Which boiler for a 3 bedroom bungalow with 2 bathrooms

[functor]

I did the "online test" at the worcester website and it recommended http://www.worcester-bosch.co.uk/homeowner/boilers/gas-boilers/greenstar-42cdi which seems to be the most expensive and biggest boiler in the range? Isn't it a bit of an overkill?! I previously had a Baxi Solo PF 50 and it heated the central heating system fine - but I had trident water heaters for the bathrooms (don't ask.. was like this when I got it..)

Edit: This one looks more reasonable: http://www.worcester-bosch.co.uk/homeowner/boilers/gas-boilers/greenstar-30cdi ?

 

[D_Hailsham]

The 42CDi has been suggested because you have two bathrooms. The selector assumes that you want both baths filling at the same time and that you have sufficient flow to enable this to happen.

Presumably you have chosen a combi as you want to get rid of the water heaters and don't want to install a hot water cylinder. Whether this is feasible depends on the incoming cold water flow rate and pressure.

If you want two baths filling at the same time from a combi boiler you need a flow rate of at least 20 litres/min for a 35C temperature rise. Neither boiler suggested would be able to meet this requirement.

You can easily check you flow rate at the cold tap in the kitchen using a litre-marked bucket and a watch. If you have a garden tap straight off the incoming mains, check the flow with garden tap open and then with it closed. The lower flow rate is the significant one.

The old boiler is 15kW, so your heating requirement is about this, but may be lower if the insulation has been improved since the boiler was installed. You can check what you now need by using the Whole House Boiler Size Calculator.

You should also check that your radiators will be suitable as they will give off less heat when fed from a condensing boiler. This is because a condensing boiler runs at lower temperatures. Us the information in the Stelrad Elite Catalogue to find the outputs of you rads (assuming they are modern steel ones). You should use the page headed 40Δt as this takes into account the lower outputs when fed from a condensing boiler.

 

[functor]

I already have a water tank, but I wanted to remove it in order to convert the loft into a room - do you think it is better to use a water tank and forget about a combi?

EDIT: Not sure why the showers are not fed from the water tank, presumably because the previous owners wanted instant hot water from the shower.

 

[D_Hailsham]

I already have a water tank, but I wanted to remove it in order to convert the loft into a room - do you think it is better to use a water tank and forget about a combi?

The tank in the loft is the cold water tank. I was thinking of the hot water cylinder, usually in the airing cupboard.

If you just want to get rid of the tank(s) in the loft, the best solution is to replace the hot water cylinder by an unvented cylinder. This takes
cold water straight from the mains, heats it to 60C and then stores it. When you turn on the hot tap, the pressure of the incoming cold water forces the hot water out of the tap. You then get the advantage of mains pressure hot water without the need to heat the water instantly.

This means that you do not need a very high output boiler to heat the water. How big you need depends then on how much water you draw and how long there is available to reheat the cylinder before the next usage. If the HW cylinder is of sufficient size you may be able to get three 10 minute showers before the temperature in the cylinder had dropped sufficiently for the boiler to come on to reheat the water.

Not sure why the showers are not fed from the water tank, presumably because the previous owners wanted instant hot water from the shower.

It could be because the cold water tank would not provide sufficient pressure. Incoming mains cold water will have a pressure of at least 1 or 2 bar (33 to 66ft head) while a cold water tank in a loft above a bathroom will only have a pressure of 0.3 bar (11ft head).