Wood Pellet Boiler Review - Simple 25

This wood pellet boiler review is for the “Slimpel 25” and has been carried out on my own boiler. I’ve not received anything in return for the review and I paid the full market price when I purchased it in Dec 2014.

Slimpel 25 Wood Pellet BoilerSlimpel 25 Boiler

Although I did get a £2,500 grant towards the cost of the boiler as soon as it was installed (which anyone can get in N.Ireland) and I also get Renewable Heat Incentive (RHI) payments for the next 7 years, but more about that later.

Like many of you, the main reason my wife and I chose to install a wood pellet boiler was to try to reduce the amount of money we were spending on heating our home by taking advantage of the RHI payments.

After significant research on many of the wood pellet boilers available (and failing to find a decent boiler review ) we chose the Slimpel 25 from Green Energy Engineering (GEE).

Our reasons for buying the Slimpel 25 biomass boiler:

  • We had very limited space in which to install a biomass boiler as it had to fit through a standard doorway (and go down 5 steps) to get into our back garden and it needed to  fit into a boiler house which is 2 m wide x 2 m deep. The Slimpel 25 is 970mm wide, 730mm deep and 1830mm high.

  • It doesn’t need a buffer tank – meaning the cost of installation is less.

  • GEE have a significant market share of wood pellet boiler sales in Northern Ireland (and mainland UK) so they must be doing something right.

  • It’s a modulating boiler - so it can provide up to 25kW of heat when required, but once the house gets up to temperature the boiler can reduce its output right down to 5kW.

  • They happily took me to see existing customer’s boilers in operation.

  • I also thought it would be nice to get free heating for the next seven years and be able to write this boiler review. 

Once we’d decided on the boiler I thought it would also be a good idea to use our experiences to write a wood pellet boiler review to share with my readers and hopefully help you make the right choice when it comes to choosing a pellet boiler.


Our boiler was pretty quick to install, it arrived in the morning on the back of a trailer and after a little manhandling through the door in our garden wall, it was in and working by the end of the day.

Prior to it arriving GEE gave me full details of what I needed to have done for them arriving such as extending the power supply and the flow and return pipes to the new boiler house.

All in all it was very quick and much more painless than I’d expected.

Slimpel 25 Features:

Ash Cleaning:

The Slimpel 25 is the smallest wood pellet boiler that GEE supply and does without some of the features on the larger models such as the automatic ash removal.

My boiler's ash tray has to be removed manually to be emptied approximately every 2 weeks (depending on usage). 

I find that I empty mine every 2 weeks in the winter when the boiler is on for around 14hrs per day and every few months in the summer when it’s only heating the domestic hot water for 2-3hrs per day.

Some of the larger models have an automatic ash removal function, but realistically you’ll still have to remove them to clean around them occasionally.

Cleaning the tray is pretty straight forward. You simply unscrew two locating knobs, lift out the tray, empty it and sweep around the bottom of the boiler where the tray sits before reinstating it.

I turn the boiler off for about 2 hrs before attempting to remove the ash tray to allow it to cool down.

Once the tray is cool it takes about 5 minutes to clean out the tray, brush the burn pot with a steel brush and turn the boiler back on.

Slimpel 25 Ash TrayAsh Tray

Automatic Ignition:

As with most biomass boilers it has automatic ignition, which simply means a small electric heat gun pre-heats the fuel in the burn pot in order to get the combustion process going.

Baffle Cleaning:

The Slimpel 25 has a manual lever located on the top of the boiler which is used to bang the baffles and shake the ash off them.

Slimpel 25 Baffle Cleaning HandleSlimpel 25 Baffle Cleaning Handle

Some of the higher models in the range have automatic baffle cleaning but the manual system is very simple. Each time I’m at the boiler to load pellets (typically every 2 days) I push it down and let it go about four or five times to shake the baffles and keep them clean.


The Slimpel 25 has full modulation, meaning the boiler will initially fire at a high load of up to 25kW until it has the water in the system up to temperature. Then it can modulate its output down to around 5kW to reduce the amount of pellets used.

At present oil boilers don’t have modulating burners, primarily due to cost implications, so they’re either on full or off.

However it should be noted that some of the manufacturers would argue that there is very little efficiency advantages to be had from an oil fired boiler with a modulating burner.

Separate Hot Water Control:

Although we don’t use this feature (as we have a three zone system) the Slimpel 25 is also capable of managing your domestic hot water separately from the heating if required.

Control Panel:

Slimpel 25 Control PanelSlimpel 25 Control Panel

The digital control panel is simple and clear offering information such as:

  • outside temperature
  • water temperature
  • whether the boiler is in the starting, cleaning or heating phase etc.
  • It also shows the boiler output in kW’s so you can see if it has modulated down or if it’s having to work at full capacity etc.

To be honest I haven’t had to do anything with the control panel since it was installed, other than getting an idea of what the output is or the air temperature etc.

Inbuilt 120kg Hopper:

As we don’t have enough room for a bulk hopper we manually feed the boiler every few days (depending on the season) so the inbuilt 120kg hopper comes in very handy.

Slimpel 120kg HopperIn Built 120kg Hopper

As you’ll see below in the winter we use between 30 - 40kg of pellets per day depending on how cold it is outside. (Jan 2016's numbers should be taken in context because at the time of compiling the chart, we were under 2 weeks into January).

This is potentially the only real downside of our installation as it means I have to fill the boiler every two or three days in the winter and once a week in the summer. But in my opinion it’s a small price to pay for free heating.

Slimpel Pellet Usage ChartWood Pellets Used So Far

I don’t see the manual loading as a major issue as it only tales a few minutes. But it is little more hassle than a gas or oil boiler which you can pretty much ignore. But then again they're not free.

Alternatively, if you’ve got the room, you could have a 3.5 ton bulk hopper installed which allows you to forget about your boiler to an extent (apart from ash cleaning). Plus bulk pellets are cheaper to buy than the 10kg bags.

Typically I pay around £240/ton (delivered) for my 10 kg bags whereas 3 ton of bulk pellets are typically around £210/ton (delivered).

What self-respecting wood pellet boiler review would be complete without a graph showing our pellet usage so far.

Bulk HopperBulk Wood Pellet Hopper

My Slimpel 25 Boiler Review Conclusion:

Having lived with the boiler for over a year now and kept records for the purposes if this boiler review throughout the journey, I’m really impressed with it.

It’s been fault free and GEE have been very professional. Any time I’ve had any questions they’ve answered them really quickly.

For more updates on our Slimpel wood pellet boiler review click here.

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