Radiators slow to heat.

[theboygazza]

Hello everyone.

This may sound like a naive question, but should radiators take longer to heat up when the boiler is heating the water at the same time
I've had my pump changed and a new three port valve fitted because it wasn't switching between the two. I've therefore had my immersion on the electric so the boiler only had to heat the rads.

Since going over to hot water on the gas the rads seem a bit sluggish - taking over and hour to heat. Before they only took about twenty minutes. If I switch the hot water off the rads fire up straight away.

It's a Potterton suprima 40 boiler with Danfoss equipment throughout. The system is about 20 years old but the boiler is working fine and water and rads do all come on fine once they heat up. Thanks in advance.

 

[stem]

The hot water cylinder can often be piped in 22mm straight from the motorised valve, with a 22mm return to the boiler. As a result, the majority of the water will take the easiest route throught the large diameter pipe and back to the boiler, rather than the far longer, more resistive route around the radiator circuit much of which is probably 15mm pipe (or less).

Ideally, there should be a balancing gate valve somewhere. Usually in the return from the hot water cylinder to the boiler, this operates like the lockshield on a radiator and can be used to reduce the flow through the hot water cylinder, thus diverting more water to the radiators. Having said that, even if there isn't a valve fitted, or it's fully open, normally it only takes a short time to get the cylinder hot again, so the motorised valve will divert all of the flow to the radiators.

The easiest solution is to set the hot water to come on 30 minutes before the radiators, so that it's fully heated before the radiators come on. That way the rads get full flow from the start.

 

[theboygazza]

The easiest solution is to set the hot water to come on 30 minutes before the radiators, so that it's fully heated before the radiators come on. That way the rads get full flow from the start.[/quote]

Cheers for your help. The only problem is the Danfoss controls are old and only allow you to put heating and water on at the same time. Probably best to set the system to come on half and hour earlier while we are at work, so we don't notice the lower temerature at the start. Also, I read that the immersion thermostat should be about 60 degrees, is that right? I'm guessing if it's set too high it will take longer for the valve to switch over to rads only.

 

[stem]

Also, I read that the immersion thermostat should be about 60 degrees, is that right? I'm guessing if it's set too high it will take longer for the valve to switch over to rads only.

The immersion heater is completely different. The immersion has an electrical heating element (shown with blue top below) and is not connected to the boiler/radiator system at all. The coil of pipe shown at the bottom is what is heated by the boiler.

If you are referring to the cylinder thermostat that controls the motorised valve, then 60 degrees is a common setting.

Untitled

• stem
• 9 Mar 2010
• 1

 

[theboygazza]

Yes sorry I was referring to the Danfoss dial at the bottom of the tank that connects to the valve. Presumably the higher this is set the longer it will take for the valve to switch over to rads only? Sorry for being a bit slow on this one, but I'm learning fast!

 

[stem]

Presumably the higher this is set the longer it will take for the valve to switch over to rads only

Correct! A few degrees either way won't make a huge difference though.

 

[theboygazza]

Cool. I'll double check the temperature on there tonight. I've set the system to come on at 4 instead of 4:30 which should give it a fighting chance. I think the problem was we were coming home at 5, having a shower, washing the dishes and putting more pressure on the hot water side when it had only just got the water up to the right temperature and the rads were still warming up. I was more concerned that there was a problem with the system.