Rads not as hot when heating water in copper cylinder

newfy

Hi ,

I have new zoned heating..its 2 zones and a new gas boiler! ...I can turn hot water and radiators on individually...

when I turn the heating on only...the radiators get very hot....but when I turn the heating and hot water on together the radiators don’t get near as hot....and the hot water cylinder stat also dosnt get as hot as it would on a 1 hour boost if the hot water had been running on its own off the boiler!

is this normal...is it because the boiler is heating the rads and hot waters that the rads won’t get as hot ??

cheers

newfy

anyone

kirving

Ruling out any problem with the install, you're correct. Boilers for the most part are just set on or off. When they are on, they output X amount of heat until the timer or the thermostat cuts it off. You can usually manually turn it up, but as you've said the rads or water get very hot on their own, so unlikely to be the issue.

Having a boiler that can heat up your house, and a big tank of water at the same time would be a bit over-engineered, and cost a little more to buy in the first place. It's also a bit easier to make a less powerful boilers more efficient, because there is less heat energy to try and control and prevent escaping of the flue.

The theory at least is that you can get away with the smallest, most efficient boiler possible, even in winter, because newer houses are better insulated. That said, and I'm not a heating installer, but I've been in a fair few houses where the boiler has been undersized. Why? Either the customer didn't want to spend more, the installer wanted to submit the cheapest quote, or the sizing was miscalculated based off a theoretical BER. (Most BER's are completely made up IMO. There's a process, but it's fudged almost always, again IMO.)

I'd suggest running the water for an hour, or an then turning on the heating rather than trying to do both together. Running the new boiler for 2 hours almost certainly wont cost as much as running the old one for 2 hours.

John.G

With zoning then as you say bring the HW demand only on for say a hour before CH is required, it also helps to bring the CH on as well approx 10C before the cylinder reaches its setpoint temperature as the cylinder coil will only be emitting ~ 20/25% of its rated output which can lead to boiler cycling, the boiler then has 80% or more of its output available for CH. It is quite common now to rate the boiler on CH requirements only in view of the above.

It used to be common practice to have a gate valve on the cylinder coil return which was left ~ 1/2 to 1 turn open and so throttled the flow/heat input to the cylinder but i think this is actually "forbidden" now on new installs for some reason or other (legionella?).

newfy

Ok thanks for the reply!

Can you recommend what temperature to set the gas boiler! My boiler is currently set at 70 degrees! Should I bump this up? Turn it down ?