Back boiler integration

funkey_monkey

Hi, we have just installed a back boiler stove into our home and there is a problem whereby the excess hot water is not being dumped into the radiators unless the central heating is turned on in order to turn on the room thermostats and per the pump.

My expecting was that excess hot water would be sent to the radiators and after that it would be dumped. However the hot water tank does not seem to be able to regulate it self and only did so when the heating is activated. In this instance the pump is turned on and pumps the water, but importantly the oil burner is not turned on as it is not required.

Should the thermostats and pump be independent of the central heating or should there be a switch in the controls to allow them to be permanently powered, regardless of the state of the central heating controller, in order to handle the back boiler?


Thanks.

funkey_monkey

I should also say that the boiler does not send to the excess to any dump radiators.
I presume this is because it is a pumped CH system and not gravity fed?

listermint

Get the plumber back who installed the back boiler.

For my money people shouldnt be getting these installed in this day and age, they are inefficient and not worth the money for them. Waste of cash.

In your case you should have had a seperate new pump installed with thermostatic control off the back boiler to pump the water into the system. It sounds like this was not done. It also sounds like a half arsed job depending on what you asked the installer to do.

funkey_monkey

It was installed as the stove would be the main source of heat in the house and excess heat would be used to warm the water. The new stove was being added and for the sake of a small additional outlay then hot water is also provided whilst it is on.

listermint

If as you say its the main source of heat then the inference is that it was plumbed into the Radiators.

As the main source of heat and being plumbed into the radiators then it should have its own circulation pump.

I.e the Job was not done correctly if there was no pump installed with it.

Get the plumber back.

funkey_monkey

Ahh sorry - that was a typo. It will be the main source of heat in the main living area. Excess heat goes into a tank to heat water. When water goes over the tank thermostat value, it should dump into 2 radiators towel radiator and a normal radiator. Problem is that tank continually heats tank without dumping when the thermostat value is exceeded.

Both radiators are opened.

The only way I can dump the excess HW from the tank is to turn on heating to power the CH pump.

my3cents

Sounds like we are getting there.

It sounds like the back boiler is only good for heating the hot water but if you aren't using it then you need to dump some of the heat into a couple of rads to keep the temp of the back boiler at safe levels so it doesn't explode.

Your hot water is being heated without a pump by thermosyphon action, hot water from the boiler goes up to the hot water cylinder where heat is extracted the water cools and it then drops back to the boiler to get heated again. Thats exactly what should happen.

Now you need to refine the system so that two rads that will have to be above the level of the boiler use any excess heat up. If you are in a bungalow then tough luck it won't work without a pump.

If you are in a two story house then the thermosyphon action can be extended to heat upstairs rads your only problem seems to be that either the thermostat isn't working or it or pipework attached to it doesn't facilitate the thermosyphon.

Disclamer - I'm not a plumber but plumbed in our Range Cooker for hot water with no pump so had to work out how to get it to work myself and its subsequently been checked by a plumber.

Answer really is get the plumber back but it shouldn't be too much of a problem as he has the main bit working right.

funkey_monkey

Should the expansion tank in attic be where this excess goes to? If so, what is the point of the dump radiators as they will need a pump to operate as such.

my3cents

funkey_monkey wrote: »

Should the expansion tank in attic be where this excess goes to? If so, what is the point of the dump radiators as they will need a pump to operate as such.

No hot water should go into the expansion tank in the attic.

There is no excess water only excess heat. The header tank should do virtually nothing except keep a head water in the system allow for expansion and top it up as required.

Now if your water gets too hot it boils and can end up in the expansion tank but that is only an emergency safety feature.

Its simple physics hot water rises cold drops. The top of the boiler (hotest water) is connected to a pipe that goes to the top of the coil in the HWC (Hot Water Cylinder) and the bottom of the HWC coil is connected to the bottom of the boiler. Water warms raises and circulates through the pipes with cooler water coming back down to the bottom of the boiler. The cooler water can still be quite hot.

When the hot water in the return pipe gets too hot because the HWC is getting up to temperature a valve can open that also allows the water heated by the boiler to go into the rads at a suitable height above the boiler again by the process of hot water raising and cold dropping.

Could the issue be the boiler is too big for a HWC and two rads?

my3cents

Here is a simple explanation of how your hot water should be heated

https://www.stovefittersmanual.co.uk/articles/connecting-a-wood-burning-stove-to-central-heating/

The cold fill part from the bottom of the header tank is omitted in the diagram.

