I have a Polar F11 Heart Rate Monitor. The watch won't pick up the heart rate from the sender (00 is displayed), but my heart rate shows on a Polar enabled Gym machine, e.g. xtrainer. I've changed the battery in the sender.

Language settings should be specified in your user's manual.
See General Settings, Page 12-13
Read it. If you don't have one, download it from here

http://www.polar.com/e_manuals/FT7/Polar_FT7_user_manual_English/manual.pdf

See video

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u may have started wearing a Polar F11 Fitness heart rate monitor to help you keep track of your pulse during cardiovascular exercise. The monitor consists of a strap with electrodes that you attach to your chest, which transmit heart rate data to a receiver worn on your wrist like a watch. The wrist unit stores data and settings that you can customize, such as your weight, height and target heart rates. However, you may wish to reset the unit and start over with new settings. You can reset your Polar F11 by following a few easy steps.
Instructions

1. • 1 Press any button to activate the Polar F11 Fitness heart monitor unit.
     
   • 2 Press all buttons on the Polar F11 Fitness heart monitor simultaneously, except the "Light" button. There are five buttons and the "Light" button is at the top left side of the unit. Continue to hold the buttons until the display fills with digits.
     
   • 3 Press the "OK" button, which is located on the right side of the unit in the middle of the up and down buttons. The Polar F11 Fitness heart monitor will reset.
     

You may have started wearing a Polar F11 Fitness heart rate monitor to help you keep track of your pulse during cardiovascular exercise. The monitor consists of a strap with electrodes that you attach to your chest, which transmit heart rate data to a receiver worn on your wrist like a watch. The wrist unit stores data and settings that you can customize, such as your weight, height and target heart rates. However, you may wish to reset the unit and start over with new settings. You can reset your Polar F11 by following a few easy steps.
Instructions:

Press any button to activate the Polar F11 Fitness heart monitor unit.

• 2 Press all buttons on the Polar F11 Fitness heart monitor simultaneously, except the "Light" button. There are five buttons and the "Light" button is at the top left side of the unit. Continue to hold the buttons until the display fills with digits.
  
• 3 Press the "OK" button, which is located on the right side of the unit in the middle of the up and down buttons. The Polar F11 Fitness heart monitor will reset.
  

Click on my profile (escapement), then click on Tips and Tricks. There you will find link to all POLAR watch manuals. Rate me, please.

try this link for the manual.
polar f11 heart rate monitor manual Google Search .
However, you may wish to reset the unit and start over with new settings. You can resetyour Polar F11 by following a few easy steps. Press any button to activate the Polar F11 Fitness heart monitor unit. Press all buttons on the Polar F11 Fitness heart monitor simultaneously, except the "Light" button.

How to Reset a Polar F11 ' Healthfully

https://healthfully.com/reset-polar-f11-7176556.html reset polar f11 heart rate monitor Google Search

Before you try to change the battery:

Paste this link into your browser and download manual:

http://www.polar.fi/en/support/downloads?product=&category=User+manuals&documenttitle=User+Manual+for+F11+in+English+9%2F2007-&document=/gip/PKBStoGIP.nsf/web_cat/C225742500419A8AC225732F0049BE55

This manual helped a lot to other Polar users.

Do not forget to rate, please.

Paste this link into your browser and download manual:

http://www.polar.fi/en/support/downloads?product=&category=User+manuals&documenttitle=User+Manual+for+F11+in+English+9%2F2007-&document=/gip/PKBStoGIP.nsf/web_cat/C225742500419A8AC225732F0049BE55

Do not forget to rate, please.

You have to change battery.

I' advise to start with battery check. If the battery is ok, then you will need a professional to find the cause of malfunction.

no it shouldnt be cause the heart rate would be based off your pulse

im not a heart specialist but ive had my share of heart polups and a faulty valve in my heart
i was told by my doctor that the heart rate is monitored by bloodpreasure threw the veins pulse so i dont think that the size of the watch would make a difference this is in my opinion based off my own docs words hes the doc what can i say

but recommend that you speak with your doctor about heart monitor watches they know more indepth about these devices or your local pharmacy

i hope this helped you out