail to disclose? , i simply asked for a boiler rating to supply 3 hot water tanks , whether commercial or domestic has no bearing to my question.
 
R

roomtolet

gasafengineer";p="2052856 said:
Some time soon you will get the google guys on here telling you 6kw will be plenty enough to heat those tanks.quote]

A little confused at this comment, would you care to expand on it?
 
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Cylinder manufacturers are often very optimistic about their cylinders' recovery rate. Sure, to heat 900kg of water from 10*C to 60*C will need approx 180,000 KJ. To do this in 30 minutes (1800 seconds) takes a rate of 100 kJ/s or 100kW. I would say that most cylinder coils in pressurised cylinders are no bigger than 10kW capacity, and that is when cold. As the water heats up the coil power is reduced, maybe to as low as 3kW as domestic water approaches 60*C and deltaT is more like 15K. This means you may only need a 30 or 40kW boiler dedicated to hot water generation.
Please note the correct use of lower case k for kilo, and upper case for Kelvin
However, the good part is that acceptable hot water may be as low as 45*C, so the customer will assume the water has recovered much sooner than the 30 minutes design time.

Consult a competant person with design experience of this type of project, as has been said before here, experience is that which you receive just after you need it.

Also, with constant (assumed), and large hot water requirements, have you considered supplementing the gas boiler with solar panels? Presumably your occupancy levels peak in the summer months when solar generation is at its peak. Be seen by your customers as green too, whilst saving money (and saving a little gas for my kids)

MM
 
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How about those Andrews type gas fired HW cylinders , seen several sports centres / schools with this set up ??
 
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whether commercial or domestic has no bearing to my question.
It does if you try to commission a commercial boiler on a domestic meter; no way it will pass. The same goes for the pipework needed for this set up.
But like I said before, your RGI will advice you on this. Oops, sorry for assuming again. Do you have a suitably qualified installer to do this job safely and legally? Which type of gasmeter will feed this lot?
 
R

roomtolet

Tanks in question are heatrae sadia , looking @ the coil rating on line it suggests 25.4 kw , what it does not specify is recovery rate in relation to deltaT.

Thanks for the promt responce so far, much obliged.

Matthew.
 
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Use an unvented with plate heat exchanger recovery also known as semi instantaenous supply.
The plate dumps the boiler heat direct into the top of the cylinder thus when the cylinder energy is expended it continues with the boilers dhw output capacity direct to the taps.
 

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