What is an Injector T and why might I need one?
What is an Injector T and why might I need one?
5 mins

What is an injector t and why might I need one?

Anybody who researches stoves with back boilers and radiators ends up reading about the injector T. Read on and find out more…

The manual is designed to be used alongside Document J of the Building Regulations coupled with any advice provided by your local Building Control. You should not rely on this manual alone. In the case of boiler stoves you will also need to familiarise yourself with Document G of the Building Regulations and any other relevant plumbing regulations.

So we have looked at the primary system and the heat sink radiator. As already suggested this is the foundation of any solid fuel system.

Adding radiators is quite a simple process. Let’s start with a scribble:

Four port boiler. Primary circuit is in black. Radiator flow from stove is in blue, return in orange. For the purpose of clarity the heat sink radiator is not shown on this diagram. Note that it is possible that your radiator circuit might act as a gravity circuit even without the pump on – hot stove water flows up the radiator circuit’s flow pipe through one or more rads and back to the stove. This is usually not desirable (radiators on when one doesn’t want them on). To prevent this one can add a two-port valve next to the pump (closes the circuit completely when the pump is off but opens the circuit when the pump is on).

If your boiler stove has four ports on the rear then this makes life easy. Connect your flow and return to the two spare ports (primary circuit on other two already) and add a pump to the return pipe. The primary circuit is not affected by the pump because the stove is acting as a buffer between the two circuits (whether pump is on or off the gravity circulation continues as if the pump were not there). Both thermostats are connected to the pump.

The thermostat on the PRIMARY CIRCUIT RETURN switches the pump ON when the water returning from the heat sink is hot enough – this ensures that the pump only comes on when the stove is lit and prevents cold water being pumped around the stove which is not good as it encourages condensation within the stove. This thermostat is set at approx. 35-40 degrees centigrade. If the heat sink is a cylinder rather than a radiator then the thermostat ensures the pump does not come on until the cylinder has plenty of hot water (if set at 35-40 degrees). So although very simple in design we do have a “hot water priority” system.

The thermostat on the PRIMARY CIRCUIT FLOW switches the pump ON only if the flow pipe from the stove gets particularly hot. It is set at approx. 85 degrees centigrade. This “overheating” of the flow pipe can take place if the stove is burning out of control (e.g. the door has been left slightly ajar or the user has overloaded the stove with fuel). When the pump comes on the system is cooled (water passing through the radiators).

Unlike the primary circuit, the radiator circuit, due to being pumped, can follow any kind of path (up, down, multiple bends etc.) with trapped air being the only concern where pipes travel “up and over” (avoid this or ensure that at the top is some method of air release or collection).

Top tip: Running radiator pipes under floors is often fraught with problems. Joists are in the way and Building Regulations might not allow notching or drilling. One way around this is running pipes up into the loft (often following a chimney breast) and then dropping down from the loft into rooms below (often in corners where boxing is less obtrusive). Make sure your header tank is well above all of your pipes.

Let’s look at what to do if your stove only has two ports on the rear:

For the purpose of clarity the heat sink radiator is not shown on this diagram.

 Now the pump DOES affect the gravity circulation.

The radiator flow pipe can branch off anywhere along the flow of the primary circuit. The radiator return must enter the primary return in the manner shown above: via an injector T. An injector T is a simple copper plumbing fitting that allows water to be pumped through it whilst sucking in water from its top opening. So in our scribble the pump pushes water into the stove whilst water is sucked in from the pipe coming in from above (this being the primary return). The injector T ensures that the gravity circuit is assisted by the pump and not interfered with. If the injector T is not in place then the pump pushes water into the stove and UP the primary return (and is interfering). Then this happens:

NO injector T. For the purpose of clarity the heat sink radiator is not shown on this diagram.

What happens here is that the water flow within the cylinder is reversed. Now a proportion of the cool water returning from the rads (rads have stolen heat) bypasses the stove and travels through the cylinder, stealing heat from the cylinder before travelling back trough the rads. It will still “work” but the cylinder will not benefit whilst the pump is on and may cool somewhat.


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Costs correct as of April 2023:

Approx. costs if you have a chimney and fireplace ready to use: £750-£1,000 (save £500 by self-installing).

Approx. costs if you have a chimney but need the fireplace "opening up": £1,600-£2,200 (save £1200 by self-installing).

Approx. costs if you do not have a chimney and need a clip-together flue: Shed £475-£700. Bungalow £1500. 2-storey house £2500. Save £1,000-£1400 by self-installing.

Above figures include labour and materials but no appliance.

We, of course, advise you to purchase your stove and materials from Stovefitter's to ensure quality goods are installed (some installers use budget materials to increase margin). If you buy your stove from us (rather than your local small shop or installer) we have a lot more power when approaching manufacturer's with a warranty issue. Why is that? Because we buy many hundreds of stoves a year from these brands.

We do not fit stoves.

But we know a few who do!

Google: Hetas installers

Hetas are the trade body of registered UK installers.

Most installations will require that you slide a chimney liner down your chimney (flexible metal tube 5" or 6" in diameter). Do you have a narrow chimney and want to lessen the risk that a liner might not go down your chimney? Then make sure your chosen stove can use a 5" liner.

Must I line my chimney? Best read this article but most likely the answer is yes. Do I have to fit a chimney liner?

DEFRA-Exempt wood burning stoves with a 5″ collar can usually be fitted to a five inch liner rather than the usual 6″ minimum, making the installer's job much less stressful.


I seriously suggest any self installer fits a 5" liner unless they know their chimney is large enough for a 6"!

