solar cylinder

baldieman

Hi,
Have been looking at a solar thermal system with a typical 300L s/s cylinder.
The problem is both coils are at the bottom of the tank, that is the central heating coil. Now, correct me if I'm wrong, but this would make the whole system very inefficient would it not? Your central heating is working to heat the whole tank and in the morning when the sun gets up, the temp. in the cylinder is already at 60 degrees. The solar manifold has to exceed 80 degrees before delivering useful heat. Any thoughts?

kevymoss

Hello
What make is the cylinder. Normally the coil is the primary heat source ike a boiler. The solar coil should be at the bottom.
Whereabouts are you based. There are loads of different cylinders available so if you pm me I could help you
Thanks
Kevin

quentingargan

It looks to me like this is a twin coil cylinder. The specs for a solar cylinder are very different. You are right - the solar should heat water at the bottom of the cylinder only, and the heating coil should heat the top 120L to 140L.

Also, the solar coil should have a very high surface area. There are cheap "solar" cylinders that look fine, but the solar coil only has a minimal surface area. You need AT LEAST 0.25 sq m of coil surface area for each 1sq m of panel. If you are using 6 sq m of panels, you need at least 1.5 sq me of coil area.

I can pm you details of suitable cylinders Q

MiniGolf

If your heating/hot water from your boiler is properly controlled with a cylinder thermostat and motorised valve arrangement or similar the boiler will NOT put any heat into the cylinder if it is already up to temperature. Also the maximum water temperature of hot water coming from a hot tap should be no more than 60 degrees! This is to prevent the risk of scalding.:eek:
The fact that both coils are near the bottom is not really an issue if you have the proper controls that give electrical "interlock" to the boiler although they generally are one above the other.

quentingargan

MiniGolf wrote: »

If your heating/hot water from your boiler is properly controlled with a cylinder thermostat and motorised valve arrangement or similar the boiler will NOT put any heat into the cylinder if it is already up to temperature. Also the maximum water temperature of hot water coming from a hot tap should be no more than 60 degrees! This is to prevent the risk of scalding.:eek:
The fact that both coils are near the bottom is not really an issue if you have the proper controls that give electrical "interlock" to the boiler although they generally are one above the other.

Hmmm. Doctors differ and patients die, but the point here is that if the heating system cuts in first thing in the morning and brings the cylinder up to temperature, the solar is redundant for the rest of the day. Thats why the solar coil is at the bottom and the other coil is at the top - so you can have hot water available to the consumer and cold water available for the panel to work on.

I would suggest with the buffer that you allow the cylinder to go to 85 degrees. This is safe enough provided you have a thermostatic mixing valve on the outlet of the domestic cylinder at the other location and/or controls to ensure that the buffer can't bring the other cylinder above 60.

Price Check

Delay the boiler coming on until the solar has time to decide if its coming on (timeclock) and then use a pipe-state on the solar return to see if its doing anything, if it is leave the boiler off.

baldieman

Price Check wrote: »

Delay the boiler coming on until the solar has time to decide if its coming on (timeclock) and then use a pipe-state on the solar return to see if its doing anything, if it is leave the boiler off.

That sounds like an interesting solution, how practical is it? is it complicated or expensive?
ps. thanks guys

quentingargan

Price Check wrote: »

Delay the boiler coming on until the solar has time to decide if its coming on (timeclock) and then use a pipe-state on the solar return to see if its doing anything, if it is leave the boiler off.

Good idea, but arguably, you don't need the boiler to heat the cylinder in the morning at all. It should still be hot from the night before. So simply try tweaking the timer on the motorised valve to this cylinder and cut it. However, not sure if you can do this because it is also the buffer tank for the central heating, isn't it?

Dixy

The OP'ers main problem is wasted energy.
Most people have the boiler coming on early morning to heat the house up when everyone gets out of bed but this also results in the cylinder being heated to temp and so when everyone leaves for work and the solar starts working, the cylinder is already fully heated and doesnt require any more.
Delaying the boiler from coming on is one option but that doesnt heat the house on a cold early morning.
I would suggest controlling the cylinder's motorized valve on the heating circuit with a second clock instead of a stat, keeping the valve closed in the morning when the boiler is on allowing the boiler to just do rads and then open when the solar comes on to heat the cylinder.
The cylinder should still be hot enough from the previous evening to allow for showers in the morning and then heating again during the day from the solar saving expensive gas.

baldieman

Dixy wrote: »

The OP'ers main problem is wasted energy.
Most people have the boiler coming on early morning to heat the house up when everyone gets out of bed but this also results in the cylinder being heated to temp and so when everyone leaves for work and the solar starts working, the cylinder is already fully heated and doesnt require any more.
Delaying the boiler from coming on is one option but that doesnt heat the house on a cold early morning.
I would suggest controlling the cylinder's motorized valve on the heating circuit with a second clock instead of a stat, keeping the valve closed in the morning when the boiler is on allowing the boiler to just do rads and then open when the solar comes on to heat the cylinder.
The cylinder should still be hot enough from the previous evening to allow for showers in the morning and then heating again during the day from the solar saving expensive gas.

Ok, the plot thickens, Is it fare to say that it is a badly designed cylinder and would be a much simpler system to have the central heating coil at the higher part of the tank?

Dixy

Personally i think its a very poor design.
There is no need for the coil being heated by the gas boiler to be anywhere near the bottom of the cylinder and the solar coil should be as low down as possible to allow it to heat the entire cylinder to temp.
The gas boiler should only be heating the top half to allow some hot water if its a very poor day for the solar.