It's for a 5 bed 70's house
Before spending money on a large boiler, you should get the house insulated - loft and cavity walls. This will have a significant effect on the size boiler required and your annual running costs.
There are many cheap/free deals going at the moment. See
Energy Savings Trust Grant Search
- Installing Vallaint Ecogen 37Kw
That is seriously oversized, even for a five bed house.
Use the
Boiler Size Calculator to get a more reasonable size. If you know how big your future extension will be, you can add this on assuming the highest level of insulation.
- Installing Vaillant UniStor 210L megaflow
Has anyone bothered to check the incoming cold water flow rate and pressure to the house? You will need at least 20 litres/min.
You can check this at the kitchen cold tap using a bucket and a watch. If there is a garden tap off the mains, do the test with the garden tap closed and then with it fully open. The smaller the difference in filling time, the better.
The Unistor 210 has a 21 minute heatup time (assuming 70% is at 15C) at a primary flow rate of 15litres/min. This is equivalent to a 21kW boiler so, again a 37kW boiler is oversized. However most cylinder thermostats have a 10C differential, meaning that it will start reheating the water when the temperature in the cylinder has dropped to about 50C. This is equivalent to about 25% cold water. This is approximately equivalent to a ten minute shower.
gasafengineer said:
ianniann said:
That house, 90% of the time, will need only a kW or two of heating.
Where are all these 2kw boilers installed?
What is priceless, and very worrying, is that Gasafengineer does not seem to understand what Ianniann means, so I will explain it to him simply.
The size of a boiler is determined by the requirement to heat the house from -1C to 21C, i.e in the winter.
The number of days in a year when the temperature is actually down to -1C are very few. So, for the majority of the year the boiler will be oversized.
The only points which are debatable are the 90% and 2kW figures, which could be a slight exaggeration. An analysis of the weather data in my region (SE) shows that the boiler will be producing less than 50% of the calculated output for about 42% of the year.