What is the best chimney liner? Silvacore 904 (we sell it so of course we will say that ;-). What is the best chimney liner?

Will your stove require an air vent within the room (some stone walls are very difficult to drill)?

5kW or under and wood burning stoves often do not require an air vent (new builds always require an air vent).

What is the maximum output in kW of your "5kW" wood burning stove? The majority of manufacturers just specify the “nominal output” and this figure means very little in real life. The nominal is a figure the manufacturer chooses to sell the stove at - the stove is capable of reaching at least this output with one fuel load. Nominal means "capable of". But it is not the maximum.

Check out the size of the area where the logs will go (firebox size) as this varies enormously. The kW output is completely dependant on the amount of logs burning at any one time - more logs burning equals more heat. If you can fit three logs in stove A and just two logs in stove B then stove A will be capable of throwing out 33% more heat.

DO NOT TRUST MANUFACTURERS’ kW RATINGS as manufacturers specify what output they desire to sell the stove at and testing allows for much “playing with the figures”. This is why you can get very small 5kW stoves (e.g. Aga Little Wenlock) and very large 5kW stoves (e.g. DG Ivar 5 by Dik Geurts which is actually rated 5kW but has a MUCH larger firebox than the Ekol Crystal 5 by Ekol Stoves). A Crystal 5k might get to 5kW and not be capable of any higher whilst a DG Ivar, despite being rated at 5kW, can get to 8kW with a full fuel load.

Note that, over time, one might damage the internal firebricks of a stove by running at a higher load than the manufacturer's suggest. Firebricks are easily replaceable.

Will your wood burning stove fit in your recess WITH the required air gaps around it? This is obviously not an issue if your stove will be freestanding.

Air gaps to non-combustible materials (brick, stone etc.) are usually "as close as you like" legally but manufacturers will sometimes specify a recommendation. This recommendation is there to allow heat to escape from the recess into the room - so you get the heat benefit rather than the heat soaking into the building structure and being lost. If no gap to non-combustibles recommended then we suggest 50-100mm air gap left and right of stove, 50mm behind and 100mm above.

Are you in a Smoke Control Area (usually built up areas)?

Choose your stove accordingly.

A stove must be DEFRA-Approved if you wish to burn wood in a smoke control area.


In simple terms if a stove has an efficiency rating of 70% then 30% of the heat from your logs goes up the chimney.

If a stove has an efficiency rating of 90% then only 10% goes up the chimney.

So think of this in terms of how many logs you have to chop/buy.

Example: A Saltfire Peanut 5 by Saltfire Stoves in Dorset has an efficiciency of 80%.

A tall chimney (6m or more) that is lined will be happy with an efficient stove.

Efficiency importance can be said to be overrated and anything between 75% and 85% is fine. Go much higher and performance can actually suffer (smoke in room when opening door to reload, blackening of glass).

Many modern stoves can go on 12mm thick hearths. Others require full, 5″ thick constructional hearths. All of the stoves we sell state whether or not a 12mm hearth is suitable.More about hearths for wood stoves here.

Can you can talk to somebody on the phone should you need to after the wood burning stove has been delivered, especially if you are self installing? Will the staff at “wesellzillionsofstoves.com” be able to assist with any installation issues? What if there are any problems after install?

Do yourself a favour before ordering stoves or materials on the Internet: Go to Trustpilot and type in the company name before you buy. Some companies advertising at the top of search engines are not good news - check for yourself.

When striving to find thebest 5kW wood burning stovesyou will likely be bewildered by the choice. There are many to choose from. The question I get asked most in our shop is “why should I pay <£1,000> for this one when this other one is just <£500>?”. Here is the very simple answer:the cheaper wood stoves are made in Chinaor Eastern Europe whilst the more expensive are made in Western Europe (or sometimes the USA). Here are a few examples where a more expensive stove might excel over a cheaper stove:

  • Aesthetics (more time spent on design)
  • Hinges (sometimes hidden on more expensive stoves)
  • Better quality glass
  • Thicker steel (longer life)
  • Improved door locking mechanisms
  • Longer warranty
  • Improved controllabilty of flame due to more resource invested on design of air flow within stove
  • Brushed steel fittings instead of cheapy chrome look

Open and close the door on a cheap Chinese stove. Then open and close the door on a DG stoveArada stovesWoodford stovesHamlet stoves or Saltfire stoves. You’ll understand the difference.

Stove pricing reminds me of wine pricing. A £20 bottle of wine is not double the quality of a £10 bottle of wine (the drinking experience might be improved by 20% as an example). We are talking “the law of diminishing returns here. They are all “fire in a metal box” at the end of the day.

Yes. However, there are specific regulations and restrictions in place to address air pollution concerns, particularly in areas designated as Smoke Control Areas. In these areas, only approved "smokeless" fuels or exempt appliances, such as Defra-approved wood-burning stoves, can be used. These stoves are designed to burn wood more efficiently and produce fewer emissions.

All the stoves we sell are DEFRA approved and Eco-design approved and suitable for all areas of the UK.


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A stainless steel tube, slides down a brick/stone chimney to provide a smooth and safe route for smoke.

All of our stoves are approved by DEFRA to burn wood in all UK locations including Smoke Control Areas (towns and cities). Not all stoves are, so be careful if buying elsewhere.

All of our stoves are ECODESIGN approved to be sold in the UK. Not all stoves are, so be careful if buying elsewhere. ECODESIGN is mandatory by law since January 2022.

The base your stove sits on.

If the chimney is the polo mint then the flue is the hole.