Then with a heat sink rad https://www.stovefittersmanual.co.uk/articles/heat-sink-radiator/ note there is no thermostat in this one.

funkey_monkey

my3cents wrote: »

Could the issue be the boiler is too big for a HWC and two rads?

The issue is that the heat sink radiators are never activated. Tank heats as expected from the boiler. Heat sink rads (I've checked and they have no TRV's) do not warm up.

my3cents

funkey_monkey wrote: »

The issue is that the heat sink radiators are never activated. Tank heats as expected from the boiler. Heat sink rads (I've checked and they have no TRV's) do not warm up.

Then get your plumber back, as a test to see whats happening or not you could ask him to remove the thermostatic valve then the rads should heat up along with the water, obviously the water will heat more slowly. If the rads still don't heat then they aren't installed correctly.

funkey_monkey

They heat up with the central heating turned on. They just don't heat up with the back boiler alone.

funkey_monkey

How should the excess heat be supplied to the dump radiators?

my3cents

They are a safety feature so there should really be no valves in the system. The problem with that is they are then sharing the heat with the HWC but if you are producing enough heat that shouldn't be an issue.

Basically they should work as in the explanation I linked to earlier https://www.stovefittersmanual.co.uk/articles/heat-sink-radiator/ you can get away with rads at the same level as the stove but the hot feed pipe has to rise up above the rads and then feed them from above and the return needs to stay as low as possible to get the temperature difference to set up a thermosiphon you also need to use a larger diameter feed pipe.

funkey_monkey

Well this is a single story property.
We are on the plumbers list for a return. Not sure when though. If it requires something beyond a simple tweak it will be into next year now.

From what you say, I don't know what can be changed without butchering the setup.

my3cents

If the rads are all fed from below there is no way in hell that they will work as heat sink rads unless they are mounted on the ceiling.

A good traditional plumber could get this to work but you really need at 28mm pipe running through the roof space and pipes down the wall to the rads you want working without a pump running.

I've seen rads heat that are below the level of the stove (no pump) with some clever plumbing but it needs a degree of skill you may not find from someone who is used to oil boilers and pumps. With an oil boiler a pump is fine because you have immediate on/off control if anything overheats with a solid fuel stove you can't just put it out if the water in it starts boiling and the power fails which is why the job needs doing by someone who understands what they are doing.

Any chance you have one of the modern stoves with the safety feature that allows it to be plumbed in with an oil boiler?

funkey_monkey

my3cents wrote: »

Any chance you have one of the modern stoves with the safety feature that allows it to be plumbed in with an oil boiler?

The only literature I can access states that it is an indirect back boiler. What is this safety feature - although I genuinely doubt that this stove would have it.

my3cents

funkey_monkey wrote: »

The only literature I can access states that it is an indirect back boiler. What is this safety feature - although I genuinely doubt that this stove would have it.

There are some stoves now that have like a internal heat sink that is attached to a cold water source (tank in the roof) so that if the heat goes over a certain limit they can be cooled down. Same stoves can be integrated with a closed central heating system but thats not an idea thats caught on much here as it goes against traditional best practice and leaves the potential however small for something to go wrong.

funkey_monkey

Nope, not got that.

listermint

It might be an idea to map out the system around the Stove / copper tank / boiler on a page and see if anyone has an idea.

funkey_monkey

Spent some time poking about in the cupboard where the hot water tank is located.
Deep in at the back behind a pile of clothes was a thermostat on the stove return pipe which was set to max, over 90deg.

I turned it down to 50deg and the pump then turned on. This dumped water around zone 2 (bedrooms).

I've set both stats now to 60deg.

What would cause the pump to send the water to zone 2? How is this done? I expected it to only pump out to the two dump radiators.

funkey_monkey

I am wondering if I have a false expectation of how the heating system works.

What is the purpose of the boiler flow thermostat?

What I would like the system to do is to maintain the tank temperature at around 65degC to 75degC and circulate the excess to the radiators in at least zone 2.

What seems to be happening is that once per evening the tank will circulate for a brief period of time (normally just enough to circulate the water fully and then turn off, never to turn back on again. Meanwhile the temp keeps rising.

If I turn on the zone 2 central heating it will circulate the water as I expect. Is my problem that I have confused the central heating function with the safety feature of dumping of overheated water from the tank? My problem with this is that I thought there was two 'always on radiators' for this feature. However, when I screw the boiler flow thermostat down to a low temp, then all zone 2 gets circulated for approx 1 - 2 minutes and then it switches off competely.

Also, what temp should the boiler flow thermostat normally be set to? I had it set to 55degC tonight (tank thermostat set to 60degC), but the tank thermometer was reading 78degC.