Sanity check..., gravity hot water and pumped radiators, should they work together

[Matstones]

I hope someone can comment on my setup and let me know if I have a problem or not !

I have old gas central heating system, all was working:

gravity fed hotwater heating
radiators via pump.

We then had back boiler fitted to wood burner and replacement cylinder with 2 coil element type fitted by professional plumber, so water can be heated by gas or wood.

Can't seem to heat water from boiler

Can gravity work fast enough to transfer the heat, from cold.

When I try there is lots of scary banging and sound of water rushing through pipes, the input of cylinder gets too hot to touch, the bottom exit gets slightly warm, Water clearly not flowing through it properly.

Even with the radiators on, hot water doesn't seem to get heated, but as the pump is pulling water through radiator circuit it's hard to see why it would flow down into the hot water circuit, but no banging or venting but water does not get heated

As a layman I can think of three possibilities

1/ The water heats up too quickly to create circulation current, boils and vents off
2/ There is a Honeywell valve on exit - I replaced the head, and checked the spigot turns, this seems to be OK, and does activate when powered - and tried locked open - seems to be ok but could be blocked
3/ An air block in hot water heating element

What I want is to be able to heat water from boiler without radiators being on....

Should gravity be enough for water heating only

As there are two parallel circuits (rads/pumped+ water/gravity), it's hard to see how they can both exist together...

If you made it this far - thanks for reading - any comments very welcome

Matt

 

[Matstones]

I still remain confused! You say the "gravity fed wood burner" circuit is connected to the rear coil and is working. But then you say the " gravity DHW " isn't working? There is no such term as that in a heating system. Your coil that isn't heating must be from another boiler, - gas I think you said? Gravity circuits not allowed on new gas or oil installs now.
But as you are not a plumber, you could make a mistake.
The job has clearly not been done by someone who understands the basics of connecting to a cylinder coil. Even if that front coil was on a pumped circuit, it is wrong and air will be trapped in top of coil.
I would get the plumber back who did the job to see what he says is wrong with his work. The job probably needs another plumber who is experienced in gravity circuits and dual heating link ups to check all the pipework.

There are two heat sources and two coils both for domestic hot water (DHW), wood and gas.

The system was installed many years ago, I did test it then, but not really used gas for hot water much due to wood burner and unlimited free wood. We also use the immersion heater as we have PV solar power that provides free electricity. (sometimes!)

Anyway I slackened off the 28mm input to cylinder and air hissed out and carefully heated up the system to create more pressure.

It seems to all work now - I can hear the boiler boil but it doesn't vent, and both ends of the coil now get hot soon after system is turned on.

After about 5 minutes the boiler turns off due to thermostat control, and when cools due to heat transfer it comes on again. I guess this regular on/off/on/off is an unavoidable fact for gravity fed systems as there is not much water in the transfer system when the radiators aren't being used

So a massive thanks to everyone who commented.

I now have a working system and a much better understanding of how my C-Plan system works

 

[Chuck]

Anyway I slackened off the 28mm input to cylinder and air hissed out and carefully heated up the system to create more pressure.

That was a dangerous thing to do. You are lucky you didn't injure yourself.

It seems to all work now - I can hear the boiler boil

You shouldn't normally be able to hear water boiling inside a 'boiler'.

both ends of the coil now get hot soon after system is turned on.

One end should get hot, typically 75°C. The other should start cold and gradually rise to 60°C over a period or an hour or so, at which point the boiler should stop supplying heat.

After about 5 minutes the boiler turns off due to thermostat control, and when cools due to heat transfer it comes on again. I guess this regular on/off/on/off is an unavoidable fact for gravity fed systems as there is not much water in the transfer system when the radiators aren't being used

If the system is installed and working correctly the cylinder should heat constantly without the short-cycling you observe.

In my opinion, your system is not properly installed and is neither safe nor efficient. You really should get a competent heating engineer to review it and fix any issues.

Also, it doesn't normally make financial sense to use PV-generated energy to heat water if you can use gas instead and sell the surplus electricity.

 

[Matstones]

That was a dangerous thing to do. You are lucky you didn't injure yourself.



You shouldn't normally be able to hear water boiling inside a 'boiler'.



One end should get hot, typically 75°C. The other should start cold and gradually rise to 60°C over a period or an hour or so, at which point the boiler should stop supplying heat.



If the system is installed and working correctly the cylinder should heat constantly without the short-cycling you observe.

In my opinion, your system is not properly installed and is neither safe nor efficient. You really should get a competent heating engineer to review it and fix any issues.

Also, it doesn't normally make financial sense to use PV-generated energy to heat water if you can use gas instead and sell the surplus electricity.

Points taken

However with solar power you are paid for all the power you generate, whether you use it locally or feed it back to grid, so not a choice to sell or use, you either sell and use or sell and don't use...

So on a sunny day it produces 3Kw. I am paid for 3Kw regardless. I have choice to dump it in immersion heater, or if I don't use it it flows into grid. Either way makes no difference in terms of £, but one provides tank full of hot water.

 

[Chuck]

However with solar power you are paid for all the power you generate, whether you use it locally or feed it back to grid, so not a choice to sell or use, you either sell and use or sell and don't use...

Interesting. My comment was based on this:

Feed-in tariffs: get money for generating your own electricity: Overview - GOV.UK

, which seems to describe a generation-tariff (as you describe) plus a 4.85p/kWhr export-tariff, which is more than gas costs at the moment. Evidently you either don't have an export meter or have another type of feed-in agreement.

 

[Matstones]

My solar system was installed 8 years ago - the contracts were much more "interesting" then, and the contract holds for 25 years.

Would be a very different contract for a new system today

 

[Best]

Boiler is coming under pressure and forcing gravity flow to work, I reckon. You removing air was only a temporary sort of fix, but not correct or a solution. Get a heating engineer in to repipe the fault we mentioned and anything else they find wrong

 

[zzzjim]

..... Evidently you either don't have an export meter or have another type of feed-in agreement......

Another GOV scheme to get the ball rolling .
(Won't be wanting a Smart meter (IF it can cope !))
... Combining heat sources was expensive to do right ..(Safely)