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

[Best]

I am not a plumber but understand the pipe has to rise for gravity to work. Around the back of the boiler (photo doesn't show) are similar 28mm pipes for wood burner circuit which works well.

You have me confused. I thought you said the solid fuel stove won't heat the hot water?

 

[Chuck]

I am not a plumber but understand the pipe has to rise for gravity to work.

It's more complicated. One needs the correct combination of temperature gradients, pipe gradients and points of heat extraction and input for 'gravity to work'.

Whether a pipe rises or falls depends on which direction the water is travelling inside it.

 

[Matstones]

gravity fed wood burner circuit works
(can't see pipework on photo - on back of cylinder, same 28mm fittings)

gravity DHW does not seem to work - seems circuit is blocked

I understand the logic and location of the airlock on heat exchanger input at top

To remove I need to force water through the coil ?. Does it matter which direction (ie from vent, or supply in attic). Isn't there a danger I'll just flush water through the boiler, bypassing the cylinder, as it offers lower resistance

Would cracking open the bolt remove airlock - I suppose not as not much pressure to push air out, unless I heat the system, but knowing me if I did that I'd spray boiling steam into my face, so not too keen in that idea....

 

[Chuck]

If a pro fitted the cylinder you need to get them back to fix the problem. This will involve first checking that the cylinder is suitable for gravity circulation. If so then I would expect them to move the tee feeding the top part of the heat exchanger upwards to get the gradient correct. If it's not a gravity cylinder, they need to fit a circulating pump for the HW or replace the cylinder depending on which you are happy with (and paid for).

Musings: This is a strange mistake for a pro to make. They've used end-feed fittings and and the soldering looks okay but they've left an inverted trap in a gravity-circulation system. Did they assume that your boiler had a pump on the HW circuit, perhaps?

Would cracking open the bolt remove airlock - I suppose not as not much pressure to push air out, unless I heat the system, but knowing me if I did that I'd spray boiling steam into my face, so not too keen in that idea..

. . . and while you were screaming in pain water would continue to pour out of the open joint causing a remarkable amount of damage to your house in the process. Damage that probably wouldn't be covered by your home insurance policy.

 

[Best]

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.

 

[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