Combi or System boiler?

[dar2008]

I'll be moving in a few weeks time to a 5bed detached Victorian house in need of lots of TLC. One of my primary tasks is to have a new heating / hot water system installed and I'm torn between going for a high powered Combi or a System boiler with Megaflo tank. Considering Vaillant or Worcester for the boiler.

We have good mains water pressure in the area and I've always been happy with Combi boilers in preference to a heated tank - I prefer the idea of heating only the water we need rather than heating water that may or may not be used. My only criticism of Combi's is in my experience the hot tap for the bath fills rather slowly - but I put that down to my current model of Combi (Baxi 105he).

I asked my builder to quote for the heating installation based on a Vaillant Combi 937 or 838, and he questioned this suggesting I should go for a system (637) / megaflo solution because of the size of the house.

For reference the house will be mostly occupied by 2adults and 2 or 3 children.

Is it a fair assumption that the running costs of a Combi would be less than a system / megaflo type solution - assuming normal usage?

All opinions welcomed.

 

[linkyplumb]

More important than the number of bedrooms, are your hot water requirements.
How many bathrooms/ensuites etc?
Liklihood of more than one outlet being used at the same time?
As well as mains pressure, you also need to ascertain the DYNAMIC flow rate.
At the cold tap in the kitchen,time how long it takes to fill, say a 2 litre jug, whilst running another mains fed outlet.

 

[dar2008]

Thanks for the reply.

3 bathrooms plus downstairs loo.
Rare that more than one will be used at the same time.
Can't check the dynamic flow rate yet, but will be sure to do that next time I get access.

I don't expect our hot water requirements to change much from our current smaller house.

 

[bell824]

unvented tank and system boiler in this size of house with your size of fammily is the best option!(condition's permitting)

don't think you would notice much cost differance as unvented tanks lose very little heat and proper control's for it will minimise loss.

 

[Dan Robinson]

Without the prerequisite checks, my suggestion is a 170 OSO RI invented cylinder and 18kw system boiler.

The advantage of the OSO is that if you need more hot water you can fit an external expansion vessel and get more hot water capacity.

Just don't fit in the loft

 

[D_Hailsham]

don't think you would notice much cost differance as unvented tanks lose very little heat and proper control's for it will minimise loss.

I'll vouch for that.

Typical open-vent boiler and foam insulated vented cylinder.
Three people in the house.
Two or three showers every evening, usually after 10pm.
The hot water is timed for 9pm to 12.30am (for the showers) and the 12noon to 2pm (top up). So that's a maximum of 5½ hours per day - usually much less as the water temperature has to drop 10C before the cylinder stat calls for heat.

Although the tank has been cooling down overnight for six hours, the water is still hot enough in the morning when we get up and during the morning. It can begin to get cold around lunch time if a lot has been drawn off for housework. The washing machine and dishwasher are both cold fill.