Combi or pressurised tank?

Hi Guys,

I am on my second house renovation, My first was easy, replace what was there job and I made a tidy profit. My second project is a little more complicated!

I am getting the boiler replaced. As it is original 1982 job.

It is a large 4/5 bed house. 16 rads.

I was looking at a 40kw combi to get rid of water tank from current location to make better use of the space.

Should I get a pressurised tank instead? I have ample room downstairs to accommodate a cupboard for this off the kitchen area.

The house has two ensuites and a main bathroom to supply.

I like everyone else enjoy a powerful thermostatic shower and I dont think the combi will be good enough really.

All opinions greatly received.

3 Bathrooms - Unvented Cylinder.

I am a retired Heating Engineer and you enquiry about the use of a combi boiler on this second house project. I would strongly advise that you do not install a combination boiler because you will have problems trying to meet the hot water flow requirements under a high simultaneous demand. I would reccomend a sytem boiler working through an Unvented Cylinder providing there is a good mains water presure available you will need at least 3bar pressure. The other thing to check is the incoming water mains supply size as this may need to be increased. the sizing of the Unvented Cylinder should be sized carefully and based on the number of bathrooms. See Heatrae Megaflow or OVO websites. John Evans Tetbury

For a large house I would also consider a bronze circulating pump on the HWC. Instant hot water is a luxury I would pay for again.

Thanks for the advice guys, would a 125 litre tank be enough? And if i have the tank what Kw boiler would I require?

The answer is NO. If the house has 2baths plus 1 shower you will need a 210 litre Indirect Unvented Cylinder or if it has 3 baths plus one shower you will need a 250 litre. If the house is well insulated it can withstand a hot water priority control for the system. This means that when the Cylinder thermostat calls for heat and control motorised valve on the HWS primary flow will open and at the same time the control motorised valve on the heating circuit will close until the Hot water Cylinder reaches its full thermostat setting normallt 60degC. This means that a boiler can be sized based on whichever is the highest the heating load or the Hot water load. For the Hot Water Load you would require 24 Kw plus a margin of 15%. However before you use this figure you need someone to calculate the Heat Losses for each room of the House. One common heating pump needs to be sized to deal with a) The Heating Circuit and or b) The Hot water Cylinder Hope this helps John Evans

I doubt that 125 litres would be enough (unless you all take 2-minute showers). 125 litres HWC provides roughly 250 litres of shower or between 15 and 20 minutes total shower time for single shower heads, less for drench showers.

I'm tempted by a W-plan system with separate flow temperature controls for DHW and weather compensation for CH, size the boiler for the radiators only. Time the DHW and CH differently so they don't clash.